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Review

Life-History Traits of a Small Cosmopolitan Copepod (Oithona similis) in the Barents Sea: A Review

by
Vladimir G. Dvoretsky
* and
Alexander G. Dvoretsky
Murmansk Marine Biological Institute of the Russian Academy of Sciences (MMBI RAS), 183038 Murmansk, Russia
*
Author to whom correspondence should be addressed.
Biology 2026, 15(1), 27; https://doi.org/10.3390/biology15010027
Submission received: 20 October 2025 / Revised: 17 December 2025 / Accepted: 19 December 2025 / Published: 23 December 2025
(This article belongs to the Special Issue Feature Papers in 'Conservation Biology and Biodiversity' (2nd Edition))
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Simple Summary

This review highlights the important role of the small copepod Oithona similis in the Arctic ecosystem, particularly in the Barents Sea amid ongoing warming. Oithona similis is a common planktonic species whose abundance and biomass vary across different water masses, with notable populations in coastal and Arctic waters. Despite contributing less to overall zooplankton biomass compared to other copepods, it can still represent a significant portion during certain seasons. Its reproductive activity and morphological characteristics differ regionally, influenced by environmental factors such as temperature, salinity, and chlorophyll levels. The species exhibits distinct populations across the southern, central, and northern parts of the sea, each with unique life cycle durations and reproductive patterns. The highest mortality rates occur in summer, driven by predation, parasitism, and competition. These findings underscore the species’ adaptability and the influence of environmental conditions on its biology. Understanding these dynamics is crucial, as Oithona similis plays a vital role in the Arctic food web, and its responses to climate change may have significant ecological implications in this rapidly changing region.

Abstract

This review synthesizes current knowledge on the biology and ecology of the small cyclopoid copepod Oithona similis, a prevalent planktonic species in the Barents Sea, during the period of Arctic warming since the early 2000s. The region serves as an effective model system for examining the influence of different water masses on Arctic zooplankton dynamics. The highest abundances and biomass of Oithona similis are observed in Murmansk Coastal Waters (MCW) and Arctic Waters (ArW). Although its contribution to total zooplankton biomass is generally lower than that of higher copepod taxa, it can account for up to 27–35% seasonally and regionally. Ovigerous females are most abundant in Novaya Zemlya Waters (NZW) and ArW. Egg production rates exhibit a decreasing trend from south to north across the sea. Morphometric analyses reveal an increase in prosome length for both sexes, while relative antenna size diminishes from the south (MCW) to the north (ArW). The highest mortality rates occur during summer, coinciding with peak abundances of Oithona similis, its predators, and parasites, as well as increased interspecific competition. Based on morphological and reproductive parameters, three distinct populations are delineated within the Barents Sea: southern (MCW), central (Atlantic Water/Barents Sea Water), and northern/eastern (ArW/NZW), with respective life cycle durations of 11–12, 9–10, and 11 months, and typically one to two generations per year. The primary environmental drivers influencing population abundance, biomass, size, and reproduction are temperature and salinity, while chlorophyll a concentration predominantly affects mortality rates.

1. Introduction

The Barents Sea represents the largest Arctic continental shelf region. The total area of the sea is approximately 1,400,000 km2 with an average depth of 230 m [1]. The maximum depth (500 m) is registered in the western part of the Bear Island Trench [2]. The northern part of the Barents Sea ecosystem borders the Arctic Ocean, while the western, eastern, and southern boundaries are defined by the shelf break toward the Norwegian Sea, the Novaya Zemlya archipelago, and the coasts of Russia and Norway, respectively [3]. The Barents Sea is a transition zone for warm Atlantic waters entering from the Norwegian Sea and contributing to the deep-water areas of the Arctic Ocean [4,5]. In the northern part, the upper layer is occupied by Arctic waters with relatively low salinity and density partly originating from the Arctic Ocean [6]. Atlantic water is mainly confined to the southern, western, southwestern, and central parts [7]. The boundary between these two main water masses is referred to as the Polar Front [8]. It is located across the eastern slope of the Svalbard Bank and eastward from Hopen towards Novaya Zemlya [1,3].
Among all Arctic regions, the Barents Sea is considered the most productive area [9,10]. Here, phytoplankton produce 49% primary production of the total primary production on the pan-Arctic shelf [2,7,11]. Such high productivity is mainly utilized by plankton animals that are the main food resource for pelagic fish [12]. Subsequently, these pelagic fish, particularly capelin Mallotus villosus and herring Clupea harengus, are consumed by Atlantic cod Gadus morhuaand and haddock Melanogrammus aeglefinus [13,14]. For this reason, the Barents Sea includes one of the world’s largest fishing areas [15], which is located in permanently ice-free waters in the south and southwest regions [16] strongly influenced by warm waters from the Norwegian Sea [6].
There are six major water masses in the Barents Sea [2,17,18]: (a) Murmansk Coastal Water (MCW)—coastal areas of the Kola Peninsula and adjacent waters (southern part); temperature range 1–9 °C; salinity 33.8–34.7 psu; (b) Atlantic Water (AW)—central and western parts of the sea; temperature > 3 °C, salinity > 35.0 psu; (c) Arctic Water (ArW)—northern part of the sea (temperature < 0 °C, salinity 32.0–34.8 psu); (d) Barents Sea Water (BSW)—eastern part of the sea (temperature –1.5–+5 °C; salinity 34.5–35.0 psu); (e) Pechora Coastal Water (PCW)—south-eastern part = Pechora Sea (temperature –1.9–+8 °C; salinity 32.0–34.0 psu); (f) Novaya Zemlya Coastal Water (NZW)—eastern part (temperature –1.9–+8 °C; salinity 32.0–34.0 psu) (Figure 1).
Copepods are among the most frequent, abundant, and widely distributed marine organisms in the World’s Ocean [19,20,21]. These animals are a major food resource for ichthyoplankton, pelagic fishes, young demersal fishes and macrozooplankton [22,23,24,25]. Copepod assemblages link primary producers and microzooplankton to higher trophic levels [26] and therefore, play an important role in pelagic food webs worldwide [27,28]. The significant role of copepods in the functioning of the Barents Sea ecosystem makes this group a subject for extensive studies [29,30]. Most of the data on the biology of planktonic crustaceans are already available especially for larger taxa (e.g., Pseudocalanus and Calanus spp.) because they are the most important food resource for commercial fish [31,32,33]. However, results of recent studies suggest that small species may be more significant in marine food webs than that was previously considered [34,35,36].
The cyclopoid copepods of the genus Oithona occur in many marine geographical regions [37,38,39,40,41,42,43,44,45,46,47,48]. The population density and biomass as well as population structure of Oithona spp. demonstrate significant variations at interannual, seasonal, and regional scales [49,50,51,52]. Moreover, the cyclopoid copepod Oithona similis Claus, 1866 is a cosmopolitan species that inhabits circumglobally and it has been described as a eurythermal, euryhaline, and omnivorous species well adapted to a wide range of habitats [38]. It has been suggested that O. similis may be considered the most numerous copepod in the world [34]. Oithona spp. feed on smaller-sized organisms including heterotrophic or autotrophic microplankton, and copepod nauplii, and they are the preferred food of fish larvae and other zooplanktivores [53,54,55]. In the Barents Sea, O. similis represents an example of the genus Oithona that reaches maximum abundance among other small planktonic copepods [56,57,58,59,60,61,62]. O. similis appear to be less affected by seasonal changes in phytoplankton abundance than larger herbivorous copepods (Calanus spp. and Pseudocalanus spp.). This small opportunistic species remains active all year round and does not undergo diapause in Arctic regions [51,52,63,64,65,66,67].
The abundance and biomass of Oithona similis can, in certain seasons and regions of the Barents Sea, surpass those of Calanus finmarchicus [68]. Due to their high average abundance and small body size, O. similis plays a crucial role as a food source for various zooplankton taxa, including larger copepods, some macrozooplankton groups (such as chaetognaths, hyperiid amphipods, and medusae), as well as for fish larvae and planktivorous fish species, including capelin and herring. In cold years, when the biomass of C. finmarchicus declines relative to more typical or warmer years, O. similis may assume the role of the primary food source for adult fish in the Barents Sea [68]. While the biology of O. similis has been extensively studied in temperate waters [42,43,44,48,54,55,69,70,71], recent years have seen significant advances in understanding its ecology in the Barents Sea [57,58,61,62,72,73,74,75,76,77,78,79,80,81].
The Arctic environment has undergone substantial climatic changes over the past few decades [82,83]. Key factors driving significant shifts in pelagic ecosystems include rising water and air temperatures, retreating Arctic sea ice, declining sea-ice extent, alterations in oceanographic regimes, and intensified inflow of Atlantic Water [84,85]. A particularly notable phenomenon is the Atlantification of the Barents Sea, characterized by the northward expansion of boreal taxa and the concomitant migration of cold-water species toward the Arctic Ocean [86]. Additionally, warming trends have extended the growth season of phytoplankton assemblages [6,87], resulting in increased primary productivity and enhanced chlorophyll a concentrations in the Barents Sea [88]. These changes have cascading effects across all trophic levels, impacting both large zooplankton and smaller copepods [61,62,79,80,89].
This review synthesizes critical data on the biology of Oithona similis in the Barents Sea, aiming to evaluate the predominant environmental drivers affecting this species and enhance our understanding of its life history within the region. The review addresses key aspects including the distribution, abundance, biomass, reproductive strategies, mortality, morphology, and life cycle of O. similis during the recent period of Arctic warming that has characterized the early 21st century. The insights derived from this analysis may serve as a valuable resource for monitoring shifts in zooplankton assemblages in the Barents Sea in response to climatic perturbations, thereby contributing to a more comprehensive understanding of how pelagic ecosystems adapt to variations in environmental conditions.

2. Abundance and Biomass of Oithona similis

Population density of O. similis varies in a wide range depending on the location of sampling stations, water masses (environmental conditions), and season (Table 1).
In summer, the lowest values of Oithona similis abundance and biomass are generally observed in areas affected by freshwater discharge, such as the Pechora Sea and southern Kola Bay, where salinity is low and mean abundance does not exceed 100 individuals m−3 (ind. m−3). In contrast, maximum abundances are recorded in regions influenced by Arctic and Atlantic waters, particularly in frontal zones (AW–ArW, AW–BSW), as well as in coastal areas enriched by organic inputs from the shore, especially in bays near significant seabird colonies (MCW and NZW), where mean abundance can reach up to 1000 ind. m−3 [81,93]. Seasonal fluctuations in total O. similis abundance are closely linked to the location of water masses, with overall abundance typically increasing during the spring and summer, and exhibiting a marked decline in late winter.
During the summer months, O. similis constitutes 21–35% (with peaks up to 50%) of total mesozooplankton abundance in MCW, 35–45% (up to 75%) in AW, 23–48% in coastal waters off Novaya Zemlya, and 25–28% in ArW. Studies conducted in Kola Bay (coastal waters of the southwestern Barents Sea, MCW) indicate that the total abundance of O. similis varies from 109 to 256 ind. m−3 in winter, 120 to 2760 ind. m−3 in spring, 410 to 1300 ind. m−3 in summer, and 300 to 400 ind. m−3 in fall, with peak abundance recorded between 4700 and 9600 ind. m−3 during June and July [57,74]. In Kola Bay, this species comprised 15% of total abundance in spring, 25% in fall, and 35% in winter [74].
Overall, the total biomass of O. similis is considerably lower compared to larger copepods and other crustaceans, such as krill and hyperiids, in the Barents Sea [32,33]. The contribution of O. similis to the total zooplankton biomass exhibits considerable variability depending on the season, water mass, and specific year of study (Table 1). Notably, O. similis is absent in certain estuarine areas characterized by extremely low salinity, such as the delta of the Pechora River and the mouth of Kola Bay [58,90]. In other regions of the Barents Sea, the species’ proportion of total biomass ranges from 0.1% to 27.3% during summer (see Table 1). In specific seasons, such as winter 2010 in BSW, O. similis may account for as much as 35.4% of total zooplankton biomass [95,96]. Similar trends have been documented in the White Sea, where the relative biomass of O. similis fluctuated between 0.1% and 30% (mean 3.1%) during the summer periods of 2001 and 2008 [105]. In the Kara Sea, O. similis represented 0.2–11.1% of the total zooplankton biomass during winter [106], while accounting for 0.3–41.0% (mean 6.1%) in the summer of 2012 [107]. For comparison, in Godthåbsfjord (Greenland waters), the relative density of Oithona spp. (predominantly O. similis) peaked in summer, coinciding with maximum seasonal abundance [52]. There, the seasonal cycle of Oithona spp. density exhibited clear succession patterns, with low abundance recorded in late winter to early spring, surging to peak levels from July to August (3000–7330 ind. m−3, recalculated from original data by Zamora-Terol et al. [52]). Conversely, in Kongsfjorden (Svalbard waters), O. similis was present year-round, with maximum values observed during winter (November) and lowest abundances recorded in summer [63]. In Isfjorden, a pronounced peak in O. similis abundance was detected in September (5000–6500 ind. m−3), while the minimum occurred in March to April [108].
Environmental forcing significantly influences zooplankton communities across the World’s Oceans [2,7,11,19]. In the Barents Sea and other Arctic regions, the response of plankton assemblages is contingent upon regional, seasonal, and climatic conditions [33,51,52,65,75,98,100,109,110]. Within coastal waters (MCW), Oithona similis exhibits a strong negative correlation with salinity, while its abundance positively correlates with water temperature [57,58]. Conversely, in PCW, salinity positively affects both abundance and biomass, whereas temperature shows a negative correlation with these parameters. Interestingly, in AW, total abundance of O. similis was also negatively correlated with temperature [57,58]. This observation likely results from elevated water temperatures during the study periods, which are unfavorable for O. similis, as the species is adapted to colder conditions.
In Arctic regions, total abundance of O. similis generally increases as water temperature decreases; however, in some instances, no significant relationships were observed between environmental variables and indices of O. similis [57,58]. Similar trends have been noted in other research. For example, a comparison of long-term datasets from the North Atlantic and Mediterranean Sea revealed that O. similis abundance was negatively correlated with water temperature only in the Mediterranean, where the species exhibited an optimal temperature range of 15 to 20 °C—beyond which higher temperatures limited O. similis occurrence [70]. In the North Atlantic, the mean abundance of O. similis was lower than in the colder waters of the Barents Sea and adjacent Arctic regions. Notably, peak density of O. similis near Plymouth was recorded in March, with 286 ind. m−3 for females and 737 ind. m−3 for copepodites [48]. In Svalbard waters, sea water temperature was identified as the primary factor controlling 62% of the variance in O. similis abundance [108]. Latitudinal analyses indicate a preference for colder waters, suggesting ecological implications for studying the effects of climatic forcing on zooplankton assemblages and their phenology [70,71].
Food availability also influences O. similis abundance in the Arctic and other areas [42,53,54,55]. However, no correlations have been found between O. similis population density and phytoplankton biomass, as measured by chlorophyll a concentrations (Chl-a), in the Barents Sea [57]. Similarly, Castellani et al. [70] observed only a weak positive correlation between O. similis abundance and Chl-a in the North Atlantic and Mediterranean. This lack of correlation may stem from the feeding behavior of O. similis, which prefers to consume ciliates (e.g., Strombidium spp. and Myrionecta spp.) and other protozooplankton, although phytoplankton are not completely disregarded as a food source [42,53,54]. Seasonal lipid analyses of O. similis have indicated high levels of 18:1 (n-9) fatty acids throughout the year, suggesting a generally omnivorous, carnivorous, or detritivorous diet in the Arctic Kongsfjorden (Svalbard waters) [63].
Thus, the contribution of Oithona similis to total zooplankton abundance in the Barents Sea is substantial. However, its proportion in the total zooplankton biomass has been relatively low in the context of recent climatic changes in the Arctic. Temperature emerges as the primary factor influencing O. similis abundance in the Barents Sea. The species’ relative importance in zooplankton assemblages tends to diminish during warming periods compared to colder years, likely due to the unfavorable conditions associated with higher water temperatures.

3. Reproduction of Oithona similis

In summer, Oithona similis females comprised 45% of the total population abundance across the Barents Sea, with the exception of the northern regions (ArW and BSW). This proportion increased as one moved from the southern coastal waters (MCW) to the central Atlantic waters (AW), then decreased again in the northern regions (ArW) [57]. A similar trend was observed for males; however, copepodite IV exhibited a latitudinal decrease from MCW to AW, peaking in ArW. The sex ratio of O. similis varied between 1:3 and 1:8, averaging around 1:5. Notably, environmental factors did not influence the sex ratios, and inter-annual variations were minimal [57].
During the summer, ovigerous females (those carrying egg sacs) were found in all regions of the Barents Sea, with their relative abundance ranging from 5% (in AW) to 52% (also in AW) [73]. Mean values showed an increasing trend from the south (MCW and AW) to the north, with particularly high proportions in the northern shelf waters (NZW) and ArW (Table 2). Overall, lower numbers of ovigerous females were observed in southern coastal waters during the summer, indicating a spatial pattern in the reproductive behaviors of O. similis [73].
The female reproductive effort (FRE), defined as the ratio of female carbon mass to egg carbon mass, was found to be lowest in ArW, AW, and BSW in 2007 (25%). In contrast, maximum FRE values were recorded in MCW and AW in 2005, ranging from 45% to 47% [57,73]. Higher values of female reproductive effort were noted in MCW, whereas the northern regions exhibited consistently low FRE (Table 2).
A latitudinal trend in egg diameter was observed for O. similis in the Barents Sea, with the smallest diameters recorded in PCW and NZW, intermediate values in MCW and AW, and the largest diameters found in ArW and BSW. Egg diameters varied from 44 to 70 µm during the summer seasons [73]. Comparisons of mean egg diameter did not reveal significant differences between summer and autumn periods [57].
Clutch size (CS), defined as the number of eggs per sac, was relatively stable across different regions in summer, ranging from 18 to 27 eggs, with a mean of 21 to 24 eggs (Table 2). Maximum clutch sizes were observed in the coastal waters (MCW, NZW, and PCW) [73]. In autumn 2007, CS varied from 16 to 32 eggs per sac (Table 2).
Egg production rates (EPR) during the summer ranged from 0.09 to 1.84 eggs per female per day, with the highest rates occurring in MCW and AW [73]. A significant decline in mean EPR was evident from the southern to the northern Barents Sea (Table 2). Interestingly, EPR in autumn was higher than in summer, varying between 0.9 and 1.89 eggs per female per day [57]. Specific egg production rates (SEPR) for O. similis ranged from 0.002 to 0.045 day−1 during summer, with peak values observed in MCW and AW [73]. Mean SEPR was comparable in PCW, NZW, and ArW, while BSW showed intermediate values (Table 2). In autumn, SEPR values in MCW exceeded those recorded in the summer [58].
According to ANOVA and multiple comparisons, significant differences in the reproductive characteristics of O. similis were identified across the Barents Sea during summer (Table 3). The most pronounced differences were observed between the Arctic (ArW, BSW) and Atlantic (AW, MCW) regions [73]. Coastal areas also differed significantly from open sea sites (Table 3).
The relative abundance of ovigerous females of Oithona similis showed no significant correlations with fluctuations in environmental factors [57,73]. However, female FRE, CS, EPR, and SEPR were positively correlated with average water temperature, while a negative relationship was found with egg diameter [73]. An increase in the proportion of ovigerous females and egg diameter was associated with higher salinity levels, whereas FRE decreased with increasing salinity. Overall, temperature and salinity accounted for 50–93% of the total variance in female reproductive parameters. Additionally, Chl-a positively impacted CS and SEPR in O. similis [73].
Reproductive patterns of Oithona spp. have been studied in other Arctic and sub-Arctic regions. In Greenland waters, ovigerous females ranged from 30% to 80% from March to August 2010, with a relative abundance of 12% to 56% observed during the winter-spring period [51]. The mean clutch size for Oithona spp. was reported to be 20–27 eggs per female, which positively correlated with temperature during the summer [52]. In contrast, the mean clutch size during winter-spring ranged from 14 to 38, peaking during the post-bloom period [51]. Mean EPRs were recorded as 0.09–0.30, 0.09–0.91, and 0.13–1.65 eggs per female per day during winter, spring, and summer, respectively [51,52]. The mean SEPR was highest at 0.038 day−1 in summer, while winter and spring values did not exceed 0.019 day−1 [51]. Both EPR and clutch size showed positive correlations with water temperature, but no significant correlations with Chl-a during the summer. EPR increased significantly with protozooplankton biomass during the spring period [52]. In Balsfjord, Norway, mean clutch sizes of O. similis were found to be 8–9 in spring and 17–23 in summer during 2017–2018 under experimental conditions. The SEPR of O. similis varied from 0.02 day−1 in spring to 0.12 day−1 in summer and correlated with surface temperature [111].
Consequently, the reproduction of O. similis in the Barents Sea and adjacent Arctic regions is primarily influenced by fluctuations in water temperature. Furthermore, temperature has been identified as a key factor driving fecundity in Oithona spp. across other polar regions [45,49]. Experimental studies have indicated that EPR in Oithona is related to egg size, salinity, and temperature in various Arctic regions [41]. Food concentration also affects reproductive parameters, with protozooplankton density having a more significant impact than phytoplankton biomass, particularly in Arctic waters. This trend appears to be even more pronounced in temperate waters such as the North Sea and North Atlantic Ocean [42,54,55,69]. Recent investigations utilizing DNA barcoding have highlighted the dietary plasticity of Oithona similis in the Barents Sea. Phytoplankton constituted a significant portion of their winter-spring diet (up to 43%), while animal food items were more prevalent in other seasons [112]. As omnivorous organisms, Oithona spp. can sustain their activity year-round and are capable of reproducing throughout all seasons, as they are not limited by seasonal fluctuations in phytoplankton density. Oithona spp. play a crucial role as important copepods during winter and autumn when larger calanoid copepods are in diapause [38,68]. Therefore, O. similis is vital for high-Arctic ecosystems during periods of low activity and density of herbivorous zooplankton.

4. Mortality of Oithona similis

Instantaneous mortality rates for the stage pairs copepodite IV to copepodite V (MC4-C5) and copepodite V to adult (MC5-A) of Oithona similis were examined using a vertical life table approach [78,113,114]. The stage-specific daily mortality rates exhibited significant variations across different water masses, with distinct spatial patterns observed during periods of population peaks (Table 4).
Overall, the mortality rate for the stage pair copepodite IV to copepodite V (MC4-C5) varied from 0.003 to 0.335 day−1 during the summer period [115]. The highest rates were recorded at certain coastal sites within MCW and PCW (Table 4). In contrast, the minimum MC4-C5 rates were found in northern areas (ArW), ranging from 0.03 to 0.12 day−1, with low mean estimates (Table 4). Analysis of MC4-C5 dynamics in MCW revealed significant seasonal differences, with higher mortality observed in autumn (ANOVA, p < 0.05) (Table 4).
Spatial variations in the mortality of O. similis, particularly for copepodite V to adult (MC5-A), were documented in the Barents Sea. Maximum summer mortality rates were recorded in MCW at 0.382 day−1, while minimum levels were observed in ArW at 0.001 day−1 [57,115]. Mean values in AW, BSW, NZW, and PCW were similar during the summer period (Table 4). Regionally, mortality rates were generally higher in MCW compared to other water masses during the summer population growth phase, with C5-A summer mortality rates lower than those observed in autumn in MCW (Table 4).
The primary factors influencing mortality rates of O. similis in the Barents Sea are water temperature and salinity [78,115]. Temperature plays a significant role in high-salinity regions (ArW, AW, BSW), while salinity appears crucial in estuarine regions (MCW and PCW). A pronounced increase in mortality rates was associated with warm water areas (MCW), and there was a clear trend of decreasing mortality rates moving northwards. Salinity was negatively correlated with mortality rates, suggesting lower survival potential for O. similis in areas with reduced salinity, such as the Pechora River Delta and the mouth of Kola Bay [57,78,115]. Seasonal variations were also noted in Kola Bay. Here, MC4-C5 mortality rates were recorded as 0.005, 0.004, 0.004, and 0.002 day−1 for winter, spring, summer, and autumn, respectively. For MC5-A, the mortality rates were 0.058, 0.079, 0.228, and 0.120 day−1 for females, and 0.162, 0.207, 0.453, and 0.288 day−1 for males [57,115]. Thus, summer mortality rates for MC5-A were 2–3 times higher than in other seasons. In Kola Bay, no significant differences in MC4-C5 mortality rates were found among the four seasons, although winter estimates were higher than in other seasons. Overall, O. similis reached peak mortality levels in the summer in Kola Bay (Table 2). These values surpass mortality estimates from Arctic locations (e.g., Disco Bay, MC5-A at 0.003 day−1 [116]), Antarctic regions (e.g., Scotia Sea with MC4-C5 at 0.008 day−1, MC5-A females at 0.018 day−1, and males at 0.110 day−1 [114]), and temperate waters (e.g., North Sea, MC5-A females at 0.003 day−1 [113]). This variation is likely due to inter-annual differences in the structure of local pelagic communities, from microplankton to ichthyoplankton, and variations in hydrological regimes.
Water temperature was identified as the most critical factor affecting mortality rates of O. similis in Kola Bay. Moreover, a positive relationship was observed between Chl-a concentrations and mortality rates in this fjord. High temperatures and phytoplankton biomass may create favorable conditions for O. similis, potentially leading to increased abundance. However, higher abundance can also result in increased mortality due to intraspecific competition for space and food resources, as well as heightened vulnerability to parasites and predators, especially in favorable seasons [19,57,58,78,117]. Nevertheless, no direct relationships between mortality rates and Chl-a concentrations were found. Being a typical marine species, O. similis is adversely affected by low salinity levels, which can significantly contribute to its mortality, particularly in PCW.
In other Arctic regions, mortality rates also exhibited strong spatial and temporal fluctuations. For instance, in the White Sea in July 2001, mean summer MC4-C5 and MC5-A mortality rates for O. similis were 0.194 day−1 and 0.127 day−1, respectively [115]. In the northern part of the White Sea in July 2008, MC4-C5 mortality averaged 0.034 day−1, while MC5-A values ranged from 0.040 to 0.207 day−1, yielding a mean of 0.095 day−1. Similarly to the Barents Sea, the overall mortality of O. similis was positively correlated with water temperature and negatively correlated with salinity. In Disco Bay (Greenland waters), mortality rates were comparatively low (<0.06 day−1) across all developmental stages of O. similis in June 2001 [116].
In summary, the mortality of O. similis fluctuates significantly in the Barents Sea and other Arctic regions, with peak values generally associated with nutrient-rich, warm waters or freshened environments. Recent warming trends in the Arctic may be influencing the mortality patterns of O. similis, potentially impacting copepod assemblages in the region.

5. Morphological Variability and Populations of Oithona similis

In the southern Barents Sea (MCW), the modal prosome length (PL) is calculated to be 405 µm for males, and 450 µm and 465 µm for females. In the southeastern part (PCW), specimens with prosome lengths of 375 µm and 405 µm were observed in males, with females reaching up to 480 µm. In the central (AW, BSW) and eastern (NZW) regions, males exhibited a prosome length of 420 µm, while females were found in the 495 µm and 510 µm size classes [57,72]. In the northern and northeastern areas (ArW), the modal PL for males was found to be 465 µm and for females, it was 510 µm. Overall, the average prosome length of both sexes increased from south to north (Table 5).
Clear latitudinal trends in antennule length, number of setae, and relative antennule length were observed in both male and female O. similis, with each parameter decreasing as latitude increased (Table 5). The highest values were recorded in the southern regions, while the smallest were found in the northern areas.
In the Barents Sea, salinity and Chl-a explained very little of the variance in the morphological parameters of O. similis [57,72]. In contrast, the number of setae, absolute setae length, and both absolute and relative antennule lengths were positively correlated with temperature. Conversely, body size and relative setae lengths were negatively correlated with this environmental variable. At high latitudes, animals tend to grow more slowly, live longer, exhibit larger sizes, and have lower reproductive rates compared to their counterparts at lower latitudes [118,119,120]. An increase in cell size in a low-temperature environment is considered a general biological mechanism affecting body size in marine crustaceans [121]. Spatial variations in absolute and relative antennule and setae lengths reflect the adaptations of O. similis to different environmental conditions. A higher relative surface area of appendages and other morphological structures at lower latitudes—where water density is lower than at high latitudes—is crucial for maintaining neutral buoyancy [57,58].
Populations of O. similis were delineated using non-metric cluster analysis based on Euclidean distances between log10-transformed reproductive parameters, as well as discriminant analyses on the morphological parameters of these copepods [57,72]. Both methods identified three distinct populations (Figure 2).
Population 1, which includes O. similis individuals from the southern and central regions, corresponds to AW and MCW. This population shows an intermediate female prosome modal size of 480 µm, situated between the smaller boreal form and the larger Arctic form [57,72,73]. Population 2 is composed of individuals from the eastern part of the Barents Sea, associated with NZW and BSW. This population represents a transition between northern and southern sector populations [38]. Population 3 occurs in the northern Barents Sea and consists of larger O. similis individuals, with a modal female prosome length of 510 µm, from the ArW zone. This population aligns closely with the Arctic–Okhotsk Sea group described by Shuvalov [38].
A phylogeographic study of O. similis s.l. populations from the Arctic Ocean, the Southern Ocean and its northern boundaries, the North Atlantic, and the Mediterranean Sea—based on two gene fragments—revealed seven distinct, geographically delineated mitochondrial lineages, with divergences among lineages ranging from 8% to 24% [122]. In the Arctic Ocean, only one lineage of O. similis was identified, indicating low genetic variability of the species in this region [122]. Another study examined seven populations of O. similis in the Arctic and North Atlantic Oceans to investigate allele expression variation in natural populations [123]. The authors found low genomic differentiation across all populations studied. Thus, the three morphological groups, distinguished by their morphology and reproductive traits, can be viewed as local populations adapted to the specific environmental conditions of the primary water masses in the Barents Sea (coastal, Arctic, and Atlantic).

6. Life Cycles of Oithona similis

The life cycle of Oithona similis exhibits specific characteristics across different populations in the Barents Sea, particularly studied in Kola Bay (southern Barents Sea, MCW). In early winter, adults comprised 52% of the population, while late copepodites (CIV and CV) accounted for 48%, with nauplii absent [57,74]. As winter progressed, the relative abundance of adults decreased to 32%, and the late-stage copepodites increased to 68%. By the end of winter, CIV and CV dominated the population (80%). In spring, the abundance of adults rose to 71%, while CV numbers decreased by 9%, and CIV were absent in mid-May. Mass reproduction occurred in June, resulting in a high abundance of nauplii (up to 97%). By late June, all copepodite stages were present, though nauplii remained the most numerous (95%). In the latter half of the summer, nauplii numbers decreased from 87% to 49%, while the proportion of older copepodites and adults increased by 25% and 26%, respectively [57,73,74]. Mass spawning of a new generation was noted in September, with all stages represented in samples, including a majority of nauplii (70–73%). By early October, naupliar abundance dropped to 7%, CIV and CV reached 36%, and adults made up 57% of the population. By the end of autumn, the population comprised 60% adult males and females, and 40% older copepodites [57,58,73,74].
Seasonal variations in PL and reproductive characteristics of O. similis were documented throughout the year in Kola Bay [74]. The average sizes for CV and adults peaked in May. Female PL decreased by August but rose again by December. Males were the smallest in September, with their PL increasing in November and remaining stable during winter. Similar PL variability patterns were observed in CV. The species reproduces year-round in Kola Bay, with ovigerous females found across all seasons and two major peaks in July (39%) and September (28%). Mean clutch size varied from 20 to 26 eggs per female, peaking in June, September, and November. The mean egg diameter was highest in May and smallest during July–August. EPR and SEPR were lowest in May and between October and December (<0.04 eggs female−1 day−1 and <0.001 day−1), while maximum values reached 2.2 and 1.2 eggs female−1 day−1 (0.046 and 0.023 day−1) in July (autumn generation) and September (summer generation), respectively [57,73,74]. Environmental factors significantly influenced the population characteristics of O. similis in Kola Bay. Mean abundance of all stages (with the exception of CIV and CV) increased with water temperature. Adult and stage-CV copepodites’ PLs were negatively correlated with water temperature and positively correlated with salinity. Abundances exhibited negative correlations with salinity, except for CV. The proportion of ovigerous females, EPR, and SEPR increased significantly with rising water temperature and decreasing salinity [73,74].
In summary, in the southern Barents Sea (MCW), O. similis abundance is low from December to May, predominantly consisting of older copepodites and few adults [124]. By late June, adult male and female abundances increase due to molting of V-stage copepodites from the previous year. Mass reproduction occurs at the end of June, characterized by high numbers of females with egg sacs, nauplii, and young copepodites. By July, all developmental stages are present, and the reproduction period concludes in late September, with males nearly absent outside of summer. Mass molting of nauplii into young copepodites occurs in late September, allowing these copepodites to overwinter until May. A small portion may continue to develop into adults by November, capable of reproducing during winter months (March–April) [57,74]. Thus, the life cycle of O. similis in this region spans 11–12 months (Figure 3A).
In the central Barents Sea (AW and BSW), there is a noticeable transition from overwintering CIV stages to CV stages at the beginning of May. Shortly after, the first adult specimens appear as spawning occurs in June, leading to a peak in reproducing females in July. Nauplii reach their peak abundance in early August, and by the end of September, they develop into CI–CIII and subsequently CIV–CV stages, which dominate the population from November to May. A small number of these stages may continue developing into adults during winter [57]. The life cycle of O. similis in this region lasts approximately 9–10 months (Figure 3B).
In the northern Barents Sea (ArW), developmental stages show strong seasonality. A shift from younger to older copepodites and adults is evident by the beginning of July. First egg-bearing females are observed at the end of July, with mass spawning occurring in early August. By mid-August, the population shifts to a predominance of nauplii, which transition into young copepodites of the next generation by October. Most of these individuals overwinter and do not spawn until August of the following year, although some copepodites may develop into adults and spawn in winter [58,72,73]. Thus, in Arctic waters, the life cycle lasts about 11 months (Figure 3C).
Across other Arctic regions, the life cycle of O. similis generally follows a pattern similar to that in the Barents Sea, with variations primarily in spawning times and abundance peaks. For example, in Kongsfjorden (northwestern Svalbard waters), O. similis reproduces year-round, with key reproductive periods in May–June and August–September [63]. Maximum abundance occurs in November, while minimum abundance is in June, possibly due to colder temperatures compared to Kola Bay. In Kola Bay, the life cycle of O. similis resembles that of the White Sea, where mass spawning occurs in May–June and July–August [57]. Developmental times for the first and second generations in the White Sea are 2 and 9–10 months, respectively [125]. Madsen et al. [65] reported that all life stages of Oithona spp. are present year-round in Disco Bay, with female populations comprising 40% during spring and summer. Two reproductive peaks have also been noted in western Greenland waters (June and August–September) [126]. In Godthåbsfjord (Greenland waters), adults and late copepodites are most abundant during winter and early spring, with stage composition shifting in May–June as younger stages emerge. Adult females of Oithona spp. constituted 20–83% of the population from April to June but decreased to 25% in summer when younger stages dominated. Adult males peaked in April–May (10–22%), while CIV–V stages exhibited peaks in April (15–58%) and July (13–40%). Nauplii were minimal (0–5%) in March–April but increased to 20–30% by early May, peaking at 50% in late May–June [52]. Contrarily, only one generation is present in the Greenland Sea [127], with few new generations maturing into adults. In Antarctic seas, O. similis also forms two generations annually [128], a pattern observed in Canadian Arctic waters [129] and northern Bering Sea regions [130]. However, only one generation is likely to occur in the Laptev Sea year-round [131].

7. Conclusions

Oithona similis demonstrates significant plasticity in body size, antennule length, and reproductive parameters (including egg diameter and production rates), allowing it to thrive across various water masses in the Barents Sea. This copepod prefers higher salinity and lower temperatures, displaying a seasonal life cycle with peaks in abundance and reproduction during the summer. In coastal areas affected by freshwater runoff, salinity is the primary factor influencing the abundance, reproductive rates, and mortality of O. similis. In contrast, temperature plays a more critical role at other locations. The relationship between temperature and population parameters can be both positive and negative, varying based on water mass characteristics, seasonal changes, and hydrological anomalies. Chlorophyll a concentrations appear less critical to O. similis than temperature and salinity. Further research is necessary to investigate the distribution patterns and seasonal abundance of late developmental stages of O. similis, utilizing appropriate sampling methods. Although prior studies in other regions indicate the significance of microplankton as a food source, there is limited information on microzooplankton prey and feeding behaviors specific to O. similis in the Barents Sea. Long-term studies have suggested the existence of three distinct populations of O. similis, but genetic research is essential to confirm this. Furthermore, routine monitoring of O. similis populations in the Barents Sea is critical given the ongoing shifts in climatic conditions in Arctic waters.

Author Contributions

V.G.D.: Conceptualization, Formal analysis, Methodology, Investigation, Data curation, Validation, Writing—original draft; A.G.D.: Investigation, Data curation, Formal analysis, Validation, Project administration, Software, Visualization, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the Ministry of Science and Higher Education of the Russian Federation.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Rudels, B. The Physical Oceanography of the Arctic Mediterranean Sea: Explorations, Observations, Interpretations; Elsevier: Amsterdam, The Netherlands, 2021. [Google Scholar]
  2. Jakobsen, T.; Ozhigin, V.K. (Eds.) The Barents Sea: Ecosystem, Resources, Management: Half a Century of Russian-Norwegian Cooperation; Tapir Academic Press: Trondheim, Norway, 2011. [Google Scholar]
  3. Loeng, H.; Drinkwater, K. An overview of the ecosystems of the Barents and Norwegian Seas and their response to climate variability. Deep-Sea Res. II 2007, 54, 2478–2500. [Google Scholar] [CrossRef]
  4. Schauer, U.; Loeng, H.; Rudels, B.; Ozhigin, V.K.; Dieck, W. Atlantic water flow through the Barents and Kara Seas. Deep-Sea Res. I 2002, 49, 2281–2298. [Google Scholar] [CrossRef]
  5. Carmack, E.; Barber, D.; Christensen, J.; Macdonald, R.; Rudels, B.; Sakshaug, E. Climate variability and physical forcing of the food webs and the carbon budget on panarctic shelves. Prog. Oceanogr. 2006, 71, 145–181. [Google Scholar] [CrossRef]
  6. Dvoretsky, A.G.; Dvoretsky, V.G. Fluctuations of net primary production along a standard transect in the Barents Sea and their relationships with environmental factors. Environ. Res. 2026, 288, 123177. [Google Scholar] [CrossRef]
  7. Wassmann, P.; Reigstad, M.; Haug, T.; Rudels, B.; Carroll, M.L.; Hop, H.; Gabrielsen, G.W.; Falk-Petersen, S.; Denisenko, S.G.; Arashkevich, E.; et al. Food webs and carbon flux in the Barents Sea. Prog. Oceanogr. 2006, 71, 232–287. [Google Scholar] [CrossRef]
  8. Oziel, L.; Sirven, J.; Gascard, J.C. The Barents Sea frontal zones and water masses variability (1980–2011). Ocean Sci. 2016, 12, 169–184. [Google Scholar] [CrossRef]
  9. Sakshaug, E. Biomass and productivity distributions and their variability in the Barents Sea. ICES J. Mar. Sci. 1997, 54, 341–350. [Google Scholar] [CrossRef]
  10. Reigstad, M.; Carroll, J.; Slagstad, D.; Ellingsen, I.; Wassmann, P. Intra-regional comparison of productivity, carbon flux and ecosystem composition within the northern Barents Sea. Prog. Oceanogr. 2011, 90, 33–46. [Google Scholar] [CrossRef]
  11. Sakshaug, E. Primary and secondary production in the Arctic Seas. In The Organic Carbon Cycle in the Arctic Ocean; Stein, R., Macdonald, R.W., Eds.; Springer: Berlin/Heidelberg, Germany, 2004; pp. 57–81. [Google Scholar]
  12. Sakshaug, E.; Bjørge, A.; Gulliksen, B.; Loeng, H.; Mehlum, F. Structure, biomass distribution, and energetics of the pelagic ecosystem in the Barents Sea: A synopsis. Polar Biol. 1994, 14, 405–411. [Google Scholar] [CrossRef]
  13. Sakshaug, E.; Johnsen, G.; Kovacs, K. (Eds.) Ecosystem Barents Sea; Tapir Academic Press: Trondheim, Norway, 2009; 587p. [Google Scholar]
  14. ICES. Working Group on the Integrated Assessments of the Barents Sea (WGIBAR). ICES Sci. Rep. 2022, 4, 1–235. [Google Scholar]
  15. Gjøsæter, H. Pelagic fish and the ecological impact of the modern fishing industry in the Barents Sea. Arctic 1995, 48, 267–278. [Google Scholar] [CrossRef]
  16. Eide, A. Climate change, fisheries management and fishing aptitude affecting spatial and temporal distributions of the Barents Sea cod fishery. Ambio 2017, 46, 387–399. [Google Scholar] [CrossRef] [PubMed][Green Version]
  17. Loeng, H. Features of the physical oceanographic conditions in the central parts of the Barents Sea. Polar Res. 1991, 10, 5–18. [Google Scholar] [CrossRef]
  18. Ozhigin, V.K.; Ivshin, V.A. Water Masses of the Barents Sea; PINRO Press: Murmansk, Russia, 1999. (In Russian) [Google Scholar]
  19. Raymont, J.E.G. Plankton and productivity of the Oceans, Volume 2: Zooplankton; Pergamon Press: Oxford, UK, 1983; 824p. [Google Scholar]
  20. Woodd-Walker, R.S.; Ward, P.; Clarke, A. Large-scale patterns in diversity and community structure of surface water copepods from the Atlantic Ocean. Mar. Ecol. Prog. Ser. 2002, 236, 189–203. [Google Scholar] [CrossRef]
  21. Cornils, A.; Sieger, R.; Mizdalski, E.; Schumacher, S.; Grobe, H.; Schnack-Schiel, S.B. Copepod species abundance from the Southern Ocean and other regions (1980–2005)—A legacy. Earth Syst. Sci. Data 2018, 10, 1457–1471. [Google Scholar] [CrossRef]
  22. Hsieh, C.H.; Chen, C.S.; Chiu, T.S. Composition and abundance of copepods and ichthyoplankton in Taiwan Strait (western North Pacific) are influenced by seasonal monsoons. Mar. Freshw. Res. 2005, 56, 153–161. [Google Scholar] [CrossRef]
  23. Reynolds, C.S. A changing paradigm of pelagic food webs. Int. Rev. Hydrobiol. 2008, 93, 517–531. [Google Scholar] [CrossRef]
  24. Dalpadado, P.; Yamaguchi, A.; Ellertsen, B.; Johannessen, S. Trophic interactions of macro-zooplankton (krill and amphipods) in the Marginal Ice Zone of the Barents Sea. Deep Sea Res. II 2008, 55, 2266–2274. [Google Scholar] [CrossRef]
  25. Bernal, A.; Olivar, M.P.; Maynou, F.; de Puelles, M.L.F. Diet and feeding strategies of mesopelagic fishes in the western Mediterranean. Prog. Oceanogr. 2015, 135, 1–17. [Google Scholar] [CrossRef]
  26. Armengol, L.; Franchy, G.; Ojeda, A.; Santana-del Pino, Á.; Hernández-León, S. Effects of copepods on natural microplankton communities: Do they exert top-down control? Mar. Biol. 2017, 164, 136. [Google Scholar] [CrossRef]
  27. Mauchline, J. The Biology of Calanoid Copepods; Elsevier: Amsterdam, The Netherlands, 1998; Volume 33; pp. 1–710. [Google Scholar]
  28. Schukat, A.; Auel, H.; Teuber, L.; Lahajnar, N.; Hagen, W. Complex trophic interactions of calanoid copepods in the Benguela upwelling system. J. Sea Res. 2014, 85, 186–196. [Google Scholar] [CrossRef]
  29. Aarflot, J.M.; Skjoldal, H.R.; Dalpadado, P.; Skern-Mauritzen, M. Contribution of Calanus species to the mesozo plankton biomass in the Barents Sea. ICES J. Mar. Sci. 2018, 75, 2342–2354. [Google Scholar] [CrossRef]
  30. Svensen, C.; Halvorsen, E.; Vernet, M.; Franzè, G.; Dmoch, K.; Lavrentyev, P.J.; Kwasniewski, S. Zooplankton communities associated with new and regenerated primary production in the Atlantic inflow north of Svalbard. Front. Mar. Sci. 2019, 6, 293. [Google Scholar] [CrossRef]
  31. Tande, K.S. Calanus in North Norwegian fjords and in the Barents Sea. Polar Res. 1991, 10, 389–408. [Google Scholar] [CrossRef]
  32. Falk-Petersen, S.; Pedersen, G.; Kwasniewski, S.; Hegseth, E.N.; Hop, H. Spatial distribution and life-cycle timing of zooplankton in the marginal ice zone of the Barents Sea during the summer melt season in 1995. J. Plankton Res. 1999, 21, 1249–1264. [Google Scholar] [CrossRef]
  33. Dalpadado, P.; Arrigo, K.R.; van Dijken, G.L.; Skjoldal, H.R.; Bagøien, E.; Dolgov, A.V.; Prokopchuk, I.P.; Sperfeld, E. Climate effects on temporal and spatial dynamics of phytoplankton and zooplankton in the Barents Sea. Progr. Oceanogr. 2020, 182, 102320. [Google Scholar] [CrossRef]
  34. Gallienne, C.P.; Robins, D.B. Is Oithona the most important copepod in the world’s oceans? J. Plankton Res. 2001, 23, 1421–1432. [Google Scholar] [CrossRef]
  35. Turner, J.T. The importance of small planktonic copepods and their roles in pelagic marine food webs. Zool. Stud. 2004, 43, 255–266. [Google Scholar]
  36. Skjoldal, H.R.; Aarflot, J.M. Abundance and biomass of copepods and cladocerans in Atlantic and Arctic domains of the Barents Sea ecosystem. J. Plankton Res. 2023, 45, 870–884. [Google Scholar] [CrossRef] [PubMed]
  37. Fish, C.J. The biology of Oithona similis in the Gulf of Maine and Bay of Fundy. Biol. Bull. 1936, 71, 168–187. [Google Scholar] [CrossRef]
  38. Shuvalov, V.S. Copepod Cyclopoids of the Family Oithonidae of the World Ocean; Nauka Press: Leningrad, Russia, 1980; 198p. (In Russian) [Google Scholar]
  39. Nishida, S. Taxonomy and distribution of the family Oithonidae (Copepoda, Cyclopoida) in the Pacific and Indian oceans. Bull. Ocean Res. Inst. Univ. Tokyo 1985, 20, 1–167. [Google Scholar]
  40. Böttger-Schnack, R.; Schnack, D.; Weikert, H. Biological observations on small cyclopoid copepods in the Red Sea. J. Plankton Res. 1989, 11, 1089–1101. [Google Scholar] [CrossRef]
  41. Nielsen, T.G.; Møller, E.F.; Satapoomin, S.; Ringuette, M.; Hopcroft, R.R. Egg hatching rate of the cyclopoid copepod Oithona similis in arctic and temperate waters. Mar. Ecol. Prog. Ser. 2002, 236, 301–306. [Google Scholar] [CrossRef]
  42. Nielsen, T.G.; Sabatini, M. Role of cyclopoid copepods Oithona spp. in North Sea plankton communities. Mar. Ecol. Prog. Ser. 1996, 139, 79–93. [Google Scholar] [CrossRef]
  43. Hansen, F.C.; Mollmann, C.; Schutz, U.; Hinrichsen, H.H. Spatio-temporal distribution of Oithona similis in the Bornholm Basin (Central Baltic Sea). J. Plankton Res. 2004, 26, 659–668. [Google Scholar] [CrossRef][Green Version]
  44. Castellani, C.; Irigoien, X.; Harris, R.P.; Holliday, N.P. Regional and temporal variation of Oithona spp. biomass, stage structure and productivity in the Irminger Sea, North Atlantic. J. Plankton Res. 2007, 29, 1051–1070. [Google Scholar] [CrossRef][Green Version]
  45. Ward, P.; Hirst, A.G. Oithona similis in a high latitude ecosystem: Abundance, distribution and temperature limitation of fecundity rates in a sac spawning copepod. Mar. Biol. 2007, 151, 1099–1110. [Google Scholar] [CrossRef]
  46. Temperoni, B.; Viñas, M.D.; Diovisalvi, N.; Negri, R. Seasonal production of Oithona nana Giesbrecht, 1893 (Copepoda: Cyclopoida) in temperate coastal waters off Argentina. J. Plankton Res. 2011, 33, 729–740. [Google Scholar] [CrossRef]
  47. Dahms, H.U.; Tseng, L.C.; Hwang, J.S. Biogeographic distribution of the cyclopoid copepod genus Oithona–from mesoscales to global scales. J. Exp. Mar. Biol. Ecol. 2015, 467, 26–32. [Google Scholar] [CrossRef]
  48. Cornwell, L.E.; Findlay, H.S.; Fileman, E.S.; Smyth, T.J.; Hirst, A.G.; Bruun, J.; McEvoy, A.J.; Widdicombe, C.E.; Castellani, C.; Lewis, C.; et al. Seasonality of Oithona similis and Calanus helgolandicus reproduction and abundance: Contrasting responses to environmental variation at a shelf site. J. Plankton Res. 2018, 40, 295–310. [Google Scholar] [CrossRef]
  49. Metz, C. Seasonal variation in the distribution and abundance of Oithona and Oncaea species (Copepoda, Crustacea) in the southeastern Weddell Sea, Antarctica. Polar Biol. 1995, 15, 187–194. [Google Scholar] [CrossRef]
  50. Paffenhöfer, G.A. On the ecology of marine cyclopoid copepods (Crustacea, Copepoda). J. Plankton Res. 1993, 15, 37–55. [Google Scholar] [CrossRef]
  51. Zamora-Terol, S.; Nielsen, T.G.; Saiz, E. Plankton community structure and role of Oithona similis on the western coast of Greenland during the winter-spring transition. Mar. Ecol. Prog. Ser. 2013, 483, 85–102. [Google Scholar] [CrossRef]
  52. Zamora-Terol, S.; Kjellerup, S.; Swalethorp, R.; Saiz, E.; Nielsen, T.G. Population dynamics and production of the small copepod Oithona spp. in a subarctic fjord of West Greenland. Polar Biol. 2014, 37, 953–965. [Google Scholar] [CrossRef]
  53. Nakamura, Y.; Turner, J.T. Predation and respiration by the small cyclopoid copepod Oithona similis: How important is feeding on ciliates and heterotrophic flagellates? J. Plankton Res. 1997, 19, 1275–1288. [Google Scholar] [CrossRef]
  54. Castellani, C.; Irigoien, X.; Harris, R.P.; Lampitt, R.S. Feeding and egg production of Oithona similis in the North Atlantic. Mar. Ecol. Prog. Ser. 2005, 288, 173–182. [Google Scholar] [CrossRef]
  55. Castellani, C.; Irigoien, X.; Mayor, D.J.; Harris, R.P.; Wilson, D. Feeding of Calanus finmarchicus and Oithona similis on the microplankton assemblage in the Irminger Sea, North Atlantic. J. Plankton Res. 2008, 30, 1095–1116. [Google Scholar] [CrossRef]
  56. Dvoretsky, V.G.; Dvoretsky, A.G. Checklist of fauna found in zooplankton samples from the Barents Sea. Polar Biol. 2010, 33, 991–1005. [Google Scholar] [CrossRef]
  57. Dvoretsky, V.G.; Dvoretsky, A.G. Biology and Role of Oithona similis in Zooplankton of Arctic Seas; Centre Russian Academy of Sciences Press: Apatity, Russia, 2011. (In Russian) [Google Scholar]
  58. Dvoretsky, V.G.; Dvoretsky, A.G. Ecology of Zooplankton Communities in the Barents Sea and Adjacent Waters; Renome: St. Petersburg, Russia, 2015. (In Russian) [Google Scholar]
  59. Dvoretsky, V.G.; Dvoretsky, A.G. Coastal mesozooplankton assemblages during spring bloom in the eastern Barents Sea. Biology 2022, 11, 204. [Google Scholar] [CrossRef] [PubMed]
  60. Dvoretsky, V.G.; Dvoretsky, A.G. Copepod assemblages in a large Arctic coastal area: A baseline summer study. Diversity 2023, 15, 81. [Google Scholar] [CrossRef]
  61. Dvoretsky, V.G.; Dvoretsky, A.G. Marine copepod assemblages in the Arctic: The effect of frontal zones on biomass and productivity. Mar. Environ. Res. 2024, 193, 106250. [Google Scholar] [CrossRef]
  62. Dvoretsky, V.G.; Dvoretsky, A.G. Local variability of Arctic mesozooplankton biomass and production: A case summer study. Environ. Res. 2024, 241, 117416. [Google Scholar] [CrossRef]
  63. Lischka, S.; Hagen, W. Life histories of the copepod Pseudocalanus minutus, P. acuspes (Calanoida), and Oithona similis (Cyclopoida) in the Arctic Kongsfjorden (Svalbard). Polar Biol. 2005, 28, 910–921. [Google Scholar] [CrossRef]
  64. Lischka, S.; Hagen, W. Seasonal lipid dynamics of the copepods Pseudocalanus minutus (Calanoida) and Oithona similis (Cyclopoida) in the Arctic Kongsfjorden (Svalbard). Mar. Biol. 2007, 150, 443–454. [Google Scholar] [CrossRef]
  65. Madsen, S.D.; Nielsen, T.G.; Hansen, B.W. Annual population development of small-sized copepods in Disko Bay. Mar. Biol. 2008, 155, 63–77. [Google Scholar] [CrossRef]
  66. Svensen, C.; Seuthe, L.; Vasilyeva, Y.; Pasternak, A.; Hansen, E. Zooplankton distribution across Fram Strait in autumn: Are small copepods and protozooplankton important? Prog. Oceanogr. 2011, 91, 534–544. [Google Scholar] [CrossRef]
  67. Barth-Jensen, C.; Daase, M.; Ormańczyk, M.R.; Varpe, Ø.; Kwaśniewski, S.; Svensen, C. High abundances of small copepods early developmental stages and nauplii strengthen the perception of a non-dormant Arctic winter. Polar Biol. 2022, 45, 675–690. [Google Scholar] [CrossRef]
  68. Degtereva, A.A. The regularities in plankton quantitative development in the Barents Sea. Trudy PINRO 1979, 43, 22–53. (In Russian) [Google Scholar]
  69. Sabatini, M.; Kiørboe, T. Egg production, growth and development of the cyclopoid copepod Oithona similis. J. Plankton Res. 1994, 16, 1329–1351. [Google Scholar] [CrossRef]
  70. Castellani, C.; Licandro, P.; Fileman, E.; di Capua, I.; Mazzocchi, M.G. Oithona similis likes it cool: Evidence from two long-term time series. J. Plankton Res. 2016, 38, 703–717. [Google Scholar] [CrossRef]
  71. Cornwell, L.E.; Fileman, E.S.; Bruun, J.T.; Hirst, A.G.; Tarran, G.A.; Findlay, H.S.; Lewis, C.; Smyth, T.J.; McEvoy, A.J.; Atkinson, A. Resilience of the copepod Oithona similis to climatic variability: Egg Production, mortality, and vertical habitat partitioning. Front. Mar. Sci. 2020, 7, 29. [Google Scholar] [CrossRef]
  72. Dvoretsky, V.G.; Dvoretsky, A.G. Morphological plasticity in the small copepod Oithona similis in the Barents and White Seas. Mar. Ecol. Prog. Ser. 2009, 385, 165–178. [Google Scholar] [CrossRef]
  73. Dvoretsky, V.G.; Dvoretsky, A.G. Spatial variations in reproductive characteristics of the small copepod Oithona similis in the Barents Sea. Mar. Ecol. Prog. Ser. 2009, 386, 133–146. [Google Scholar] [CrossRef]
  74. Dvoretsky, V.G.; Dvoretsky, A.G. Life cycle of Oithona similis (Copepoda: Cyclopoida) in Kola Bay (Barents Sea). Mar. Biol. 2009, 156, 1433–1446. [Google Scholar] [CrossRef]
  75. Dvoretsky, V.G.; Dvoretsky, A.G. Epiplankton in the Barents Sea: Summer variations of mesozooplankton biomass, community structure and diversity. Cont. Shelf Res. 2013, 52, 1–11. [Google Scholar] [CrossRef]
  76. Dvoretsky, V.G.; Dvoretsky, A.G. Summer mesozooplankton community of Moller Bay (Novaya Zemlya Archipelago, Barents Sea). Oceanologia 2013, 55, 205–218. [Google Scholar] [CrossRef][Green Version]
  77. Dvoretsky, V.G.; Dvoretsky, A.G. Structure of mesozooplankton community in the Barents Sea and adjacent waters in August 2009. J. Nat. Hist. 2013, 47, 2095–2114. [Google Scholar] [CrossRef]
  78. Dvoretsky, V.G. Seasonal mortality rates of Oithona similis (Cyclopoida) in a large Arctic fjord. Polar Sci. 2012, 6, 263–269. [Google Scholar] [CrossRef]
  79. Dvoretsky, V.G.; Dvoretsky, A.G. Summer zooplankton assemblages in the Barents Sea: Spatial variations and effects of environmental conditions as revealed from in situ and satellite data. Prog. Oceanogr. 2025, 231, 103417. [Google Scholar] [CrossRef]
  80. Dvoretsky, V.G.; Dvoretsky, A.G. Effects of water temperature on zooplankton abundance and biomass in the southwestern Barents Sea: Implications for Arctic monitoring and management. Ocean Coast. Manag. 2025, 261, 107506. [Google Scholar] [CrossRef]
  81. Dvoretsky, V.G.; Dvoretsky, A.G. Coastal zooplankton assemblages in the southern Barents Sea: A summer pattern of diversity and production. Mar. Environ. Res. 2026, 213, 107710. [Google Scholar] [CrossRef]
  82. Wassmann, P. Arctic marine ecosystems in an era of rapid climate change. Prog. Oceanogr. 2011, 90, 1–17. [Google Scholar] [CrossRef]
  83. Koenigk, T.; Key, J.; Vihma, T. Climate Change in the Arctic. In Physics and Chemistry of the Arctic Atmosphere; Kokhanovsky, A., Tomasi, C., Eds.; Springer: Cham, Switzerland, 2020; pp. 673–705. [Google Scholar]
  84. Ardyna, M.; Arrigo, K.R. Phytoplankton dynamics in a changing Arctic Ocean. Nat. Clim. Change 2020, 10, 892–903. [Google Scholar] [CrossRef]
  85. Malik, I.H.; Ahmed, R.; Ford, J.D.; Hamidi, A.R. Arctic warming: Cascading climate impacts and global consequences. Climate 2025, 13, 85. [Google Scholar] [CrossRef]
  86. Polyakov, I.V.; Alkire, M.B.; Bluhm, B.A.; Brown, K.A.; Carmack, E.C.; Chierici, M.; Danielson, S.L.; Ellingsen, I.; Ershova, E.A.; Gårdfeldt, K.; et al. Borealization of the Arctic Ocean in response to anomalous advection from sub-arctic seas. Front. Mar. Sci. 2020, 7, 491. [Google Scholar] [CrossRef]
  87. Lewis, K.M.; Van Dijken, G.L.; Arrigo, K.R. Changes in phytoplankton concentration now drive increased Arctic Ocean primary production. Science 2020, 369, 198–202. [Google Scholar] [CrossRef] [PubMed]
  88. Dvoretsky, V.G.; Vodopianova, V.V.; Bulavina, A.S. Effects of Climate Change on Chlorophyll a in the Barents Sea: A Long-Term Assessment. Biology 2023, 12, 119. [Google Scholar] [CrossRef]
  89. Wold, A.; Hop, H.; Svensen, C.; Søreide, J.E.; Assmann, K.M.; Ormanczyk, M.; Kwasniewski, S. Atlantification influences zooplankton communities seasonally in the northern Barents Sea and Arctic Ocean. Prog. Oceanogr. 2023, 219, 103133. [Google Scholar] [CrossRef]
  90. Dvoretsky, V.G.; Dvoretsky, A.G. Summer mesozooplankton structure in the Pechora Sea (south-eastern Barents Sea). Estuar. Coast. Shelf Sci. 2009, 84, 11–20. [Google Scholar] [CrossRef]
  91. Dvoretsky, V.G.; Dvoretsky, A.G. Summer mesozooplankton distribution near Novaya Zemlya (eastern Barents Sea). Polar Biol. 2009, 32, 719–731. [Google Scholar] [CrossRef]
  92. Dvoretsky, V.G.; Dvoretsky, A.G. Copepod communities off Franz Josef Land (northern Barents Sea) in late summer of 2006 and 2007. Polar Biol. 2011, 34, 1231–1238. [Google Scholar] [CrossRef]
  93. Dvoretsky, V.G.; Dvoretsky, A.G. Mesozooplankton structure in Dolgaya Bay (Barents Sea). Polar Biol. 2010, 33, 703–708. [Google Scholar] [CrossRef]
  94. Dvoretsky, V.G.; Dvoretsky, A.G. Early winter mesozooplankton of the coastal south-eastern Barents Sea. Estuar. Coast. Shelf Sci. 2015, 152, 116–123. [Google Scholar] [CrossRef]
  95. Dvoretsky, V.G.; Dvoretsky, A.G. Features of winter zooplankton assemblage in the Central Trough of the Barents Sea. Arct. Environ. Res. 2018, 18, 28–36. [Google Scholar] [CrossRef][Green Version]
  96. Dvoretsky, V.G.; Dvoretsky, A.G. Mesozooplankton in the Kola Transect (Barents Sea): Autumn and winter structure. J. Sea Res. 2018, 142, 125–131. [Google Scholar] [CrossRef]
  97. Dvoretsky, V.G.; Dvoretsky, A.G. Latitudinal variations of zooplankton community structure and productivity in the Barents Sea (summer 2013). Arct. Antarct. Res. 2018, 64, 294–310. (In Russian) [Google Scholar] [CrossRef]
  98. Dvoretsky, V.G.; Dvoretsky, A.G. Arctic marine mesozooplankton at the beginning of the polar night: A case study for southern and south-western Svalbard waters. Polar Biol. 2020, 43, 71–79. [Google Scholar] [CrossRef]
  99. Prokopchuk, I.P.; Trofimov, A.G. Interannual dynamics of zooplankton in the Kola Section of the Barents Sea during the recent warming period. ICES J. Mar. Sci. 2019, 76, i10–i23. [Google Scholar] [CrossRef]
  100. Gluchowska, M.; Dalpadado, P.; Beszczynska-Möller, A.; Olszewska, A.; Ingvaldsen, R.B.; Kwasniewski, S. Interannual zooplankton variability in the main pathways of the Atlantic water flow into the Arctic Ocean (Fram Strait and Barents Sea branches). ICES J. Mar. Sci. 2017, 74, 1921–1936. [Google Scholar] [CrossRef]
  101. Dvoretsky, V.G.; Venger, M.P.; Vashchenko, A.V.; Vodopianova, V.V.; Pastukhov, I.A.; Maksimovskaya, T.M. Marine Plankton during the Polar Night: Environmental Predictors of Spatial Variability. Biology 2023, 12, 368. [Google Scholar] [CrossRef]
  102. Gawinski, C.; Basedow, S.L.; Sundfjord, A.; Svensen, C. Secondary production at the Barents Sea polar front in summer: Contribution of different size classes of mesozooplankton. Mar. Ecol. Prog. Ser. 2024, 735, 77–101. [Google Scholar] [CrossRef]
  103. Dvoretsky, V.G.; Moiseev, D.V.; Venger, M.P.; Vashchenko, A.V.; Vodopianova, V.V. Environmental control of Arctic marine zooplankton near a large archipelago during the summer season. Cont. Shelf Res. 2025, 284, 105363. [Google Scholar] [CrossRef]
  104. Postel, L.; Fock, H.; Hagen, W. Biomass and abundance. In ICES Zooplankton Methodology Manual; Harris, R., Wiebe, P., Lenz, J., Skjoldal, H.R., Huntley, M., Eds.; Academic Press: London, UK, 2000; pp. 83–192. [Google Scholar]
  105. Dvoretsky, V.G.; Dvoretsky, A.G. Mesozooplankton structure in the northern White Sea in July 2008. Polar Biol. 2011, 34, 469–474. [Google Scholar] [CrossRef]
  106. Dvoretsky, V.G.; Dvoretsky, A.G. Distribution of the under-ice mesozooplankton in the Kara Sea in February 2002. Polar Biol. 2009, 32, 1227–1231. [Google Scholar] [CrossRef]
  107. Dvoretsky, V.G.; Dvoretsky, A.G. Regional differences of mesozooplankton communities in the Kara Sea. Cont. Shelf Res. 2015, 105, 26–41. [Google Scholar] [CrossRef]
  108. Balazy, K.; Boehnke, R.; Trudnowska, E.; Søreide, J.E.; Błachowiak-Samołyk, K. Phenology of Oithona similis demonstrates that ecological flexibility may be a winning trait in the warming Arctic. Sci. Rep. 2021, 11, 18599. [Google Scholar] [CrossRef]
  109. Dvoretsky, V.G.; Dvoretsky, A.G. Macrozooplankton of the Arctic—The Kara Sea in relation to environmental conditions. Estuar. Coast. Shelf Sci. 2017, 188, 38–55. [Google Scholar] [CrossRef]
  110. Dvoretsky, V.G.; Dvoretsky, A.G. Summer macrozooplankton assemblages of Arctic shelf: A latitudinal study. Cont. Shelf Res. 2019, 188, 103967. [Google Scholar] [CrossRef]
  111. Barth-Jensen, C.; Koski, M.; Varpe, Ø.; Glad, P.; Wangensteen, O.S.; Præbel, K.; Svensen, C. Temperature-dependent egg production and egg hatching rates of small egg-carrying and broadcast-spawning copepods Oithona similis, Microsetella norvegica and Microcalanus pusillus. J. Plankton Res. 2020, 42, 564–580. [Google Scholar] [CrossRef]
  112. Flo, S.; Svensen, C.; Præbel, K.; Bluhm, B.A.; Vader, A. Dietary plasticity in small Arctic copepods as revealed with prey metabarcoding. J. Plankton Res. 2024, 46, 500–514. [Google Scholar] [CrossRef]
  113. Eiane, K.; Ohman, M.D. Stage-specific mortality of Calanus finmarchicus, Pseudocalanus elongatus, and Oithona similis on Fladen Ground, North Sea, during a spring bloom. Mar. Ecol. Prog. Ser. 2004, 268, 183–193. [Google Scholar] [CrossRef]
  114. Hirst, A.G.; Ward, P. Spring mortality of the cyclopoid copepod Oithona similis in polar waters. Mar. Ecol. Prog. Ser. 2008, 372, 169–180. [Google Scholar] [CrossRef]
  115. Dvoretsky, V.G.; Dvoretsky, A.G. The mortality of the planktonic copepod Oithona similis Claus, 1866 (Copepoda: Cyclopoida) in the Barents and White seas. Russ. J. Mar. Biol. 2011, 37, 123–131. [Google Scholar] [CrossRef]
  116. Thor, P.; Nielsen, T.G.; Tiselius, P. Mortality rates of epipelagic copepods in the post-spring bloom period in Disko Bay, western Greenland. Mar. Ecol. Prog. Ser. 2008, 359, 151–160. [Google Scholar] [CrossRef]
  117. Viitasalo, M.; Kiørboe, T.; Flinkman, J.; Pedersen, L.W.; Visser, A.W. Predation vulnerability of planktonic copepods: Consequences of predator foraging strategies and prey sensory abilities. Mar. Ecol. Prog. Ser. 1998, 175, 129–142. [Google Scholar] [CrossRef]
  118. Clarke, A. Life in cold water: The physiological ecology of polar marine ectotherms. Oceanogr. Mar. Biol. Annu. Rev. 1983, 21, 341–453. [Google Scholar]
  119. Clarke, A. Temperature, latitude and reproductive effort. Mar. Ecol. Prog. Ser. 1987, 38, 89–99. [Google Scholar] [CrossRef]
  120. Conover, D.O. Seasonality and the scheduling of life history at different latitudes. J. Fish Biol. 1992, 41, 161–178. [Google Scholar] [CrossRef]
  121. McLaren, I.A.; Sevigny, J.M.; Corkett, C.J. Body size, development rates, and genome size among Calanus species. Hydrobiologia 1988, 167–168, 275–284. [Google Scholar] [CrossRef]
  122. Cornils, A.; Wend-Heckmann, B.; Held, C. Global phylogeography of Oithona similis s.l. (Crustacea, Copepoda, Oithonidae)—A cosmopolitan plankton species or a complex of cryptic lineages? Mol. Phylogenet. Evol. 2017, 107, 473–485. [Google Scholar] [CrossRef]
  123. Laso-Jadart, R.; Sugier, K.; Petit, E.; Labadie, K.; Peterlongo, P.; Ambroise, C.; Wincker, P.; Jamet, J.L.; Madoui, M.A. Investigating population-scale allelic differential expression in wild populations of Oithona similis (Cyclopoida, Claus, 1866). Ecol. Evol. 2020, 10, 8894–8905. [Google Scholar] [CrossRef]
  124. Dvoretsky, V.G.; Dvoretsky, A.G. Estimated copepod production rate and structure of mesozooplankton communities in the coastal Barents Sea during summer–autumn 2007. Polar Biol. 2012, 35, 1321–1342. [Google Scholar] [CrossRef]
  125. Prygunkova, R.V. Certain peculiarities in the seasonal development of zooplankton in the Chupa Inlet of the White Sea. Explor. Fauna Seas 1974, 13, 4–55. (In Russian) [Google Scholar]
  126. Ussing, H.H. The Biology of Some Important Plankton Animals in the Fjords of East Greenland; C.A. Reitzels: Copenhagen, Denmark, 1938; Volume 100; pp. 1–108. [Google Scholar]
  127. Digby, P.S.B. The biology of marine plankton copepods of Scoresby Sound, East Greenland. J. Anim. Ecol. 1954, 23, 298–338. [Google Scholar] [CrossRef]
  128. Atkinson, A. Life cycle strategies of epipelagic copepods in the Southern Ocean. J. Mar. Syst. 1998, 15, 289–311. [Google Scholar] [CrossRef]
  129. Hopcroft, R.R.; Clarke, C.; Nelson, R.J.; Raskoff, K.A. Zooplankton communities of the Arctic’s Canada Basin: The contribution by smaller taxa. Polar Biol. 2005, 28, 198–206. [Google Scholar] [CrossRef]
  130. Shaginyan, E.R. Population dynamics and age structures of Oithona similis (Claus) and Pseudocalanus elongatus (Boeck) in the Bay of Oliyutorsk. Izv. TINRO 1982, 106, 123–126. (In Russian) [Google Scholar]
  131. Lischka, S.; Knickmeier, K.; Hagen, W. Mesozooplankton assemblages in the shallow Arctic Laptev Sea in summer 1993 and autumn 1995. Polar Biol. 2001, 24, 186–199. [Google Scholar] [CrossRef]
Biology 15 00027 g001
Figure 1. Barents Sea: location of main water masses [2,17,18]. MCW—Murmansk Coastal Water, AW—Atlantic Water, ArW—Arctic Water, BSW—Barents Sea Water, PCW—Pechora Coastal Water, NZCW—Novaya Zemlya Coastal Water.
Figure 1. Barents Sea: location of main water masses [2,17,18]. MCW—Murmansk Coastal Water, AW—Atlantic Water, ArW—Arctic Water, BSW—Barents Sea Water, PCW—Pechora Coastal Water, NZCW—Novaya Zemlya Coastal Water.
Biology 15 00027 g001
Biology 15 00027 g002
Figure 2. Population structure of Oithona similis in the Barents Sea [58]. Populations were delineated based on the morphology and reproductive features: 1—southern, 2—central, 3—northern.
Figure 2. Population structure of Oithona similis in the Barents Sea [58]. Populations were delineated based on the morphology and reproductive features: 1—southern, 2—central, 3—northern.
Biology 15 00027 g002
Biology 15 00027 g003
Figure 3. Schematic diagrams showing the life cycling of Oithona similis in the Barents Sea [57,58,73,74]. (A)—southern part (MCW), (B)—central part (AW and BSW), (C)—northern part (ArW). Black arrows indicate main reproduction/generations, dotted arrows indicate continuous reproducing throughout the year. Cop—copepodites, Np—nauplii, F—Females, M—Males, ooo—ova.
Figure 3. Schematic diagrams showing the life cycling of Oithona similis in the Barents Sea [57,58,73,74]. (A)—southern part (MCW), (B)—central part (AW and BSW), (C)—northern part (ArW). Black arrows indicate main reproduction/generations, dotted arrows indicate continuous reproducing throughout the year. Cop—copepodites, Np—nauplii, F—Females, M—Males, ooo—ova.
Biology 15 00027 g003
Table 1. Abundance (individuals m−3), biomass (mg dry mass m−3), and relative biomass (contribution to the total zooplankton biomass, mean/min-max, %) of Oithona similis in the Barents Sea estimated from net sampling (Juday or WP2 nets, mesh size 168–180 μm). Water masses: MCW—Murmansk Coastal Water AW—Atlantic Water, ArW—Arctic Water, BSW—Barents Sea Water, PCW—Pechora Coastal Water, NZW—Novaya Zemlya Coastal Water.
Table 1. Abundance (individuals m−3), biomass (mg dry mass m−3), and relative biomass (contribution to the total zooplankton biomass, mean/min-max, %) of Oithona similis in the Barents Sea estimated from net sampling (Juday or WP2 nets, mesh size 168–180 μm). Water masses: MCW—Murmansk Coastal Water AW—Atlantic Water, ArW—Arctic Water, BSW—Barents Sea Water, PCW—Pechora Coastal Water, NZW—Novaya Zemlya Coastal Water.
Water
Mass		YearSeasonAbundanceBiomassRel. BiomassReference
AW2001Summer4930.320.7/0.2–1.0[90]
PCW2001Summer1000.070.9/0.0–5.8[90]
MCW2004Summer5540.316.5/0.2–25.4[57]
AW2004Summer9020.5114.0/1.3–27.3[57]
MCW2005Summer9510.616.5/2.6–12.6[57]
AW2005Summer6640.431.3/1.2–1.4[57]
MCW2006Summer660.052.4/1.0–6.0[73]
AW2006Summer1660.121.9/0.1–2.8[73]
NZW2006Summer730.063.6/0.2–9.5[91]
ArW2006Summer2030.151.0/0.9–1.0[92]
MCW2007Summer13150.931.4/0.3–5.2[73]
AW2007Summer2740.172.0/0.1–9.0[73]
NZW2007Summer12310.812.9/0.4–5.4[73]
ArW2007Summer8140.473.8/1.3–6.4[74]
MCW2007Autumn3660.262.2/2.2–6.4[58]
MCW2007Winter190.019.4/3.4–15.3[58]
MCW2007Spring3310.231.1/0.1–3.6[58]
MCW2008Summer16501.205.0/1.3–11.7[93]
MCW2009Summer2310.548.0/0.2–16.9[77]
AW2009Summer8351.776.5/0.3–16.0[77]
ArW2009Summer7681.226.3/0.2–16.9[77]
NZW2010Summer7820.811.0/0.1–2.3[76]
PCW2010Winter4921.001.7/0.1–3.5[94]
BSW2010Winter1400.1213.2/2.8–35.4[95]
MCW2010Summer10602.73.0/0.6–6.2[61]
AW2010Summer8141.92.5/0.1–5.0[61]
ArW2010Summer12032.75.1/1.4–8.8[61]
NZW2010Summer3851.03.3/0.1–11.2[61]
MCW2011Summer15903.35.1/1.2–14.4[81]
MCW2011Autumn810.080.5/0.1–1.5[80]
AW2011Autumn3390.260.4/0.2–0.6[80]
ArW2011Autumn11840.990.4/0.1–16.1[80]
MCW2012Winter530.061.6/1.4–1.8[96]
AW2012Winter1070.110.3/0.0–0.6[96]
ArW2012Winter6240.570.4/0.1–0.9[96]
PCW2012Summer28642.83.9/0.6–6.4[60]
MCW2013Summer880.083.0/0.5–5.6[97]
AW2013Summer2560.260.3/0.1–0.6[97]
BSW2013Summer1580.171/0.3–2.4[97]
ArW2013Summer1900.215.8/0.7–16.1[97]
ArW2015Winter2270.231.4/0.7–2.4[98]
MCW2013Summer880.090.3/0.1–0.6[79]
AW2013Summer2570.261.0/0.3–2.4[79]
BSW2013Summer1580.175.8/0.7–16.1[79]
ArW2013Summer1910.211.4/0.7–2.4[79]
MCW + ArW2013Summer480.04 *0.7/0.1–2.2[99]
MCW + ArW2014–2017Summer499–9990.38–0.86 *0.1 [99]
AW2001–2014Summer2450.70.5/0.4–0.6[100]
BSW2015Summer1780.20.6/0.1–2.5[62]
ArW2015Summer2720.30.7/0.3–1.6[62]
NZCW2016Spring1060.131.4/0.4–2.5[59]
BSW2016Spring2200.261.8/0.7–2.5[59]
AW2019Winter1880.181.18[89]
AW2019Summer10391.353.04[89]
ARW2019Winter1880.181.18[89]
ARW2019Summer9440.631.1/0.1–2.5[89]
AW2021Winter1500.149.05[89]
AW2021Spring450.061.42[89]
AW2021Summer5570.882.48[89]
ARW2021Winter3070.452.62/0.5–8.2[89]
ARW2021Spring2130.493.17/1.7–6.3[89]
ARW2021Summer3110.801.85/1.4–2.3[89]
AW2021Winter1350.10.8/0.2–2.8[101]
AW2021Summer3400--[102]
ArW2021Summer600--[102]
NZCW2022Summer20892.67.3/0.8–18.2[103]
Note: * recalculated assuming 1 mg wet mass = 0.2 mg dry mass [104].
Table 2. Reproduction characteristics (mean values) of Oithona similis in the Barents Sea [57,58,73,74]. Water masses: MCW—Murmansk Coastal Water, AW—Atlantic Water, ArW—Arctic Water, BSW—Barents Sea Water, PCW—Pechora Coastal Water, NZW—Novaya Zemlya Coastal Water. OF—proportion of ovigerous females (%), D—egg diameter (μm), Fec—fecundity (eggs per sacs), RE—reproductive effort (%), EPR—egg production rate (eggs per female day−1), SEPR—specific egg production rate (day−1).
Table 2. Reproduction characteristics (mean values) of Oithona similis in the Barents Sea [57,58,73,74]. Water masses: MCW—Murmansk Coastal Water, AW—Atlantic Water, ArW—Arctic Water, BSW—Barents Sea Water, PCW—Pechora Coastal Water, NZW—Novaya Zemlya Coastal Water. OF—proportion of ovigerous females (%), D—egg diameter (μm), Fec—fecundity (eggs per sacs), RE—reproductive effort (%), EPR—egg production rate (eggs per female day−1), SEPR—specific egg production rate (day−1).
Water MassYearSeasonOFREDFecEPRSEPR
AW2001Summer33.733.063.023.00.880.026
PCW2001Summer28.635.148.723.90.680.010
MCW2004Summer21.939.248.725.80.910.012
AW2004Summer15.337.750.425.30.410.006
MCW2005Summer36.544.264.320.81.110.033
AW2005Summer22.944.865.321.20.550.017
BSW2005Summer12.344.264.420.80.190.006
MCW2006Summer30.332.750.022.31.280.019
AW2006Summer31.331.350.522.01.100.017
BSW2006Summer30.529.856.321.40.720.015
NZW2006Summer31.129.951.722.00.570.009
ArW2006Summer31.526.967.621.00.330.011
MCW2007Summer31.728.052.220.61.340.021
AW2007Summer49.525.257.818.61.580.034
BSW2007Summer47.029.957.321.71.430.030
NZW2007Summer35.830.053.822.10.640.011
ArW2007Summer35.327.068.021.30.310.010
MCW2007Autumn31.334.255.323.11.350.028
MCW2008Summer33.033.659.623.11.240.031
Table 3. Comparisons of reproduction characteristics (mean values) of Oithona similis in the Barents Sea [57,58,73]. Water masses: MCW—Murmansk Coastal Water AW—Atlantic Water, ArW—Arctic Water, BSW—Barents Sea Water, PCW—Pechora Coastal Water, NZW—Novaya Zemlya Coastal Water. OF—proportion of ovigerous females (%), D—egg diameter (μm), Fec—fecundity (eggs per sacs), RE—reproductive effort (%), EPR—egg production rate (eggs per female day−1), SEPR—specific egg production rate (day−1).
Table 3. Comparisons of reproduction characteristics (mean values) of Oithona similis in the Barents Sea [57,58,73]. Water masses: MCW—Murmansk Coastal Water AW—Atlantic Water, ArW—Arctic Water, BSW—Barents Sea Water, PCW—Pechora Coastal Water, NZW—Novaya Zemlya Coastal Water. OF—proportion of ovigerous females (%), D—egg diameter (μm), Fec—fecundity (eggs per sacs), RE—reproductive effort (%), EPR—egg production rate (eggs per female day−1), SEPR—specific egg production rate (day−1).
ComparisonsReproductive Parameters
OFDFecREEPRSEPR
Kruskal–Wallis TestH10.571.242.87080.961.5
p0.06<0.001<0.001<0.001<0.001<0.001
Pairs of water mass with significant differencesBSW-PCW, MCW, AW, NZW, ArWPCW-AW, MCW, BSW, ArWBSW-PCWArW-AW, PCW, MCW, BSWArW-PCW, AW, MCWNZW-AW, MCWBSW-PCW, MCW, AW, NZW, ArW
PCW-BSWNZW-BSW, ArWArW-MCW, PCWNZW-PCW, MCW, BSWBSW-AW, MCWPCW-AW, MCWPCW-BSW
Tukey–Kramer TestMCW-BSWAW-PCW, BSW, ArWNZW-AW-ArW, BSWNZW-AW, MCWArW-AW, MCWMCW-BSW
p < 0.05AW-BSWMCW-PCW, BSW, ArWAW-PCWPCW-ArW, NZWPCW-ArW, AW, MCWBSW-MCWAW-BSW
NZW-BSWBSW-PCW, NZW, AW, MCW, ArWMCW-ArWMCW-ArW, NZWAW-ArW, BSW, NZW, PCWAW-NZW, PCW, ArW, MCWNZW-BSW
ArW-BSWArW-PCW, NZW, AW, MCW, BSWPCW-BSW, ArW, AWBSW-ArW, NZW, AWMCW-ArW, BSW, NZW, PCWMCW-NZW, PCW, ArW, BSW, AWArW-BSW
BSW-PCW, MCW, AW, NZW, ArWPCW-AW, MCW, BSW, ArWBSW-PCWArW-AW, PCW, MCW, BSWArW-PCW, AW, MCWNZW-AW, MCWBSW-PCW, MCW, AW, NZW, ArW
Table 4. Mean mortality rates (day−1) of Oithona similis in the Barents Sea [57,58,78].
Table 4. Mean mortality rates (day−1) of Oithona similis in the Barents Sea [57,58,78].
Water MassYearSeasonPair
Copepodite IV-VCopepodite V-Adults
AW2001Summer0.0110.113
PCW2001Summer0.1130.056
MCW2004Summer0.0440.105
AW2004Summer0.0240.060
MCW2005Summer0.0780.088
AW2005Summer0.0050.084
BSW2005Summer0.0060.073
MCW2006Summer0.0980.070
AW2006Summer0.0670.072
BSW2006Summer0.0760.022
NZW2006Summer0.0300.012
ArW2006Summer0.0120.045
MCW2007Summer0.0620.319
AW2007Summer0.0610.114
BSW2007Summer0.0750.214
NZW2007Summer0.0860.140
ArW2007Summer0.0500.030
MCW2007Autumn0.1070.206
MCW2008Summer0.0670.151
MCW2009Summer0.2180.005
AW2009Summer0.1350.001
ArW2009Summer0.0540.001
MCW (Kola Bay)2005Winter0.0050.087
MCW (Kola Bay)2005Spring0.0040.109
MCW (Kola Bay)2006Summer0.0040.300
MCW (Kola Bay)2005Autumn0.0020.154
Table 5. Latitudinal trends in morphological variability of Oithona similis in the Barents Sea (modified from [57,58,72]).
Table 5. Latitudinal trends in morphological variability of Oithona similis in the Barents Sea (modified from [57,58,72]).
ParameterWater Mass
MCWPCWAWArW
X ± SDRangeX ± SDRangeX ± SDRangeX ± SDRange
Male
La, µm925 ± 34868–980883 ± 28840–952811 ± 46756–952799 ± 26756–868
Ns36 ± 330–4036 ± 330–4034 ± 328–3834 ± 228–36
Ls, µm68 ± 264–7267 ± 264–7165 ± 459–7064 ± 359–69
La/Lc, %223 ± 3217–231231 ± 5222–245190 ± 7171–204177 ± 5155–188
Ls/La, %7.0 ± 0.36.6–7.57.3 ± 0.26.9–7.67.9 ± 0.46.8–8.48.1 ± 0.27.5–8.4
Female
La, µm863 ± 39812–924830 ± 30784–896806 ± 48728–896783 ± 30700–808
Ns37 ± 332–4036 ± 330–4034.2 ± 3.130–4034 ± 230–36
Ls, µm65 ± 164–6965 ± 164–6664 ± 164–6565 ± 164–66
La/Lc, %183 ± 10166–210176 ± 10156–200160 ± 5150–173153 ± 6140–164
Ls/La, %7.9 ± 0.47.3–8.88.1 ± 0.27.7–8.58.1 ± 0.36.8–8.88.2 ± 0.47.5–9.0
Note. MCW—Murmansk Coastal Water, PCW—Pechora coastal waters, AW—Atlantic Water, ArW—Arctic Water, Lc—total cephalothorax length, La—total antennule length, Ns: total number of setae on both antennules, Ls—total setae length, La/Lc—relative antennule length, Ls/La—relative setae length.
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Dvoretsky, V.G.; Dvoretsky, A.G. Life-History Traits of a Small Cosmopolitan Copepod (Oithona similis) in the Barents Sea: A Review. Biology 2026, 15, 27. https://doi.org/10.3390/biology15010027

AMA Style

Dvoretsky VG, Dvoretsky AG. Life-History Traits of a Small Cosmopolitan Copepod (Oithona similis) in the Barents Sea: A Review. Biology. 2026; 15(1):27. https://doi.org/10.3390/biology15010027

Chicago/Turabian Style

Dvoretsky, Vladimir G., and Alexander G. Dvoretsky. 2026. "Life-History Traits of a Small Cosmopolitan Copepod (Oithona similis) in the Barents Sea: A Review" Biology 15, no. 1: 27. https://doi.org/10.3390/biology15010027

APA Style

Dvoretsky, V. G., & Dvoretsky, A. G. (2026). Life-History Traits of a Small Cosmopolitan Copepod (Oithona similis) in the Barents Sea: A Review. Biology, 15(1), 27. https://doi.org/10.3390/biology15010027

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