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Fishes 2017, 2(3), 15; doi:10.3390/fishes2030015

Article
Recognition and Distribution of Two North Atlantic Gadiculus Species, G. argenteus and G. thori (Gadidae), Based on Otolith Morphology, Larval Pigmentation, Molecular Evidence, Morphometrics and Meristics
1
Joost van den Vondelstraat 30, Winterswijk 7103 XW, The Netherlands
2
Department of Fish and Shellfish, Pinngortitaleriffik (Greenland Institute of Natural Resources), Kivioq 2, Post box 570, Nuuk 3900, Greenland
*
Author to whom correspondence should be addressed.
Academic Editor: Maria Angeles Esteban
Received: 28 July 2017 / Accepted: 3 August 2017 / Published: 29 August 2017

Abstract

:
The silvery pout genus Gadiculus consists of small aberrant codfishes with several extinct and currently only one recognized extant species. The oldest representatives of a Gadiculus lineage known from otoliths are Early Miocene in age. Fossil evidence has showed Gadiculus to originate from older genera diverging early from other true cods of the family Gadidae. As adult specimens of different species have been found to be highly similar and difficult to distinguish based on meristic and morphometric data, the number of species in this gadid genus has been controversial since different larval morphotypes were first discovered some 100 years ago. For almost 70 years, Gadiculus thori and Gadiculus argenteus have been considered subspecies only, with a distribution in the Northeast Atlantic Ocean including the Mediterranean. In this study, we resolve the long-standing issue of extant Gadiculus not being monotypic. New results in the form of distinct adult otoliths and molecular data unambiguously show two species of Gadiculus present—in agreement with larval morphotypes. Morphometric, meristic and molecular characters, as well as larval pigmentation are discussed in addition to present and past geographic distributions of the two taxa from distributions of fossil otoliths. At present, the cold-water species Gadiculus thori (northern silvery pout) is distributed in cold-temperate and subarctic latitudes in the Northeast Atlantic, including a new range extension off Southeast Greenland. Gadiculus argenteus (southern silvery pout) occurs in warmer waters and is distributed in the warm-temperate East Atlantic and Mediterranean. Fossil otoliths show that both species often co-existed in the Mediterranean from the Late Pliocene to the Middle Pleistocene.
Keywords:
taxonomic revision; otolith; Cox1 barcode; larval melanophore pattern; morphometrics; meristics; (palaeo)geographic distribution; Gadidae classification

1. Introduction

Silvery pouts of the genus Gadiculus are the smallest extant cods in the family Gadidae. Only one species, Gadiculus argenteus (Guichenot 1850), is currently recognized with a second species described Gadiculus thori (Schmidt 1913) currently considered a synonym [1]. Most gadid species are economically important, although silvery pouts have limited commercial value due to their small size and predominantly artisanal use in some Mediterranean countries. This becomes evident considering the relatively limited number of studies present on these fishes compared to other, larger gadids. However, Gadiculus have high regional abundances and have been identified as important forage prey in trophic ecosystem dynamics [2]. Juveniles and adults of Gadiculus feed almost exclusively on various groups of pelagic crustaceans, but also eat small fish [3,4,5,6].
Gadiculus fishes are meso- to bathypelagic and show gregariousness, forming large schools in the deeper parts of the shelf and above the continental slope—usually at depths between 100 and 1000 m. However, some differences have been reported concerning depth distributions between northern and southern populations, with the former mainly between 125 and 400 m [6] and the latter mainly between 200 and 400–500 m [7]. Their occurrence in the colder, deeper waters in the more southern Atlantic waters and in the Mediterranean is likely an overlooked factor concerning depth distributions. The younger stages of Gadiculus fishes are known to inhabit relatively shallow depths. Gadiculus fishes have currently been observed in the northeastern Atlantic from off the North Cape of Norway and the western part of the Barents Sea [8] southwards to Morocco, including the western and central Mediterranean [9]. Observations are also present from the Adriatic Sea [10], the entire Aegean Sea, Thracian Sea [11], off the Turkish coast [12], off the Syrian coast [13], and off the coast of Israel [14]. However, no records from the tropical or north-western Atlantic have ever been reported.
Guichenot [15] (1850) described Gadiculus argenteus from the Mediterranean coast of Algiers erecting the new genus Gadiculus. The establishment of the genus was justified due to its large eye/body ratio compared to other gadids, i.e., the eye diameter is longer than the snout, although less than 40% head length. In addition, Gadiculus shows a unique oblique mouth that is angled steeply upwards. Other distinguishing characteristics of Gadiculus are the large deciduous scales and the sensory canals with seven large open pits (mucous cavities) on the dorsal margin of the head (Svetovidov) [16]. Subsequently, Schmidt [17] described Gadiculus thori from the North Atlantic Ocean based on the following characteristics:
  • Different melanophore patterns in post-larvae of the two species. The post-larvae of G. argenteus show three transverse pigmented bars, whereas G. thori only exhibit one (Figure 1).
  • At the same stage of development, the post-larvae of G. thori are, in general, larger than those of G. argenteus (Figure 1).
  • At the same stage of development, the post-larvae of G. thori are slender compared with G. argenteus, which are stouter and shorter (Figure 1).
  • Different number of vertebrae in the two species. Schmidt found that G. thori has 41–43 (usually 42), whereas G. argenteus has 39–41 (usually 40).
  • Geographic distribution. The number of G. thori specimens declines drastically from Ireland in the North to the French Atlantic coast. Conversely, G. argenteus occurs in increasing numbers going south from the mouth of the river Gironde along the Atlantic east coast.
Subsequently, Svetovidov [18] reclassified the two taxa as subspecies, a classification that subsequently became common usage. Mercader and Vinyoles [1] went even further and synonymized the two subspecies as one indistinguishable eastern North Atlantic species based primarily on external morphometric and meristic characters of adult specimens.
The objective of the present study was to resolve the taxonomic status of extant Gadiculus by carefully comparing all characteristics available, including old and new data that were not taken into consideration by the revision of Mercader and Vinyoles [1].

2. Materials and Methods

2.1. Otoliths

Gadiculus thori: 213 specimens, Standard Length (SL) 101–147 mm or Total Length (TL) 111–164 mm, June 26th 1977, Kvinnherad Fjord, Norway (59°58′ N, 5°59′ E), catch-depth ca. 170 m, collected and identified by (Coll/ID) T. Bakke, otoliths extracted by P. Gaemers; three specimens Trondheim Fjord, Norway, Coll./ID G. Van der Velde.
Gadiculus argenteus: 10 specimens, SL 59–104 mm, Barcelona, Spain, Mediterranean Sea, catch-depths 220 and 385 m, sagittae extracted, Coll./ID C. Allué; two right sagittae, off Casablanca, Morocco, southeast Atlantic Ocean, catch-depth 350 m, Coll./ID D. Nolf; one specimen off Mallorca, Spain, Mediterranean Sea, Coll./ID P. Gaemers; one pair of sagittae, off Agadir, Morocco, Coll./ID P. Gaemers; one pair of sagittae, Mediterranean Sea (fish market, Leiden, The Netherlands), stomach content of Conger conger, Coll./ID P. Gaemers.
The aforementioned otoliths were deposited in the collection of P. Gaemers.
Furthermore, sagittae of 46 specimens of Gadiculus argenteus (TL 82–145) from off the Portuguese coast, catch-depth 140–401 m, mainly 370–401 m, Coll./ID C.A. Assis, 40 specimens in the collection of C.A. Assis and 6 specimens in the collection of P. Gaemers.
Otoliths of Gadiculus are easy to dissect due to their large size, with otolith length (OL) approximately 5% of the TL. There are two possible reasons for this: the smallest species in a genus or family tend to have the largest otoliths, and the size of the eyes and the otoliths are correlated (P. Gaemers, data not shown). The otoliths were dissected by approaching the fishes from the ventral side. The uncovered neurocranium is cut at the sagittal plane with a sharp knife. After the removal of the brain the otoliths could easily be taken out of the two halves of the skull. This method is more time-consuming than a transversal cut on the dorsal part of the head of the fish, which is the usual procedure in collecting otoliths in fisheries surveys, but provides the best chance to retrieve the otoliths intact. The otoliths were stored dry. In contrast to fossil otoliths, it is difficult to take photographs of recent otoliths that show the superficial morphology in sufficient detail, because of their white colour, their gloss and, frequently, their transparency. Therefore, the otoliths were drawn with the aid of a drawing mirror on a Wild M5 binocular microscope (Joint Stock Company, Heerbrugg, Switzerland) with a low angle of incidence of the light.
Otolith terminology and nomenclature presented in the current study follows Chaine and Duvergier [19], Schwarzhans [20], Gaemers [21] and Nolf [22].

2.2. New Record of Gadiculus thori off Greenland

One specimen of Gadiculus thori was caught by R/V Pâmiut, Greenland Institute of Natural Resources (GINR), leg 4, haul 72, August 11, 2012, in Denmark Strait Southeast Greenland at 64°19′ N, 36°45′ W and deposited at the Zoological Museum University of Bergen as ZMUB 16483 (tissue JYP#952) (Figure 2). Coll./ID J.Y. Poulsen. The specimen was caught during a routine survey with a non-closing Alfredo III trawl probing Greenland halibut abundances; therefore, the catch depth is uncertain. The bottom and fishing depth was 419–424 m with bottom temperature of 3.81 °C. This fish specimen from the Denmark Strait, Southeast Greenland, was digitally X-rayed at the Australian Museum using the industrial X-Ray model EXR 150-23 BW (Seifert Systems, Sydney, Australia), and examined under a stereomicroscope (Zeiss, model 475052-9901, West Germany). Morphological measurements were taken with a digital calliper to the nearest 0.1 mm, listed in Table 1 and compared to data by Raitt [23].

2.3. Molecular Analyses

The new record off Greenland of Gadiculus thori (ZMUB 14683) was Cytochrome Oxidase 1 (Cox1) barcoded as part of the Greenland Fishes (GLF) barcoding project [24] and the sequence deposited in the BOLD repository [25] as GLF136. Additional materials included for molecular comparisons were either newly generated (20 specimens) as part of the GLF project (see [24] for laboratory works) or downloaded (76 specimens) from BOLD. We calculated uncorrected distances for the two taxa including the smallest interspecific distance (barcoding gap) that potentially indicates species delimitation based on Cox1 DNA sequences (Meier and Paulay [26] and Meier et al., 2008 [27]). In addition, the mitogenome (complete mitochondrial genome) was determined for the specimen according to mitogenomic laboratory work [28]. Newly generated sequences are available at DDBJ (DNA Data Bank of Japan), EMBL (European Molecular Biology Laboratory, Heidelberg, Germany) or GenBank as LC146692–711 (Table 2), and G. thori mitogenome as AP018148. The Cox1 sequences are also available from BOLD, either individually (GLF) or as a single dataset including all 96 barcoded specimens used in this study DS-PGJP (data set P. Gaemers & J. Poulsen, Table 2). Catch localities of all Gadiculus specimens used for molecular comparisons are depicted in Figure 3, corresponding also with Table 2, including metadata found in the BOLD repository. Attempts at obtaining samples from the greater regions of the Bay of Biscay were not successful.

3. Results

3.1. Otolith Characteristics

Gadiculus otoliths are flat (when on their longitudinal side), having a short and high oval outline, with basically a broadly rounded anterior margin and a primarily tapering posterior end. They have a pseudobiostial sulcus opening and the sulcus type is homosulcoid (terminology from Schwarzhans) [20]. The sulcus is wide and rather shallow (Figure 4). The colliculi are short, filling only a small part of the ostium and the cauda. The pseudocolliculum is very long. Clear differences in otolith morphology in G. thori and G. argenteus have been recognized since Gaemers [36,37], although his inclusion of G. thori in the fossil genus Gadichthys is currently considered erroneous.
The outline of G. argenteus otoliths is stoutly pear-shaped, thus short and high (Figure 5). It can also be described as drop-shaped. The posterior end is bluntly pointed, forming a wide angle, and may be somewhat indented in the largest otoliths. In cases where the posterior end is truncated it is running obliquely to the longitudinal axis of the otolith. The anterior end is regularly rounded with a ventral part that is slightly more prominent than its dorsal part. In full-grown otoliths an indentation often occurs at the anterior end, separating the rostrum from a somewhat shorter antirostrum. In smaller otoliths, this indentation is absent or is only small and shallow. The otoliths rarely show clear dorsal angles—the ones depicted in Figure 5D,G with a distinct postdorsal angle are unusual exceptions. The otoliths show little variation in shape, and variability is mainly limited to the depth of the ornamentation and the length-height ratio in specimens of equal size (Figure 5). Juvenile otoliths tend to be more slender compared to the adult ones and allometry in their length-height ratio during growth is relatively small.
The outline of G. thori otoliths shows considerable changes throughout ontogeny. Juvenile otoliths are regularly pear-shaped and are often difficult to distinguish from those of G. argenteus. Adult otoliths have an irregular oval outline. In larger otoliths, the posterior end usually becomes more truncated, often ending with a clear indentation. The truncation at the posterior end is running perpendicular to the longitudinal axis of the otolith. Sometimes the posterior end differs considerably between the left and right otoliths from the same specimen, resulting in a truncated otolith that is much shorter and higher than the otolith without this truncation (Figure 6A,B). Larger otoliths often develop a clear indentation at the anterior end as well, with a clear rostrum and antirostrum. The most extreme example shows a very deeply and sharply indented anterior margin and a strongly truncated and indented posterior margin (Figure 7I). Adult otoliths sampled from the same fish population are found to display an extensive variability in the general outline (Figure 7). Often, the dorsal margin before and/or after the predorsal angle is concave, accentuating the dorsal angles. The otoliths of G. thori show very strong allometric growth concerning the length-height ratio, with the juvenile otoliths being slender compared with the adult ones.
Summary of the most distinct differences of the adult otoliths in Gadiculus: G. argenteus otoliths are more regularly drop-shaped with normal, less conspicuous dorsal angles, whereas those of G. thori are clearly truncated at the posterior end and/or having a variable dorsal margin with usually more prominent and irregular angles. There are no apparent overlaps between the shapes of the adult otoliths in the two Gadiculus species despite variation observed within both species.

3.2. Molecular Analysis

A Kimura-2-parameter (K2P) neighbour-joining cladogram is presented in Figure 8, including all recognized Gadidae taxa except Eleginus nawaga (Pallas, 1814), and rooted with the Phycidae (for the classification levels of gadiform family groups in this study, see Section 4.8). Two distinctly different groups corresponding to G. thori and G. argenteus are found with little intraspecific variation between individuals (Figure 8). After unambiguous alignment the following values are observed for 650 base pairs of the Cox1 DNA sequence; the maximum intraspecific variation between G. thori specimens was 0.62%, the maximum intraspecific distance between G. argenteus specimens was 0.31% and the barcoding gap (smallest interspecific distance, i.e., substitutions between the two species excluding the few random one-specimen substitutions) is 1.54%. We note that the barcoding gap seems appropriate in the case of Gadiculus despite low values observed. However, the clear structure in the variation of the barcodes delimiting the two species is the informative data in this particular case (Figure 8). We note that the random substitutions witnessed in single G. thori specimens that results in a variation of 0.62%, could potentially be from sequencing/editing errors that we have no chance of verifying. The two different groups correspond to a geographical separation that is illustrated in Figure 3 by the samples used in the northern and southern Northeast Atlantic Ocean, the latter including the Mediterranean Ocean. The genus Micromesistius is found as a sister group to the Gadiculus lineage with the longest branch in the tree being Gadiculus (note a three-fold shortening of the branch for practical purposes). Gadiculus and Micromesistius taxa constitute a sister group to Trisopterus (see Section 4.4). The group comprising Gadiculus, Micromesistius and Trisopterus is the sister group to the remaining true codfishes in the family Gadidae. The Lotidae family is found to be a sister family to the Gadidae, although the burbot Lota lota is rendering this family non-monophyletic (see Section 4.8). Molecular results not directly related to Gadiculus and/or classification (see below) will not be discussed further, except for the trisopterine fishes, as the results are highly similar to previous molecular studies on Gadiformes [39]. The mitogenome DNA sequence of G. thori consists of 16,713 base pairs and includes the 2 rRNA genes, the 22 tRNA genes and the 13 protein coding genes as observed in vertebrates. In addition, it shows a 258 base pairs intergenic T-P spacer sequence between tRNAs Thr and Pro, the T-P spacer being observed in all gadiform taxa [39].

4. Discussion

4.1. Size and Age

The difference in size between the two Gadiculus species appears to be much larger than previously reported: Heessen et al. [6] show that the TLmax (maximum TL) for G. thori (200 mm) is considerably larger than that for G. argenteus (150 mm). Several differences related to morphology between these species can be explained by this difference in size. The relatively larger head and eyes in the smaller species, G. argenteus, was statistically validated by Pope in Raitt [23]. The differences observed in several proportions are normal for closely related species of different sizes. The same is true when larger and smaller specimens of a species are compared with one another: during ontogeny allometric changes occur. The most important parts and organs of an animal (like for instance the head and the eyes) need to be relatively larger in smaller specimens, causing allometry. Likewise, the otoliths of G. argenteus are relatively larger than those in G. thori. Gaemers [40] observed the same relationship between the relative size of the otoliths of Trisopterus capelanus (Lacepède, 1800) and the larger T. luscus (Linnaeus, 1758). In addition, Schmidt’s [17] observations, that the post-larvae of G. thori are larger and slenderer compared with post-larvae of G. argenteus at the same stage of development, is in accordance with the difference in maximum size.
According to Albert [5], the largest G. thori individuals are at least 170 mm TL and Heessen et al. [6] reported 200 mm for northwestern European specimens, results based on a large number of specimens with broad geographic sampling. The TLmax of this species in the latter, comprehensive study was firmly based on fish populations with gradually declining length distribution towards the maximum size [41]. Stergiou and Politou [42] reported a maximum TL of 140 mm for G. argenteus in Greek Mediterranean waters. Tuset et al. [43] illustrated an otolith from a 145 mm TL specimen caught on the Atlantic coast off the Iberian Peninsula. The maximum TL of this taxon generally agreed upon is usually 150 mm [9], in accordance with a specimen recorded by Vassilopoulou [44] from the northern Euboian Gulf, Greece.
Age determinations using cross sections of whole G. argenteus otoliths from the southern Aegean Sea, showed that the oldest fishes were two years of age when ranging between 90 and 121 mm [45]. Therefore, the largest known fishes of this species are at least 3 years old. Age determination of G. thori otoliths remains to be fully established, although separation of sex and partitioning into 5 mm length classes revealed three year-classes of northern Atlantic specimens [5]. Therefore, this indicated that this species rarely lives to be older than three years, but the largest known fishes reaching 170–200 mm TL, which were not included in Albert’s study are likely to be older, reaching at least 4 years of age.
It could be possible that G. argenteus remains smaller due to environmental factors varying between the Mediterranean and the Atlantic Ocean, for example in Merlangius merlangus (Linnaeus, 1758) that attains a much smaller maximum TL in the Adriatic Sea compared with specimens from the North Sea and Atlantic Ocean, which was not due to overfishing [46,47]. However, G. argenteus has not been observed to grow larger outside the Mediterranean. If the two Gadiculus taxa were to be considered a single species, G. argenteus would have to attain a larger size northwards in the Atlantic. All available data on the size of Gadiculus fishes indicate size difference between the two taxa.

4.2. Morphometric and Meristic Data

The most important studies related to the status of Gadiculus fishes concerning morphological characteristics are: Schmidt [17], Raitt [23], and Mercader and Vinyoles [1]. However, the omission of Raitt’s thorough investigations in the latter is here noted and is problematic. The most distinctive characteristics found by Raitt (and validated by Pope’s statistics in Raitt) are:
  • The relatively larger eye in G. argenteus in relation to SL
  • The relatively larger head in G. argenteus in relation to SL
  • The number of vertebrae in G. thori (39–43)—39 and 40 rarely observed—and G. argenteus (37–41)—37 and 41 rarely observed
  • The number of D3 fin rays in G. thori (15–17) and G. argenteus (11–16)
  • The number of A1 fin rays in G. thori (15–18)—18 rarely observed—and G. argenteus (11–16)—11 rarely observed
Mercader and Vinyoles [1] included only six specimens of G. thori, whereas Raitt’s study consisted of 645 specimens of this species and included vertebral counts that are missing in Mercader and Vinyoles [1]. Raitt counted the fin rays of 85 specimens of G. thori and of 65 G. argenteus specimens. Mercader and Vinyoles studied 69 specimens of G. argenteus (48 from the Mediterranean and 21 from the Atlantic) including fin-ray counts. Raitt had only two specimens of G. argenteus available from the Atlantic and no fin-ray counts were given.
The variability in the number of A1 fin rays of G. thori (12–18) in Mercader and Vinyoles (1, Table 5) is extreme considering the small number of specimens included for this species. Raitt noted 15–18, using a much larger data set. The A1 fin ray counts for G. thori in Svetovidov [16], also incorporated into Table 5 in [1], disagree with those listed in Svetovidov [18,47]. The original data of Svetovidov [18,48] are in agreement with Raitt’s counts, thereby proving the data listed by Svetovidov [16] to be erroneous as he exchanged the A1 ray counts of G. thori for those of G. argenteus. The low A1 fin rays counts of G. thori given by Mercader and Vinyoles have apparently been copied from Svetovidov [16] and must be disregarded.
The large variability of the G. argenteus D3 fin ray counts (12–20) in Mercader and Vinyoles [1] is peculiar and may also be incorrect. None of the five studies they referenced included data similar to these, and nor did Raitt [23] who found numbers greater than 18. Additionally, Mercader and Vinyoles unfortunately did not list each specimen for each discrete character, which is customary in most elaborate taxonomic studies, making the use of their data difficult. Therefore, it is not possible to note which counts are rare and which specimens are within the overlap ranges. The high fin ray counts of the D2 and A2 for G. argenteus in Mercader and Vinyoles [1] were similarly not found by other authors and are also questionable.
All dorsal and anal fin ray counts of the two taxa overlap. According to Raitt [23], the most distinctive fins for the separation of taxa are the D3 and A1. The fin ray counts in Mercader and Vinyoles [1] are ambiguous. The interspecific differences of the D3 and A1 were found to be greater by Raitt than noted by the latter study, although these characteristics alone are not sufficient to distinguish the two taxa. These counts are informative only in combination with other characters.
With a few exceptions, individuals of Gadiculus can be distinguished to species by the number of vertebrae [23] as observations of specimens showing an overlap concerning this character are rare. The majority of G. argenteus specimens show 38–40 vertebrae and G. thori shows 41–43 vertebrae. Accordingly, the G. thori specimen caught off Greenland shows 42 vertebrae. However, it is currently believed that many species with an extensive north-south distribution often have a gradually decreasing number of vertebrae towards southern warmer waters. Hodges [49] mentions that T.N. Gill in 1863 found that the number of vertebrae in fishes is temperature dependent. This is in agreement with Wheeler and Jones [50] who found that the number of vertebrae is affected by the egg developing temperature, corresponding to Jordan’s ecogeographic rule [51]. On that note, it would be interesting to clarify the number of vertebrae for the northernmost G. argenteus specimens; however, this is beyond the scope of this study. Mercader and Vinyoles [1] did not include the number of vertebrae in their study—unfortunate as this meristic character is the best discriminator between adults of the two taxa. Raitt [23] had only two G. argenteus specimens from southern Portugal at his disposal both showing 41 vertebrae. This indicates that the number of vertebrae in G. argenteus is in fact increasing in colder waters although more data are necessary for this to be verified.

4.3. Post-Larval Pigmentation

The different pigmentation observed in the post-larvae of Gadiculus (Figure 1), being the most important reason for Schmidt [17] to erect G. thori as a new species, is an important, although often overlooked feature for the distinction of Gadiculus species.
Four stages of G. thori larvae and post-larvae (in the range 3.8–17.3 mm SL) were presented by Halbeisen and Schöfer [52], and another series by Izeta [53] that showed eight stages of larvae and post-larvae of G. argenteus from the southern Bay of Biscay (2.9–19.0 mm SL). Both studies presented the development of melanophore patterns similar to type material descriptions, thus confirming the results by Schmidt [17]. It is of note that Bay of Biscay materials showed G. argenteus present, considering this region to be near the putative boundary of the separation of the two Gadiculus species (Figure 3). However, we were unable to obtain new materials from this region, G. thori has never been found so far south and the two species have never been found together. The importance of post-larvae melanophore patterns has been demonstrated repeatedly [54,55], and is unambiguous for the separation of Gadiculus taxa. The fact that larger individuals of the two Gadiculus species are so similar does not alter the importance of post-larval pigmentation. Therefore, Mercader and Vinyoles [1] not fully acknowledging its importance is untenable.

4.4. Otolith Morphology

Otoliths are a powerful tool for distinguishing closely related species, reconstructing phylogenies, and estimating geological events [37,40]. Gadiculus otoliths show several plesiomorphic characters within the Gadidae. The presence or absence as well as the size and shape of the pseudocolliculum, a feature described by Schwarzhans [20], and the collum are very important for the identification of species and genera within the Gadidae, including phylogenetic considerations. Gadiculus and Micromesistius are the only extant gadid genera possessing a pseudocolliculum on the medial surface of their sagittae. This symplesiomorphic character supports the close relationship of Gadiculus and Micromesistius identified from molecular data (Figure 8).
Many small-sized cod species possessed a well-developed pseudocolliculum and collum in the Oligocene and Miocene (Gaemers [56,57], Schwarzhans [58,59]), but the number of species decreased in the Pliocene, reaching an all-time low number of species in the Gadidae in the present day. The gadiculine fishes were very diverse and abundant during the Oligocene and Miocene. The medially situated pseudocolliculum is apparently an ancestral character that fishes in the family Gadidae have in common with many species of the gadiform rattails, family Macrouridae. Both Gadiculus and Micromesistius otoliths have deep roots in the past: based on fossil otoliths their common ancestor appears to have lived at the beginning of the Early Oligocene, i.e., about 34 million years ago (Gaemers, in preparation). Considering the distant geological past of this common ancestor, it is, therefore, unsurprising that Gadiculus and Micromesistius have their own peculiar advanced characteristics.
The oldest common ancestor of species currently attributed to the genus Trisopterus must have lived even earlier, in the Late Eocene. It is interesting that the oldest trisopterine lineage known from the geological record also possesses a pseudocolliculum just like Gadiculus and Micromesistius [40]. This further confirmed the plesiomorphic character of collum and pseudocolliculum in the gadids.
Otolith and adult fish morphology, habitat and food preferences all show that Trisopterus esmarkii (Nilsson, 1855) is clearly different from the other Trisopterus species [40]. The genus Neocolliolus was erected for this taxon by Gaemers [36] based on its different otolith morphology. Unfortunately, this reclassification still remains to be implemented in subsequent works on Trisopterus. Establishment of the genus name Neocolliolus for T. esmarkii is supported by molecular data showing a relatively large molecular distance to its closest relative Trisopterus minutus (Linnaeus, 1758) from Cox1 barcodes (Figure 8). Based on otoliths, meristic and some external characteristics, Trisopterus minutus is more closely related to Neocolliolus esmarkii than to Trisopterus luscus (Linnaeus, 1758) and T. capelanus (Lacepède, 1800). A new genus, Allotrisopterus, is therefore introduced by Gaemers [40] for Trisopterus minutus and is confirmed by the molecular data presented in this study (Figure 8).
Otoliths of adult Gadiculus specimens identify the two species well, because they are nearly always very distinctive, at least since the Early Pleistocene. The adult otoliths of G. argenteus are essentially regularly drop-shaped, whereas those of G. thori show a distinctly truncated posterior end and/or an irregularly shaped and variable dorsal margin. In the Mediterranean, the two species already co-existed in the Late Pliocene [38] and their otoliths apparently could be distinguished in deposits formed during that period. Unfortunately, illustrations of Late Pliocene G. thori otoliths from the Mediterranean or elsewhere have not been published yet. In the light of general otolith morphology, it is remarkable that adult G. argenteus shows otoliths that are fairly uniform in shape whereas those of G. thori show high variability.

4.5. Molecular Data

Two clearly separate groups based on Cox1 barcodes within Gadiculus unambiguously support two species whether considering the molecular gap or the low intraspecific variation observed within both species (Figure 8). The molecular data are therefore in agreement with vertebral counts [23], pigmentation of post-larvae [17], geographic distribution (Figure 3) and with the differences present in the adult otoliths (Figure 3, Figure 5 and Figure 6). It is of note that the Cox1 barcodes are showing virtually no intraspecific variation within the Mediterranean/Lusitanian Atlantic and the Boreal Atlantic populations. It is also of note that the edge length of the Gadiculus lineage is the longest observed within the Gadidae compared to all other genera (Figure 8). The many substitutions delimiting Gadiculus taxa are in agreement with a relatively long history of gadiculine fishes in the Gadidae, and are therefore in accordance with the relatively complicated evolutionary history observed in the fossil otolith records.
The Cox1 barcodes show a closer relationship between Gadiculus and Micromesistius than with other Gadidae (Figure 8). This is in agreement with the results found by Teletchea et al. [60] that used two mitochondrial genes. This relationship corresponds with the otoliths of both genera that are separated from all other extant Gadidae taxa by having a pseudocolliculum.

4.6. Present Geographic Distributions

The distributional patterns of the two species are noteworthy as there is strong empirical support for a cold- and a warm-water Northeast Atlantic separation from both morphological and molecular data. This is particularly evident as Mediterranean G. argenteus specimens, without exception, cluster with specimens caught off Western Portugal and more south in the East Atlantic Ocean (Figure 3, Figure 4, and Figure 8). Therefore, separation of G. argenteus and G. thori is observed at approximately the 45° N latitude, although details remain obscure in this region as additional sampling is needed for a more accurate line of separation (Figure 3). However, separation is roughly corresponding to the boundaries of the subtropical gyre currents in the North Atlantic, originating in the warm western tropical Atlantic as the Gulf Stream and diverging to the East into the North going North Atlantic Drift and the South going Canary Current [61,62]. This divergence likely acts as a species barrier for Gadiculus spp. from either distinct temperatures in the opposite going currents or the hindrance of mixing in current swept eggs.
It is of note that G. argenteus is not tropical as it has never been reported in tropical East Atlantic waters, being confined to the warm-temperate latitudes between 20° N and 45° N. This is in agreement with other gadids that show anti-tropical distributions [9] although two distinct closely related species as observed for Gadiculus are not usually the case [63]. It is, therefore, appropriate to deem G. thori the cold-water adapted form confined to the cold-temperate latitudes between 72° N and 45° N, and G. argenteus the warm-water adapted form distributed south to off northwestern Africa. The distributions of both Gadiculus species show similarities to the distribution of Micromesistius poutassou (Risso 1827), the most closely related genus and also found throughout the eastern North Atlantic and in the Mediterranean. Similar to G. argenteus, M. poutassou appears restricted towards the south by the Canary current, an extensive system that is responsible for extensive upwelling off Northwest Africa [64]. Distributional patterns of G. thori appear similar to those found in Neocolliolus esmarkii [40].
Gadiculus thori and G. argenteus are associated with landmasses, as they have been exclusively reported from upper slope and lower shelf habitats [9]. However, we here present one specimen caught in 2012 off southeast Greenland, providing a range extension into the western part of the subarctic Atlantic [65]. The finding of G. thori off southeast Greenland means that the species was either transported by the North Atlantic Drift originating in warmer waters, very unlikely as this route is oceanic originating in the western tropical Atlantic, or distributed via the eastern Atlantic route across Iceland to Norway. The latter explanation is plausible, as Gadiculus has been reported in Icelandic waters for some time [66]. However, the route via Iceland means crossing a deep-sea barrier in the northeastern Atlantic, which is not impossible due to the pelagic mode of life of Gadiculus. Theoretically, it could also belong to a separate G. thori population of Iceland and/or Greenland. However, molecular results presented in this study show no indications of separate populations in G. thori (Figure 8). It is difficult, however, to assess whether a true distribution is present in Greenland waters as G. thori has not been observed subsequently to 2012 despite extensive yearly surveys conducted in the region by the Greenland Institute of Natural Resources (J.Y. Poulsen, personal observation). The Greenland-Iceland submarine ridge could potentially facilitate distribution of shelf-associated species such as G. thori [24]. Distributional changes for pelagic fishes are a complicated matter in subarctic Atlantic waters, as yearly seawater temperature changes are heterogeneously occurring at various depths [67]. In addition, the distinction of coastal versus oceanic habitat is difficult at times, including also a general problematic sampling and taxonomic effort in this region [68,69]. Byrkjedal and Høines [8] found Gadiculus in the southwestern part of the Barents Sea. Additionally, the recent discovery of this species in the Russian part of the Barents Sea [70] is a further extension of its area that possibly could be explained by changed temperatures in these Arctic waters.
The case of Gadiculus presents a good example of how important taxonomy is for the distribution and monitoring of taxa in relation to ocean temperature and climate changes. If organisms that show a latitudinal cline in their distributions are to be used as biological monitoring markers, for example in relation to temperature affinities, species distributions and temperature tolerance are baseline knowledge. Gadiculus has been treated as a single temperature-changing tolerant entity ranging from approximately 20° N to 70° N in the eastern North Atlantic Ocean until this study. The fact that two species were not recognized lately, with all evidence showing different temperature adaptations to have evolved in this genus, speaks volumes concerning the standard use of distributional changes without proper taxonomic assessment. Using distribution of taxa as a tool in climate change research requires a thorough investigation of the individual marker. This evidently begins with proper taxonomic considerations. With this in mind, the finding of G. thori off southeast Greenland close to the Arctic Circle is unsurprising.

4.7. Former Geographic Distributions

The present geographic distribution of G. thori and G. argenteus coupled with catch rates, already gives a clue for the existence of two species as aforementioned. An even stronger argument can be found in the geological past.
Fossil otoliths have proven that both species co-existed in the Mediterranean during parts of the Late Pliocene (younger part of the Gelasian) and Early to Middle Pleistocene (Calabrian to Ionian) [38]. The Pleistocene otoliths of both taxa in these periods are easily distinguishable and very similar to the recent ones (Figure 5 and Figure 7). Unfortunately, illustrations of Mediterranean G. thori otoliths from Late Pliocene specimens have not yet been published, but can be distinguished from Late Pliocene G. argenteus otoliths according to Girone et al. [38]. This shows that the two taxa are distinct and separated at least from c. three million years ago. Subspecies cannot co-exist in the same area without interbreeding. The fossil otoliths show that interbreeding apparently did not occur and corroborate the presence of two taxa present then and now. Hypothetically, if fossil otoliths from G. argenteus and G. thori were to be subspecies only and not species, overlapping variation would have been observed, resulting in the absence of two clearly different shape types of otoliths showing the same characteristics as observed in the two extant Gadiculus species. The size difference between the two Gadiculus species indicates that resource partitioning must have occurred in these fishes, resulting in less competition between co-existing fishes. There might also have been some depth segregation with G. thori on an average living in deeper somewhat colder water than G. argenteus.
The past co-occurrence of G. argenteus and G. thori in the Mediterranean Sea might be correlated with a much more southern course of the North Atlantic Drift and/or a weaker presence of this drift during several ice ages when arctic and subarctic environments covered a much larger area on the northern hemisphere. This would have pushed the geographical distribution of G. thori, as well as that of many other species, further southwards. The present distribution separating the two species, strongly suggests that they are vicariant species, but the fossil otoliths show that they were co-existing in the same regions during longer geological periods. This observation is important and proves that distribution is a complicated matter on multiple levels, as the fossil record and temperature tolerance are not usually known for most fishes. Otoliths indicate that sister species co-existing in the same area for long time periods is a normal phenomenon in gadids. When the earliest fossil otoliths of such closely related species co-occur in the same sediment samples and represent the beginning of lineages, they provide strong evidence for sympatric speciation. A good example of this is found for T. luscus and T. capelanus that have their origins in the Early Miocene North Sea region, and have co-existed in this region until the Late Pliocene [40]. The oldest representatives of the G. argenteus lineage are known from the late Early Miocene, although otoliths of the G. thori lineage are not yet known from before the Late Pliocene (Gaemers, in preparation). Additional data are therefore necessary to resolve whether the two extant Gadiculus species have a sympatric origin or not.
It is finally important to mention that Schwarzhans [71] described Gadiculus (Gadiculus) antipodus from the Early Miocene of New Zealand. However, this species is not a member of one of the two Gadiculus lineages leading to the two recent species, because its collum and pseudocolliculum are shorter than in Gadiculus, in addition to very small colliculi. Therefore, it is more related to the extinct genus Circagadiculus, known from the Late Oligocene to Early Miocene of the North Sea Basin [56], and should be included within that genus. The maximum size of the otoliths found by Schwarzhans also agrees with that of Circagadiculus otoliths that never attain the size of Gadiculus otoliths. This means that Gadiculus is still unknown from the southern hemisphere, but it is interesting to note that earlier gadiculines managed to cross the equator extending as far as New Zealand. Species of the genus Circagadiculus were adapted to much warmer waters than Gadiculus, because the Late Oligocene and Early Miocene North Sea was considerably warmer than at present. Temperature differences within tropical seas in that period were likely less pronounced than today. Considering the distribution in New Zealand waters, Circagadiculus antipodus probably occupied a pelagic habitat.

4.8. Classification of Codfishes in this Study

The ranking and classification of codfishes is generally not agreed upon when considering different types of evidence. This is especially evident concerning the subfamily and family levels (e.g., Endo [72]; Roa-Varón and Ortí, [73]). Concerning family levels, we have noted the gadids, lotids, gaidropsarids and phycids as distinct families due to their distinct otoliths (see Gaemers [37]), corroborated by distinctive external characteristics of the four groups. Following Howes [74] and Fahay [55], we define the Gadidae family as consisting of only the species with three dorsal fins. We note that the burbots of the family Lotidae are found non-monophyletic in this study (Figure 8), similar to results presented by Roa-Varón and Ortí [73] who used a larger dataset and more molecular markers. We have classified the two lotid clades as subfamilies: Lotinae and Molvinae. However, general morphology of Lota lota otoliths is not particularly different from those of the other lotid genera Molva and Brosme. Therefore, the observed non-monophyly of the Lotidae is unresolved at present (Figure 8). Few molecular studies of gadiform fishes are available, which is surprising considering their large-scale importance economically, and investigations of additional molecular information might reveal new results related to longer DNA sequences. However, it is encouraging that molecular and otolith data of gadiform fishes correspond well in most instances.
The gaidropsarid genus Onogadus is used in this study (Figure 8) according to evidence that supports the replacement of Gaidropsarus for Onogadus, i.e., for Onogadus argentatus (Reinhardt, 1837) and Onogadus ensis (Reinhardt, 1837) by De Buen [75]. The recognition of Onogadus is supported by several morphological characters that were described by Howes [74,76,77]. This is in accordance with otoliths that similarly support Onogadus as a distinct genus (see [78] under Platyonos).

5. Conclusions

Evidence from post-larval pigmentation, otolith morphology, number of vertebrae, molecular data, maximum size and present/former geographic distribution all support two extant species of Gadiculus. We conclude that Schmidt [17] was right in recognizing two Gadiculus species. Several morphometric and meristic measurements may introduce some ambiguity, although the number of vertebrae will usually allow for the correct identification of specimens. Table 3 shows an overview of all differences between the two Gadiculus species discussed in the present study. The questionable meristic data concerning number of vertebrae and fin rays noted by Mercader and Vinyoles [1] are excluded in Table 3.
The two Gadiculus species are not vicariant species, as their present geographical distributions would suggest. Fossil otoliths indicate that they are sister species and have co-existed during long geological periods. Therefore, it is likely that sympatric speciation occurred and Gadiculus thereby originating from a single common ancestor. Gaemers [40] also observed this pattern in other Gadidae lineages witnessed from the fossil otolith record.
For G. thori and G. argenteus we suggest northern and southern silvery pout as vernacular names, respectively.

Acknowledgments

We would like to thank T. Bakke (Norwegian Institute for Water Research, Oslo, Norway; formerly Marin Biologisk Stasjon Espegrend, Blomsterdalen, Norway) for G. thori specimens, and G. van der Velde (Biology Department, Radboud University, Nijmegen, The Netherlands) for G. thori otoliths, C. Allué (Instituto de Ciencias del Mar, CSIC, Barcelona, Spain), D. Nolf (formerly at the Royal Belgian Institute of Natural Sciences, Brussels, Belgium) and C.A. Assis (Department of Animal Biology, University of Lisboa, Portugal) for G. argenteus otoliths, N. Daan (formerly at the Rijksinstituut voor Visserijonderzoek, R.I.V.O., IJmuiden [now IMARES], the Netherlands) for fish size information, L. Lindblom & S. Thorkildsen (University of Bergen, Norway), M. Miya & T. Sado (Chiba Natural History Museum, Japan) for help with molecular lab-work, K. & E. Hjørne (www.naturporten.dk, private) for help with illustrations, T. Menne (Zoological Museum University of Copenhagen, Denmark) for cataloging, R/V Pâmiut crew, J. Nielsen (Greenland Institute of Natural Resources, Greenland) and B.H. Sunnset (MAREANO, Institute of Marine Research, Norway) for loan of photos and three anonymous reviewers for valuable comments. We owe a special thanks to I. Byrkjedal & G. Langhelle (Natural History Collections Bergen, Norway) for curating, providing materials and logistics whenever needed and to I. Chemshirova (Zoological Society London, UK) for correcting the manuscript.

Author Contributions

Otolith examinations and review of previous works was conducted by P.G.; New records and molecular works were conducted by J.P. Both author wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

TLTotal fish length in mm
TLmaxLargest known total fish length in mm
SLStandard length in mm
OLOtolith length in mm
Coll.Collector
IDPerson who originally identified
Cox1Cytochrome Oxidase 1
K2PKimura-2-parameter model
T-P spacerIntergenic non-coding region in all gadiforms between the tRNAs Threonine and Proline
ZMUBRegistration number of the Museum of Zoology, University of Bergen, Norway

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Figure 1. Illustrations of Gadiculus post-larvae at the same stage of development by Schmidt [17]. (A). Gadiculus argenteus; (B). Gadiculus thori. The post-larvae were originally used to distinguish two different species although largely neglected in subsequent works until this study. Scale bars: 1 mm.
Figure 1. Illustrations of Gadiculus post-larvae at the same stage of development by Schmidt [17]. (A). Gadiculus argenteus; (B). Gadiculus thori. The post-larvae were originally used to distinguish two different species although largely neglected in subsequent works until this study. Scale bars: 1 mm.
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Figure 2. Gadiculus thori. (A). ZMUB 14683 (registration number of Museum of Zoology, University of Bergen, Norway). Newly caught off Southeast Greenland in 2012, providing a range expansion into the northwestern Atlantic (Photo: Greenland Institute of Natural Resources, Greenland). Scale bar: 10 mm; (B). X-ray of ZMUB 14683. Meristics and morphometrics presented in Table 2; (C). Gadiculus thori newly illustrated for this study; (D). Live specimen G. thori (67°48′ N, 10°54′ E) at 227 m depth filmed during the MAREANO expedition off Arctic Norway in 2011.
Figure 2. Gadiculus thori. (A). ZMUB 14683 (registration number of Museum of Zoology, University of Bergen, Norway). Newly caught off Southeast Greenland in 2012, providing a range expansion into the northwestern Atlantic (Photo: Greenland Institute of Natural Resources, Greenland). Scale bar: 10 mm; (B). X-ray of ZMUB 14683. Meristics and morphometrics presented in Table 2; (C). Gadiculus thori newly illustrated for this study; (D). Live specimen G. thori (67°48′ N, 10°54′ E) at 227 m depth filmed during the MAREANO expedition off Arctic Norway in 2011.
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Figure 3. Catch localities of Gadiculus spp. used for molecular comparisons in this study (Table 2). Squares depict G. thori and circles depict G. argenteus with shading corresponding to specimens presented in Figure 8. The dashed line at about 45° N is denoting the approximate boundary separating G. thori and G. argenteus in the Northeast Atlantic Ocean.
Figure 3. Catch localities of Gadiculus spp. used for molecular comparisons in this study (Table 2). Squares depict G. thori and circles depict G. argenteus with shading corresponding to specimens presented in Figure 8. The dashed line at about 45° N is denoting the approximate boundary separating G. thori and G. argenteus in the Northeast Atlantic Ocean.
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Figure 4. Typical otolith of adult Gadiculus thori (Total length (TL) 144 mm, standard length (SL) 135 mm), Kvinnheradfjord, Hardanger, Norway, showing the medially positioned sulcus acusticus and associated structures. Colliculi dark grey; pseudocolliculum medium grey; very low and indistinct colliculi-like areas light grey. Scale bar: 1 mm.
Figure 4. Typical otolith of adult Gadiculus thori (Total length (TL) 144 mm, standard length (SL) 135 mm), Kvinnheradfjord, Hardanger, Norway, showing the medially positioned sulcus acusticus and associated structures. Colliculi dark grey; pseudocolliculum medium grey; very low and indistinct colliculi-like areas light grey. Scale bar: 1 mm.
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Figure 5. Typical otoliths of Gadiculus argenteus showing moderate morphological variabilities. (AG): Recent otoliths obtained off the Portuguese coast. (A) TL 143, Otolith length (OL) 7.56; (B) TL 139, OL 7.24; (C) TL 145, OL 7.24; (D) SL 104, OL 6.43 (mirror image); (E) TL 133, OL 7.70; (F) TL 133, OL 7.46 (mirror image); (G) SL 94, OL 6.19. The otoliths in E and F are from the same specimen showing asymmetry. (H,I): Fossil otoliths redrawn from Girone et al. [38], Montalbano Jonico section, Basilicata, Italy, early to mid Pleistocene. Scale bar: 1 mm.
Figure 5. Typical otoliths of Gadiculus argenteus showing moderate morphological variabilities. (AG): Recent otoliths obtained off the Portuguese coast. (A) TL 143, Otolith length (OL) 7.56; (B) TL 139, OL 7.24; (C) TL 145, OL 7.24; (D) SL 104, OL 6.43 (mirror image); (E) TL 133, OL 7.70; (F) TL 133, OL 7.46 (mirror image); (G) SL 94, OL 6.19. The otoliths in E and F are from the same specimen showing asymmetry. (H,I): Fossil otoliths redrawn from Girone et al. [38], Montalbano Jonico section, Basilicata, Italy, early to mid Pleistocene. Scale bar: 1 mm.
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Figure 6. Otoliths of a single Gadiculus thori specimen (TL 143, SL 133) showing strong asymmetries between the right (A) and left (B) side otoliths (Kvinnheradfjord, Hardanger, Norway at approximately 170 m depth). Scale bar: 1 mm.
Figure 6. Otoliths of a single Gadiculus thori specimen (TL 143, SL 133) showing strong asymmetries between the right (A) and left (B) side otoliths (Kvinnheradfjord, Hardanger, Norway at approximately 170 m depth). Scale bar: 1 mm.
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Figure 7. Otoliths of Gadiculus thori showing large morphological variability. (AJ): Recent otoliths from the Kvinnheradfjord, Hardanger, Norway. (A) TL 151, OL 7.12; (B) TL 158, OL 8.37; (C) TL 147, OL 6.88; (D) TL 151, OL 7.50; (E) TL 108, OL 6.07; (F) TL 146, OL 7.40; (G) TL 138, OL 6.65; (H) TL 164, OL 6.90; (I) TL 147, OL 6.42; (J) TL 104, OL 5.62. (K,L): Fossil otoliths redrawn from Girone et al. [38] (K) Furnari section, Sicilia, Italy, early Pleistocene; (L) Vallone Catrica section, Calabria, Italy, mid Pleistocene. Scale bar: 1 mm.
Figure 7. Otoliths of Gadiculus thori showing large morphological variability. (AJ): Recent otoliths from the Kvinnheradfjord, Hardanger, Norway. (A) TL 151, OL 7.12; (B) TL 158, OL 8.37; (C) TL 147, OL 6.88; (D) TL 151, OL 7.50; (E) TL 108, OL 6.07; (F) TL 146, OL 7.40; (G) TL 138, OL 6.65; (H) TL 164, OL 6.90; (I) TL 147, OL 6.42; (J) TL 104, OL 5.62. (K,L): Fossil otoliths redrawn from Girone et al. [38] (K) Furnari section, Sicilia, Italy, early Pleistocene; (L) Vallone Catrica section, Calabria, Italy, mid Pleistocene. Scale bar: 1 mm.
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Figure 8. Distance-based K2P neighbour-joining cladogram of the family Gadidae, including taxa of Lotidae, Gaidropsaridae and Phycidae employed as outgroups. We note that the cladogram is intended for molecular taxonomy and not an attempt at resolving Gadidae phylogeny, hence the omission of support values. All currently valid Gadidae taxa are included, except Eleginus nawaga (Pallas, 1814) (no Cox1 available). Catch locations are noted in parentheses. Numbers listed of Gadiculus spp. correspond with Figure 3 and Table 2. Double bars (\\) indicate a three-fold shortening of the branches for practical purposes only. Asterisk (*) denotes classificatory names used in this Figure that are supported by otoliths, although currently not employed in morphological and molecular taxonomy (see main text sections). It is of note that the longest branch present in the family Gadidae is representing the Gadiculus lineage, having two extant taxa and several extinct taxa. Post-larvae (Schmidt [17]) and general adult otolith morphology (this study) are included for both Gadiculus taxa. Gadiculus thori otolith: SL 150, OL 7.85; G. argenteus otolith: SL 123, OL: 7.56.
Figure 8. Distance-based K2P neighbour-joining cladogram of the family Gadidae, including taxa of Lotidae, Gaidropsaridae and Phycidae employed as outgroups. We note that the cladogram is intended for molecular taxonomy and not an attempt at resolving Gadidae phylogeny, hence the omission of support values. All currently valid Gadidae taxa are included, except Eleginus nawaga (Pallas, 1814) (no Cox1 available). Catch locations are noted in parentheses. Numbers listed of Gadiculus spp. correspond with Figure 3 and Table 2. Double bars (\\) indicate a three-fold shortening of the branches for practical purposes only. Asterisk (*) denotes classificatory names used in this Figure that are supported by otoliths, although currently not employed in morphological and molecular taxonomy (see main text sections). It is of note that the longest branch present in the family Gadidae is representing the Gadiculus lineage, having two extant taxa and several extinct taxa. Post-larvae (Schmidt [17]) and general adult otolith morphology (this study) are included for both Gadiculus taxa. Gadiculus thori otolith: SL 150, OL 7.85; G. argenteus otolith: SL 123, OL: 7.56.
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Table 1. Meristics and morphometrics of Gadiculus thori (ZMUB 14683) caught off Southeast Greenland in 2012, providing a new range extension into the Western North Atlantic (Figure 2). The specimen was compared to G. thori “northern” specimens presented by Raitt [23] from off the west coast of Scotland (ranges shown in parentheses).
Table 1. Meristics and morphometrics of Gadiculus thori (ZMUB 14683) caught off Southeast Greenland in 2012, providing a new range extension into the Western North Atlantic (Figure 2). The specimen was compared to G. thori “northern” specimens presented by Raitt [23] from off the west coast of Scotland (ranges shown in parentheses).
CharacteristicsGadiculus thori
TL126.2
SL115.9 (65.0–135.0)
% SL
Head length30.6 (26.8–35.7)
Pre-dorsal dist.31.7 (28.0–40.0)
Pre-anal dist.44.6
Pre-pelvic dist.25.5
Pre-orbital dist.9.2
Orbit11.2 (8.8–12.6)
Inter-orbital dist.7.5
1. Dorsal base11.2
1st–2nd Dorsal dist.3.3 (0.9–3.7)
2. Dorsal base12.1
2nd–3rd Dorsal dist.5.4 (2.2–6.1)
3. Dorsal base12.3
1. Anal base14.4
1st–2nd Anal dist.5.0 (2–6.9)
2. Anal base15.1
Body depth16.1 (13.8–25.8)
Caudal depth5.5
Premaxillary length12.0
1. Dorsal fin rays10 (9–13)
2. Dorsal fin rays11 (10–16)
3. Dorsal fin rays17 (15–17)
1. Anal fin rays17 (15–18)
2. Anal fin rays17 (16–17)
Vertebrae42 (39–43)
Total length (TL) and Standard length (SL) are in mm.
Table 2. 96 Specimens used for molecular comparisons of the Cox1 barcode in this study. Numbers for Gadiculus spp. correspond to Figure 8. The dataset is available as (DS-PGJP) from BOLD.
Table 2. 96 Specimens used for molecular comparisons of the Cox1 barcode in this study. Numbers for Gadiculus spp. correspond to Figure 8. The dataset is available as (DS-PGJP) from BOLD.
SpecimensRecord ID BOLDNCBIMuseumRegion, Country and Year of SamplingPositionStudy
Lotidae
Brosme brosmeGLF058LC146711ZMUB 21890SE Greenland 201364.25° N, 36.51° EThis study
Brosme brosmeSCFAC287-06KC015253ARC 25650SE Canada 200641.93° N, 65.81° E[29]
Lota lotaANGBF9234GU126680-Idaho, USA 2009-Unpubl.
Lota lotaIFCZE0693HQ961085-Ohre, Czech Republic 201050.11° N, 12.40° EUnpubl.
Molva dipterygiaGLF056LC146709ZMUB 21948SE Greenland 201364.18° N, 36.50° EThis study
Molva dipterygiaSCFAC413KC015694ARC 25589Unknown, Canada-[29]
Molva molvaGLF071LC146695No voucherSE Greenland 201366.50° N, 30.28° EThis study
Molva molvaGLF176LC146701ZMUB 22720SE Greenland 201464.27° N, 37.20° EThis study
Gaidropsaridae
Ciliata mustelaBNSFI129KJ204805MT05378NW Germany 201054.14° N, 07.90° E[30]
Ciliata mustelaBNSFI128KJ204804MT05377NW Germany 201054.14° N, 07.90° E[30]
Enchelyopus cimbriusSCAFB093KC015336ARC 24883SE Canada 200544.94° N, 66.09° E[29]
Enchelyopus cimbriusBNSFI132KJ204840MT05365NW Germany 201054.14° N, 07.90° E[30]
Gaidropsarus mediterraneusFCFPS166JQ774626MB85-005350S Portugal-[31]
Gaidropsarus mediterraneusGBGCA10850KP136735J1Bsex-80Turkey-Unpubl.
Gaidropsarus vulgarisSFM036-AF0036NW Spain 2013-Unpubl.
Gaidropsarus vulgarisGBGCA8490KJ128491NRM46985SW Sweden 200157.88° N, 11.58° EUnpubl.
Onogadus argentatusGLF114LC146708ZMUB 21814SE Greenland 201361.57° N, 40.58° EThis study
Onogadus argentatusSCAFB229KC015387ARC 26385E Canada 200669.83° N, 65.28° E[29]
Onogadus ensisSCAFB1182KC015394ARC 28289SE Canada 200744.02° N, 59.01° E[29]
Onogadus ensisGLF117LC146696ZMUC P376048W Greenland 201363.31° N, 56.31° EThis study
Phycidae
Phycis blennoidesGLF151LC146700ZMUB 22773SE Greenland 201461.42° N, 41.04° EThis study
Phycis blennoidesBIM338-P. 15193W Israel 201332.27° N, 34.36° EUnpubl.
Phycis chesteriGLF017LC146703ZMUC P375728SE Greenland 200962.12° N, 40.29° EThis study
Phycis chesteriSCFAC747KC015799ARC 25896SE Canada 200242.80° N, 63.19° E[29]
Urophycis chussSCFAC720KC016017ARC 25893SE Canada 200643.03° N, 61.61° E[29]
Urophycis chussSCFAC714KC016018ARC 25697SE Canada 200641.39° N, 66.12° E[29]
Urophycis tenuisSCFAC522KC016033ARC 25942SE Canada48.55° N, 63.07° E[29]
Urophycis tenuisSCFACB855KC016030ARC 26827SE Canada 200744.36° N, 66.50° E[29]
Gadidae
Eleginus gracilisWXYZ007-UW150495Alaska, USA 201060.99° N, 167.34° EUnpubl.
Eleginus gracilisWXYZ005-UW150494Alaska, USA 201060.99° N, 167.34° EUnpubl.
1. Gadiculus argenteusCSFOM036KJ709531CSFOM-044Sicily, Italy-[32]
2. Gadiculus argenteusFCFPS164JQ774620MB85-005348S Portugal-[31]
3. Gadiculus argenteusFCFPS133JQ774622MB85-005315S Portugal-[31]
4. Gadiculus argenteusFCFPS130JQ774618MB85-005317S Portugal-[31]
5. Gadiculus argenteusFCFPS154JQ774619MB85-005338S Portugal-[31]
6. Gadiculus argenteusFCFPW097JQ775028MB85-010501W Portugal 200540.28° N, 09.59° W[31]
7. Gadiculus argenteusFCFPW079JQ775027MB85-010519W Portugal 200540.18° N, 09.59° W[31]
8. Gadiculus argenteusFCFPW078JQ775024FCFOPB064-03W Portugal 200540.18° N, 09.59° W[31]
9. Gadiculus argenteusFCFPW076JQ775025MB85-010520W Portugal 200540.18° N, 09.59° W[31]
10. Gadiculus argenteusFCFPW077JQ775026MB85-010496W Portugal 200540.18° N, 09.59° W[31]
11. Gadiculus argenteusFCFP065JQ774831MB85-004995W Portugal 200539.08° N, 10.00° W[31]
12. Gadiculus argenteusFCFP067JQ774828MB85-004994W Portugal 200539.08° N, 10.00° W[31]
13. Gadiculus argenteusFCFP066JQ774829MB85-004998W Portugal 200539.08° N, 10.00° W[31]
14. Gadiculus argenteusFCFP069JQ774830MB85-004996W Portugal 200539.08° N, 10.00° W[31]
15. Gadiculus argenteusFCFP068JQ774832MB85-004997W Portugal 200539.08° N, 10.00° W[31]
16. Gadiculus argenteusFCFPS065JQ774623MB85-005249S Portugal-[31]
17. Gadiculus argenteusFCFPS132JQ774624MB85-005314S Portugal-[31]
18. Gadiculus argenteusFCFPS131JQ774621MB85-005318S Portugal-[31]
19. Gadiculus argenteusFCFPS134JQ774625MB85-005316S Portugal-[31]
20. Gadiculus argenteusCSFOM091KJ709532CSFOM-117Sicily, Italy-[32]
1. Gadiculus thoriGLF136LC146704ZMUB 21452SE Greenland 201264.19° N, 36.45° WThis study
2. Gadiculus thoriNAF001LC146706ZMUB 21333SW Norway 201262.04° N, 05.02° EThis study
3. Gadiculus thoriBNSFI055KJ204873MT04119N United Kingdom 201259.71° N, 00.56° W[30]
4. Gadiculus thoriBNSFI030KJ204872MT04118SW Norway 201258.22° N, 04.38° E[30]
5. Gadiculus thoriBNSFI056KJ204867MT04120N United Kingdom 201259.71° N, 00.56° W[30]
6. Gadiculus thoriBNSFI029KJ204865MT04117SW Norway 201258.22° N, 04.38° E[30]
7. Gadiculus thoriBNSFI028KJ204864MT04116SW Norway 201258.22° N, 04.38° E[30]
8. Gadiculus thoriBNSF269KJ204869MT02313SW Norway 201259.14° N, 03.13° E[30]
9. Gadiculus thoriGBGCA6718KJ128488NRM476SE Norway 200058.07° N, 10.02° EUnpubl.
10. Gadiculus thoriBNSFI057KJ204871MT04121N United Kingdom 201259.71° N, 00.56° W[30]
Arctogadus glacialisGLF145LC146697ZMUB 22974W Greenland 201468.36° N, 55.10° WThis study
Arctogadus glacialisDSFNG010-ZMUB 21027NE Greenland 201072.00° N, 21.02° WUnpubl.
Boreogadus saidaGLF148LC146698ZMUB 22936W Greenland 201469.31° N, 51.53° WThis study
Boreogadus saidaGLF105LC146694ZMUB 21932SE Greenland 201365.38° N, 30.19° WThis study
Gadus ogacGLF065LC146707ZMUB 21811SW Greenland 201360.43° N, 46.02° WThis study
Gadus ogacSCAFB565KC015369ARC 26244SE Canada 200650.05° N, 57.88° W[29]
Gadus macrocephalusFMV221JQ354100UW110223NE Pacific 2004-Unpubl.
Gadus macrocephalusUKFBI444KF929903KU 28473NE Pacific 199955.16° N, 133.99° WUnpubl.
Gadus morhuaGLF052LC146693No voucherSE Greenland 201365.28° N, 33.45° WThis study
Gadus morhuaNOFIS088-NHMO-f-541S Norway 200958.11° N, 08.13° EUnpubl.
Gadus chalcogrammusFMV536JQ354517UW150214NE Pacific 200833.87° N, 118.43° WUnpubl.
Gadus chalcogrammusABFJ129JF952737-NE Japan 2005-[33]
Merlangius merlangusANGBF9794FN689176-Iceland 2003-[34]
Merlangius merlangusANGBF9862FN689040-Black Sea, Turkey 2003-[30]
Melanogrammus aeglefinusGLF171LC146702ZMUB 22913SE Greenland 201466.35° N, 29.15° WThis study
Melanogrammus aeglefinusGLF057LC146710ZMUB 21891SE Greenland 201464.25° N, 36.51° WThis study
Microgadus proximusFMV009JQ354228UW047300NW USA 2003-Unpubl.
Microgadus proximusWXYZ011-UW 150512NW USA 201047.13° S, 122.69° WUnpubl.
Microgadus tomcodBCF621EU524129ROM-T03570SE Canada 200647.06° S, 70.42° W[35]
Microgadus tomcodSCAFB629KC015691ARC26844SE Canada44.26° S, 64.36° W[29]
Micromesistius australisFCHIL259--S Chile56.50° S, 68.62° WUnpubl.
Micromesistius australisFCHIL239--W Chile47.13° S, 75.58° WUnpubl.
Micromesistius poutassouGLF149LC146699ZMUB 22716SE Greenland 201461.10° N, 41.40° WThis study
Micromesistius poutassouBNSFI089KJ205044MT04159N United Kingdom 201257.85° N, 01.17° E[30]
Pollachius pollachiusBNSFI033KJ205137MT04178SW Norway 201258.22° N, 04.38° E[30]
Pollachius pollachiusNOFIS084-NHMO-f-537SE Norway 200958.11° S, 08.13° EUnpubl.
Pollachius virensGLF053LC146692No voucherSE Greenland 201365.28° N, 33.45° EThis study
Pollachius virensSCAFB100KC015818ARC 24890SE Canada 200542.91° N, 63.53° E[29]
Trisopterus capelanusCSFOM166KJ709669CSFOM-246Sicily, Italy-[32]
Trisopterus capelanusCSFOM165KJ709671CSFOM-245Sicily, Italy-[32]
Neocolliolus esmarkiiGLF012LC146705ZMUB 21421SE Greenland 201265.53° N, 32.36° WThis study
Neocolliolus esmarkiiGBGCA7771KJ128652NRM5415SW Sweden 200757.31° N, 11.47° EUnpubl.
Trisopterus luscusFCFP125JQ774953MB85-004867SW Portugal 200538.22° N, 08.83° W[31]
Trisopterus luscusBNSFI090KJ205243MT04227NW Germany 201153.76° N, 06.45° E[30]
Allotrisopterus minutusBNSFI133KJ205252MT05367N Germany 201054.14° N, 07.90° E[30]
Allotrisopterus minutusFCFPW193JQ775159FCFOPB086-05W Portugal 200541.62° N, 08.99° W[31]
NCBI: The National Center for Biotechnology Information, United States of America; Unpublished: BOLD records in the repository without any publication.
Table 3. Evidence supporting Gadiculus argenteus and G. thori as separate species.
Table 3. Evidence supporting Gadiculus argenteus and G. thori as separate species.
SpeciesGadiculus argenteusGadiculus thori
Maximum sizeca. 15 cmca. 20 cm
Head sizeRelatively largeRelatively small
Proportion of the eyeRelatively largeRelatively small
OL/OT ratioRelatively largeRelatively small
Shape of adult otolithsDrop-shaped
Moderately variable
Posterior end usually not truncated, but if so, truncation is oblique to long axis
Oval (irregular)
Highly variable
Posterior end usually truncated; truncation perpendicular to long axis
Postlarvae at same stage of developmentThree transverse pigmented bars
Relatively small size
Relatively stout
One transverse pigmented bar
Relatively large size
Relatively slender
Number of vertebrae37–41, usually 40
(usual range 38–40; 37 and 41 rare)
39–43, usually 42
(usual range 41–43; 39 and 40 rare)
Number of D3 fin rays11–1615–17
Number of A1 fin rays11–16 (11 rare)15–18 (18 rare)
Cox1 barcodesLittle intraspecific variationLittle intraspecific variation
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