Hydrobiology

Dive fisheries are widely practiced in coastal areas worldwide owing to their minimal equipment requirements and extensive historical background. Female divers in Korea and Japan have extensive knowledge of the local coastal environment and engage in sustainable fishing practices. However, the number of divers and their catches has been declining. In this study, long-term catch and effort data were collected to investigate the reasons for the decline in the catch of the sea urchin, Hemicentrotus pulcherrimus, in Mikuni, Sakai City, Fukui Prefecture, Japan. A significant correlation was observed between catch and effort, and the main reason for the decline in catch was a reduction in fishing effort. Fishermen have voluntarily limited their fishing efforts to prevent the depletion of natural stocks, and a decrease in the number of fishermen has contributed to this reduction. In addition, regulations on catchable size and fishing season were implemented. However, natural stocks appear to have declined. Although sea urchin fisheries have been sustainably practiced for a long time, current fishery management and regulations appear inadequate. Therefore, sustainable fishing requires identifying the factors that affect stock fluctuations and updating management practices. Full article

Because animals threatened by visually oriented predators may respond in sun-lit daytime but not at night, invertebrate responses to predatory challenges may yield varying results based on the time period within the 24 h daily cycle. We predicted that in laboratory experiments aquatic isopods exposed to kairomones from predatory fish would spend more time immobilized in daylight to avoid detection than those not exposed to kairomones but that this difference would disappear under the cover of nighttime darkness. We further predicted that isopods in the absence of kairomones would move at elevated rates in the daytime compared with night, seeking a precautionary proximity to shelters. However, contrary to our predictions, Caecidotea communis isopods exhibited consistent activity (movement rate and proportion of time spent moving) when exposed to kairomones or in the absence of such cues, at all of the three diurnal cycle periods examined. Thus, Caecidotea communis displayed cathemerality (sometimes called metaturnality), the first documented case of this behavior in crustaceans. Full article

Cold-water lakes in high-latitude regions are experiencing rapid changes in community structure and functioning associated with local and global stressors (e.g., climate change, hydropower and invasive species). However, the long-term ecological responses of cold-adapted top predators are relatively poorly monitored despite their high importance for structuring ecological communities and for the provisioning of ecosystem services. We studied long-term changes (2010–2021) in the population structure and trophic niche of two cold-adapted coregonid fishes in oligotrophic Lake Osensjøen, southeastern Norway. Our gillnet surveys indicated that the whitefish (Coregonus lavaretus) population declines simultaneously with the increasing population density of roach (Rutilus rutilus), whereas vendace (Coregonus albula) showed more stable densities. Both whitefish and vendace became increasingly dominated by small-sized individuals following the increase in coexisting roach and perch (Perca fluviatilis) populations. Our stomach content and stable isotope data indicated a marked overlap in the trophic niches of whitefish and roach, with both species showing high among-individual variation in δ¹³C and δ¹⁵N values as compared to the more specialized zooplanktivorous vendace. Our study provides further evidence that the ongoing environmental changes in high-latitude lakes may induce rapid changes in community structures and lead to the population declines of cold-adapted fishes, likely associated with strong resource competition with warm-adapted cyprinid and percid fishes. Such shifts in fish community structure may, in turn, affect the benthic and pelagic food-web compartments and reduce valuable ecosystem services such as local fisheries targeting salmonids. Full article

Trace elemental water chemistry in aquatic systems is primarily derived from the underlying geology of surrounding watersheds. Trace elements can be absorbed from the environment and become permanently incorporated into certain calcified structures of fish. Comparison of the trace elemental chemistry of these structures with the chemical signature of water from the watershed in which a fish was captured can help identify movement patterns. The goal of this study was to create a relatively fine scale baseline for the trace elemental water chemistry of the Chattahoochee, Ocmulgee, Oconee, and Savannah rivers within the Piedmont ecoregion of Georgia, U.S., and to distinguish any trace elements that may be useful in delineating different reaches. Principal component analysis was used to analyze ratios of Ba:Ca, Mg:Ca, Mn:Ca, Na:Ca, and Sr:Ca from samples in each river. Results suggest that Ba:Ca and Sr:Ca ratios may be useful in delimiting river reaches in each system. In addition, Na:Ca and Mn:Ca ratios may have some utility in further differentiating reaches in the Chattahoochee and Savannah rivers, respectively. The ability of these elements to differentiate reaches within these rivers lends support for their utility in fish microchemistry studies to help better understand and manage fish in these systems. Full article

Roots and rhizomes play diverse roles in the response of coastal wetland ecosystems to climate change through hydrobiogeomorphic and biogeochemical processes. The accumulation of living and dead belowground biomass contributes significantly to surface elevation gain, redox status through root oxygen loss and exudates, and plant transport of greenhouse gases to the atmosphere. Yet, responses of belowground biomass to global climate stressors are difficult to measure and remain poorly understood. Here, we report on the response of individual components of belowground biomass to 12 years of CO₂ enrichment in a temperate tidal marsh. In both a community initially dominated by the C₃ species Schoenoplectus americanus and another initially dominated by the C₄ species Spartina patens, elevated CO₂ increased total belowground biomass and subtly altered depth distributions of some components. In the Spartina community, this effect was the result of the direct effects of CO₂ on plant biomass allocation, while any direct response in the Schoenoplectus community was difficult to detect because of changes in the relative abundance of C₃ versus C₄ species. In the Schoenoplectus community, belowground biomass was positively related to S. americanus stem density. Compared to the C₄ community, the Schoenoplectus community had higher root and rhizome biomass and deeper rhizomes. These results highlight the importance of community composition and plant functional traits in understanding ecosystem- and community-scale responses to elevated CO₂ and their potential impacts on marsh elevation. Full article

Muskellunge (Esox masquinongy) are a recreationally and ecologically important apex predator found throughout North America. In West Virginia, the genetic structuring and diversity of native muskellunge is poorly understood. The supplementary stocking of non-native muskellunge has further complicated the issue, as the introgression of non-native alleles and prevalence of non-native muskellunge post stocking remains unclear as well. Using ddRAD sequencing, several datasets were generated to investigate the population structure and genomic diversity of muskellunge in West Virginia. Populations stocked with New York-strain muskellunge exhibited significant introgression, with genetic composition diverging from unstocked native West Virginia populations. However, one population showed greater genetic similarity to native and unstocked populations despite New York-strain prevalence, suggesting resilience against genetic alteration. Fixed SNPs between the New York and West Virginia strains were identified that can be used for broodstock screening and the enhancement of native populations. A genetically distinct population was identified in the Little Kanawha River system, with this population having the highest levels of genomic diversity among native populations as well as a high number of private alleles. However, elevated inbreeding coefficients highlight potential conservation concerns for this unique population. This study establishes a genomic baseline for muskellunge in West Virginia and underscores the importance of preserving native genomic diversity while balancing the demands of recreational fishing programs. Full article

Systemic habitat destruction over the last 100 years combined with major anthropogenic stressors such as aquatic contaminants, exotic species, and economic endeavors is driving the decline in freshwater unionid species diversity. Global temperatures continue to increase, with January 2024 being the warmest on record according to the latest report by the World Meteorological Organization. Freshwater mussels play a crucial role in aquatic ecosystems, contributing significantly to benthic processes in rivers and streams, yet they remain highly sensitive to environmental changes. This study specifically investigates the thermal tolerance of the Texas pigtoe (Fusconaia askewi) under elevated temperature conditions and explores the implications for developing effective conservation strategies in freshwater ecosystems. Eighty-four individual adult Texas pigtoe mussels were collected from the upper Sabine River near Hawkins, Texas, and taken to the University of Texas at Tyler to evaluate the effects of elevated temperature, a likely factor impacting mussels in East Texas. In the thermal tolerance study presented here, 100% survival occurred at both the control (20 °C) and the 25 °C test points. The 30 °C treatment group had an overall mortality of 14% and the 35 °C treatment group showed a mortality rate of 43% by the end of the trial, suggesting the typical summer temperatures in Texas streams will result in the loss of a portion of an otherwise healthy population. Full article

Bisphenol A, BPA, is a small molecule frequently used in large-scale plastic production. The chemical has garnered a reputation for its association with harmful human health effects, and numerous animal studies have contributed to its classification as an endocrine disruptor. Prior research has investigated the impact of the chemical on echinoderms, including seven species of sea urchin. Our project investigated the toxic effects of this chemical on two uninvestigated species: Lytechinus variegatus and Arbacia punctulata. We exposed embryos to a range of environmentally relevant BPA concentrations (1 µg/L, 10 µg/L, 100 µg/L, and 1000 µg/L) for 48 h, until the pluteus stage. Larvae were classified according to the type of abnormality they exhibited, using a light microscope, and the EC50 was determined through probit analysis and dose–response curves. We also examined isolated plutei skeletons under a scanning electron microscope to assess changes to the skeletal structure under increasing concentrations of BPA. Our results suggest BPA induces embryotoxicity and soft tissue abnormalities more severely in L. variegatus, whereas A. punctulata exhibits more resistance to these effects. The EC50 values, over 1000 µg/L for A. punctulata and approximately 260 µg/L for L. variegatus, support this. These relative values also agree with our hypothesis that sea urchin embryos in a single genus have a similar level of BPA embryotoxicity. Interestingly, under SEM examination, the A. punctulata skeletal microstructure appears to be altered as a result of BPA exposure. While the EC50s are below what has been documented in many, but not all, marine environments, longer and consistent exposure may have a more deleterious impact. These findings suggest BPA’s effects on echinoderms should be further explored with multiple forms of analysis and over the long term. Full article

The present study aimed to describe the reproductive cycle of the black-footed limpet (Patella depressa Pennant, 1777) from an intertidal rocky shore on the Algarve coast (southern Portugal). Samples were collected monthly between January 2017 and December 2018, with the species’ gametogenic cycle being described based on gonad histology and the mean gonadal index. The presence of both transitional and mosaic hermaphrodites indicates that some individuals are able to change sex (sequential hermaphroditism). Despite the occurrence of hermaphroditism, sex proportions were approximately equal, suggesting the absence of protandric sex change in this species. The population exhibited an extensive occurrence of ripe and spawning gonads throughout almost the whole study period, probably related to consecutive processes of gonadal re-ripening and partial spawning events. The reproductive dynamics of P. depressa displayed clear inter-annual differences, with a short resting period recorded in 2017 (June–August) and the absence of resting gonads in 2018. The continued monitoring of this population and collection of environmental data are required to further improve knowledge of the reproductive dynamics of this species. Such information is crucial for proposing additional management measures for the sustainable harvesting of limpets in southern Portugal. Full article

Biologists commonly use standard sampling protocols to ensure that data are comparable spatiotemporally. Data also need to be precise to allow for statistically meaningful comparisons. However, the effort needed to precisely sample desert fishes is unclear. We used a resampling approach to evaluate the effort requirements needed to precisely sample fishes among six wadeable rivers in the Gila River basin, New Mexico. We evaluated the number of samples that are necessary to obtain relative density estimates that had 25% relative standard error 80% of the time. We also estimated the effort needed to precisely characterize species richness. Our results indicate that precisely sampling fish in the Gila River basin is difficult. Sonora Sucker Catostomus insignis, Desert Sucker C. clarkii, Longfin Dace Agosia chrysogaster, and Speckled Dace Rhinicthyes osculus were generally the only species that could be precisely sampled. Characterizing the native species assemblage in the Gila River basin required between two and seven reaches, whereas the entire species assemblage could only be characterized in 50% of the study systems. The challenge of precisely sampling fish in the Gila River basin suggests that alternative sampling methods may be required to characterize changes in density or species distribution in desert Southwest systems. Full article

Tannins are plant secondary compounds that leach into soil and water. Tannin concentrations can be higher in lentic freshwater than in other aquatic ecosystems, which can result in toxicity to freshwater organisms. While the amount of plant material and the plant tissue concentrations affect aqueous tannin concentrations, little is known about which environmental conditions alter the removal and breakdown of tannins in freshwater, altering the exposure of freshwater organisms. We investigated the effects of soil, light, and temperature on aqueous tannin concentrations. Tannins degraded faster in the presence of and/or sorbed to wetland soils, silt, and clay but not sand. It is unclear whether finer soil particles or the chemical makeup of soils facilitated sorption and/or degradation. Contrary to previous work, we found no effect of ambient light over a 14-day period or sunlight over a 7-day period on tannin degradation, but tannins degraded faster in warmer water. Warmer freshwater ecosystems with greater silt or clay sediments are likely to have lower aqueous tannin concentrations with lower toxicity to aquatic organisms. This research used tannic acid, which contains gallotannins. Further research is needed to determine if similar patterns are found with other tannin types and the complex mixtures of different tannins found in most plants. Full article

An important top-down predator, the northern pike (Esox lucius), faces harsh environmental conditions in the northern boreal ecoregion. They are often managed for recreational fishing and, more recently, to create environmental offsets; strategies aimed at balancing ecological impacts by enhancing or restoring habitats. Our study examines northern pike populations in two remote boreal lakes in northern Alberta: Steepbank and Wappau. The lakes differ in size, vegetation cover, and trophic status, providing a natural experiment for investigating northern pike growth, condition, diet, and population density. Over three years (2018–2020), northern pike were sampled using gill nets. Population metrics, including growth, condition, and stomach contents, were compared between the lakes. Steepbank, a smaller, oligotrophic lake with low vegetation cover, showed lower prey fish densities compared to the larger, eutrophic Wappau, but it did not differ in northern pike catch per unit effort. Growth rates and body condition varied significantly between the lakes, with the northern pike in Wappau exhibiting faster growth and a better condition in the older age groups, while the younger northern pike in Steepbank had higher relative weights. A diet analysis revealed significant differences in prey consumption: Steepbank northern pike displayed higher rates of conspecific predation and invertebrate consumption, particularly in the younger age classes. These findings highlight how lake characteristics and prey availability shape northern pike population dynamics, offering valuable insights for lake management approaches in northern Alberta. Full article

The previously available coding region for the spiny dogfish (Squalus acanthias) AQP3-2 gene was amplified from cDNAs using PCR. Agarose gel electrophoresis gave a band of the AQP3-2 coding region, as well as multiple smaller splice variant bands. The main AQP3-2 band and the largest and most fluorescently intense pair of these splice variant bands were cloned and sequenced. Amplifications were performed on a range of tissue cDNAs, but AQP3-2 was only expressed in the kidney and brain. Quantitative PCR amplifications using pre-existing kidney cDNA from an environmental salinity acclimation experiment showed that the abundance of mRNA from both the main AQP3-2 transcript and the largest splice variant (Splice Variant 1) was lower in 120% seawater (SW) acclimated fish, although only the values for Splice Variant 1 were statistically significant. A custom-made affinity-purified rabbit polyclonal AQP3-2 antibody was produced, and this gave four bands of around the correct sizes (which were 27 and 32 kDa) for the complete AQP3-2 and Splice Variant 1 proteins. Two of the bands may have been N-glycosylated forms of these proteins. Other bands were also present on the Western blot. No bands were present when the antibody was pre-blocked by the peptide antigen. In tissue sections of the dogfish kidney, immunohistochemical localization experiments showed that AQP3-2 was expressed in the early distal tubule (EDT) and late distal tubule (LDT) nephron segments. The results suggest that AQP3-2 may be involved in cell volume regulation in the EDT and water and urea absorption in the LDT nephron segment. Full article

S. sphyraena is a widely distributed species with low commercial value and no sufficient scientific knowledge of its biology. In the present study, the age, growth, weight–length relationship, otolith morphometry, and reproduction of the species were investigated in the Eastern Ionian Sea for the first time. The von Bertalanffy growth function parameters were L_∞ = 63.65 cm, k = 0.14 year⁻¹ and t₀ = −2.01 years and Φ′ = 2.75. The negative allometric growth in weight was found. Slope b of the weight–length relationship was 2.634. For the otolith moprhometry, the variables radius, length, width, area, perimeter, roundness, circularity, form factor, rectangularity, and ellipticity were examined, which showed that the otolith shape is elongated with an elliptical and rectangular form. Six otolith variables (radius, length, width, area, perimeter, and ellipticity) showed a significant relationship with size. The sex ratio (females/males) was 1:0.74 (no statistically significant difference from 1:1), and the spawning season extended from April to June with peak values of GSI in May for females and April for males. The results of this work improve our knowledge of the species life cycle and provide basic information for species stock identification and fisheries management. Full article

The sea bivalve clam Tridacna crocea inhabiting the shallow sea of tropical and subtropical zones lives with the symbiotic alga zooxanthella in its mantle. Zooxanthellae algae perform photosynthesis and supply nutrients to T. crocea. Recently, the abundance of T. crocea has decreased rapidly due to overfishing in coastal areas in Okinawa, Japan. T. crocea culture systems for mass production will contribute to the conservation of T. crocea and thus marine ecosystems. Environmental control methods for T. crocea culture have not been established because of a lack of knowledge about the appropriate environmental conditions for T. crocea growth. The present study was initiated to obtain basic data for developing environmental control methods for T. crocea land-based aquaculture. The effects of water temperature, dissolved oxygen concentration, and photosynthetic photon flux density (PPFD) on the O₂ exchange rates of the symbiotic system of T. crocea and zooxanthella, which are indicators of photosynthesis and respiration in the system, and the effect of daily integrated PPFD on T. crocea growth were investigated. Basic knowledge was obtained for the development of optimal environmental control technology for T. crocea clam culture. The optimum water temperature and dissolved oxygen concentration for photosynthesis in this symbiotic system were 28 °C, 5–6 mgO₂ L⁻¹ and 500 μmol m⁻² d⁻¹, respectively. The optimum daily integrated PPFD for clam growth was 20 mol m⁻² d⁻¹. Full article

The evaluation of mitochondrial DNA and genetic analysis is helpful for economically significant species. Clarias gariepinus is a critical species in aquaculture. This study investigates the genetic diversity and population differentiation of C. gariepinus from 19 countries using 164 sequences of the mitochondrial DNA’s Cytochrome c oxidase I (COI) gene. The haplotype analysis revealed a total of 17 haplotypes, with a nucleotide diversity (π) of 0.012 and a haplotype diversity (Hd) of 0.87. The results of an AMOVA and fixation index indicated significant genetic variation and structure among the populations. Additionally, neutrality tests and mismatch distribution analysis supported the hypothesis of under-purifying selection in C. gariepinus. The findings suggested that the population did not experience expansion. In conclusion, the genetic analysis highlighted substantial variation among C. gariepinus populations from different locations, providing valuable insights for the global management of this species. Full article

Over the course of industrialization in the 20th century, vast emissions of air pollutants have occurred. The exhaust gasses contain sulfur and nitrogen oxides, which are converted to sulfuric acid and nitric acid in the atmosphere. This causes acid rain to enter aquatic and terrestrial ecosystems, the most serious consequence of which is large-scale forest dieback across Europe and North America. However, through various political measures, the exhaust gasses have been reduced and, thus, acid rain and forest dieback were stopped. Nevertheless, the lingering effects of this pollution are still present today and are reflected in hydrochemistry. More recently, fluctuating precipitation regimes are causing additional stress to ecosystems in Central Europe. Climatic extremes are becoming more pronounced with climate change. Substantial differences between drought years and years with regular precipitation are directly altering the discharge of springs. Now, two overlapping and interacting syndromes of environmental pressures can be studied in these small catchments at a landscape scale: (1) acidification and (2) climate change. In this long-term study, the waters of 102 forest springs, located in two neighboring forest landscapes in north-eastern Bavaria, Germany (Frankenwald and Fichtelgebirge), were investigated over 24 years (1996 to 2020). By linking changes in pH values with changes in precipitation and spring discharge, we found that pH increases with decreasing discharge and decreasing precipitation. This effect was strongest in the Frankenwald compared to the Fichtelgebirge. We hypothesize that this temporal pattern reflects the longer residence time and, in consequence, the increased buffering of acidic interflow in small catchments during periods of drought. However, this should not be misinterpreted as rapid recovery from acidification because this effect fades in times of enhanced precipitation. We recommend that fluctuations in weather regimes be considered when investigating biogeochemical patterns throughout forest landscapes. Full article

Salmonid fishes provide an important indicator of climate change given their reliance on cold water. We evaluated temporal changes in the density of stream-dwelling brook trout (Salvelinus fontinalis) from surveys conducted over a 36-year period (1988–2023) by the Maryland Department of Natural Resources in Eastern North America. Nonparametric trend analyses revealed decreasing densities of adult fish (age 1+) in 19 sites (27%) and increases in 5 sites (7%). In contrast, juvenile fish (age 0) densities decreased in 4 sites (6%) and increased in 10 sites (14%). Declining adult brook trout trends were related to atmospheric warming rates during the study period, and this relationship was stronger than the effects of land use change or non-native brown trout. In contrast, juvenile fish trends generally increased with elevation but were not related to air temperature trends or land use change. Our analysis reveals significant changes in several brook trout populations over recent decades and implicates warming atmospheric conditions in population declines. Our findings also suggest the importance of temperature for adult survival rather than recruitment limitation in brook trout population dynamics. Full article

Freshwater slugs are scarce and belong exclusively to panpulmonate Acochlidimorpha. There is a radiation of eight species of large-sized slugs living benthically in rivers on tropical Indo-Pacific Islands. In the Western Atlantic, only one small interstitial slug, Tantulum elegans Rankin, 1979, is known from the Caribbean island of St. Vincent. We recently discovered a novel species of freshwater slugs in Cuba. Here, we describe Potamohedyle espinosai n. gen. n. sp., which is the first freshwater slug in the region of the Western Atlantic with a benthic lifestyle, in 3D-microanatomical and histological detail using light and scanning electron microscopy. It shows a mix of characters from different freshwater acochlidimorph genera, such as a medium body size, the presence of an osphradial ganglion, a distal gonoduct with a muscular sphincter, a penis with a solid thorn and cuticular comb, and a basal finger with a hollow stylet. Morphological adaptations to a life in freshwater include multiplicated renopericardioducts. The taxonomic character mix justifies the establishment of a novel genus within the herein diagnostically modified freshwater family Tantulidae. A molecular phylogenetic hypothesis of riverine slugs including the first Caribbean representatives suggests that the transition to freshwater occurred once along the stemline of limnic Acochlidiidae, secondarily marine Pseudunelidae and limnic Tantulidae. Full article