In vertebrates, the olfactory bulb (OB) is the zone of the brain devoted to receiving the olfactory stimuli. The size of the OB relative to the size of the brain has been positively correlated to a good olfactory capability but, recently, this correlation was questioned after new investigation techniques were developed. Among them, the isotropic fractionator allows to estimate the number of neurons and non-neurons in a given portion of nervous tissue. To date, this technique has been applied in a number of species; in particular the OB was separately analyzed in numerous mammals and in a single crocodile species. Thus, a quantitative description of the OB's cells is available for a small portion of vertebrates. Main aim of this work was to apply isotropic fractionator to investigate the olfactory capability of elasmobranch fishes, whose traditional concept of outstanding olfaction has recently been scaled down by anatomical and physiological studies. For this purpose, the OB of two elasmobranch species, Galeus melastomus and Scyliorhinus canicula, was studied leading to the determination of the number of neurons vs non-neurons in the OB of the specimens. In addition, the obtained cell quantification was related to the olfactory epithelium surface area to obtain a new parameter that encapsulates both information on the peripheral olfactory organ and the OB. The analyzed species resulted in an overall similar quantitative organization of the peripheral olfactory system; slight differences were detected possibly reflecting different environment preference and feeding strategy. Moreover, the non-neurons/neurons ratio of these species, compared to those available in the literature, seems to place elasmobranch fishes among the vertebrate species in which olfaction plays an important role.

The ice cod Arctogadus glacialis (Peters, 1872) is one of the few fish species endemic to the Arctic. With a circumpolar distribution, the species is confined to the fjords and shelves of the Arctic seas. Biological information on A. glacialis is scarce, with genomic information restricted to microsatellites. Within the frame of the TUNU-Programme: Arctic Ocean Fishes-Diversity, Adaptation and Conservation, we studied A. glacialis at the chromosomal level to explore fish diversity and evolutionary aspects. The analysis of over 50 individuals from the Northeast Greenland fjords between latitudes 71 degrees 09 ' N and 76 degrees 42 ' N revealed a remarkable intraspecific diversity epitomized by chromosome numbers spanning from 28 to 33, the occurrence of putative B chromosomes, and diversified patterns of distribution of heterochromatin and rDNAs. The number of B chromosomes followed a latitudinal gradient from 0-2 in the north to 2-5 in the south. Considering the benthic and rather stationary life history of this species, the observed chromosomal differences might have arisen independently, possibly driven and/or fostered by the dynamics of repetitive sequences, and are being fixed in relatively isolated fjord populations. The resulting latitudinal cline we observe today might have repercussions on the fate of local populations facing the ongoing climate-driven environmental changes.

Arctic regions are inhabited by cold-adapted stenothermal or eurythermal species. Unlike in the Antarctic, eurythermal species predominate, because of opportunities for migrations to temperate latitudes. In the Antarctic sea, the modern chondrichthyan genera are scarcely represented. In contrast, in the Arctic, sharks and skates are present with about 8% of the species (Mecklenburg et al., 2011; Lynghammar et al., 2013). The distribution of the Greenland shark Somniosus microcephalus is quite wide; in fact, this species typically thrives in deep and extremely cold waters, seasonally covered by sea ice (MacNeil et al., 2012), but is also known to enter more temperate waters in the North Atlantic (Bigelow & Schroeder, 1948; Skomal & Benz, 2004). Widespread climate changes in the arctic ecosystem have led to increased attention on trophic dynamics and on the role of this apex predator in the structure of arctic marine food webs (MacNeil et al., 2012).

The olfactory organ of Chondrichthyes is characterized by a central support with several lamellae covered by a sensory olfactory epithelium. Although secondary folds are present on the lamellae in all the chondrichthyan species analyzed to date, their shape and size have not been described. We here analyze the olfactory organ of 13 elasmobranch and 1 holocephalan species, describe the shape of the secondary folds and evaluate how they contribute to the epithelial surface area. The secondary folds vary in shape and size, and they should always be considered when comparing the epithelial surface area among species; in fact, the increase of the area, due to the presence of the secondary folds, spans from 70 to 495% in the considered species. Because of the complexity of the shapes, we approach the description of the secondary folds by analyzing histological sections of the olfactory lamellae to obtain illustrative silhouettes. We introduce two indexes in order to describe a 2D-sectioned shape of the secondary folds. Considering the different numerical parameters which describe the morphology of the olfactory organ (secondary folds included), a principal component analysis elucidates the possible ecological role and phylogenetic relationship of the chondrichthyan olfactory organ.

By their faculty to transpose, transposable elements are known to play a key role in eukaryote genomes, impacting both their structuration and remodeling. Their integration in targeted sites may lead to recombination mechanisms involved in chromosomal rearrangements. The Antarctic fish family Nototheniidae went through several waves of species radiations. It is a suitable model to study transposable element (TE)-mediated mechanisms associated to genome and chromosomal diversifications. After the characterization of Gypsy (GyNoto), Copia (CoNoto), and DIRS1 (YNoto) retrotransposons in the genomes of Nototheniidae (diversity, distribution, conservation), we focused on their chromosome location with an emphasis on the three identified nototheniid radiations (the Trematomus, the plunderfishes, and the icefishes). The strong intrafamily TE conservation and wide distribution across species of the whole family suggest an ancestral acquisition with potential secondary losses in some lineages. GyNoto and CoNoto (including Hydra and GalEa clades) mostly produced interspersed signals along chromosomal arms. On the contrary, insertion hot spots accumulating in localized regions (mainly next to centromeric and pericentromeric regions) highlighted the potential role of YNoto in chromosomal diversifications as facilitator of the fusions which occurred in many nototheniid lineages, but not of the fissions.

The Antarctic notothenioid fish Dissostichus mawsoni (Antarctic toothfish) is an important piscine top predator in the Southern Ocean. Good olfactory capability has been hypothesized for this species on the basis of morphological (size of its olfactory bulb compared to other notothenioids) and behavioral (long distance migrations for food search and reproduction) traits. Here, we provide new information on the structure and function of the olfactory rosette and bulb of D. mawsoni using histology. Adult specimens (total length 136.2 +/- 11.6 cm) were collected from McMurdo Sound. The rosettes had an average of 39 lamellae, without secondary folds and with a total surface area of about 1000 mm(2). Both putative ciliated and microvillous receptor neurons were present in the sensory epithelium. Their projections to clustered glomeruli in the olfactory bulb were observed using antibodies against G-proteins. Numerous rodlet cells were observed in the epithelium and Gi2-like immunoreactivity was present in their cytoplasm. This deserves further investigation given the still-debated nature of this cell type. Through the isotropic fractionator method, we showed 116,000 cells (mg of tissue)(-1) in the olfactory bulb of D. mawsoni, a density that is similar to those found in mammals. Taken together, these data describe a well-developed olfactory system in this species, where olfaction is key sensory system.

Antifreeze glycoproteins (AFGPs) are a novel evolutionary innovation in members of the northern cod fish family (Gadidae), crucial in preventing death from inoculative freezing by environmental ice in their frigid Arctic and sub-Arctic habitats. However, the genomic origin and molecular mechanism of evolution of this novel life-saving adaptive genetic trait remained to be definitively determined. To this end, we constructed large insert genomic DNA BAC (bacterial artificial chromosome) libraries for two AFGP-bearing gadids, the high-Arctic polar cod Boreogadus saida and the cold-temperate Atlantic tomcod Microgadus tomcod, to isolate and sequence their AFGP genomic regions for fine resolution evolutionary analyses. The BAC library construction encountered poor cloning efficiency initially, which we resolved by pretreating the agarose-embedded erythrocyte DNA with a cationic detergent, a method that may be of general use to BAC cloning for teleost species and/or where erythrocytes are the source of input DNA. The polar cod BAC library encompassed 92,160 clones with an average insert size of 94.7 kbp, and the Atlantic tomcod library contained 73,728 clones with an average insert size of 89.6 kbp. The genome sizes of B. snide and M. tomcod were estimated by cell flow cytometry to be 836 Mbp and 645 Mbp respectively, thus their BAC libraries have approximately 10- and 9.7-fold genome coverage respectively. The inclusiveness and depth of coverage were empirically confirmed by screening the libraries with three housekeeping genes. The BAC clones that mapped to the AFGP genomic loci of the two gadids were then isolated by screening the BAC libraries with gadid AFGP gene probes. Eight minimal tiling path (MTP) clones were identified for B. saida, sequenced, and assembled. The B. saida AFGP locus reconstruction produced both haplotypes, and the locus comprises three distinct AFGP gene clusters, containing a total of 16 AFGP genes and spanning a combined distance of 512 kbp. The M. tomcod AFGP locus is much smaller at approximately 80 kbp, and contains only three AFGP genes. Fluorescent in situ hybridization with an AFGP gene probe showed the AFGP locus in both species occupies a single chromosomal location. The large AFGP locus with its high gene dosage in B. saida is consistent with its chronically freezing high Arctic habitats, while the small gene family in M. tomcod correlates with its milder habitats in lower latitudes. The results from this study provided the data for fine resolution sequence analyses that would yield insight into the molecular mechanisms and history of gadid AFGP gene evolution driven by northern hemisphere glaciation.

The Antarctic toothfish (Dissostichus mawsoni Norman 1937) is the largest notothenioid inhabiting high-latitude Antarctic waters, where it is an important fishery resource and plays a key ecological role at a high trophic level. Despite the considerable amount of data on D. mawsoni biology and distribution developed since the fishery began in 1997, crucial aspects of the life cycle, including spawning and early life history, remain undescribed. During the first winter longline survey to the northern Ross Sea region in 2016, ripe male and female D. mawsoni were collected for the first time, and in vitro fertilisation of eggs was performed. Here, we report on the first characterisation of D. mawsoni egg structure and initial embryonic development. The duration of the egg cleavage period was similar to that of other nototheniid species releasing pelagic eggs. The structural features of fertilised eggs, including chorion thickness and structure, support the hypothesis that eggs of D. mawsoni are pelagic. The data presented here contribute to the description of the potential habitat of the eggs of this species, and provide the first diagnostic information to recognise the eggs of D. mawsoni.

During their evolution and speciation in the Antarctic waters, notothenioid fish occupied a variety of habitats and ecological niches. The diversification led to important variations in several morphological features related to particular aspects of their ecologies. We investigated the feeding structures and biomechanics of three phylogenetically related species (family Nototheniidae) with different ecologies: the bentho-pelagic Antarctic toothfish Dissostichus mawsoni, the pelagic Antarctic silverfish Pleuragramma antarctica, and the benthic emerald rockcod Trematomus bernacchii. The suction index (SI), the mechanical advantage in jaw closing (MA), and 14 morphological traits related to their feeding activity were analyzed. Significant differences among the species were found for all the parameters considered, supporting a high level of specialization.

A new specimen of splendid alfonsino, Beryx splendens Lowe, 1834, was caught by trawling in July 2016 in the Ligurian Sea at the depth of 350 m, in the proximity of a submarine canyon. It represents the 10th documented record of B. splendens in the Mediterranean. Because of the rarity of the records in the basin, there could be doubts whether to consider or not such species as established in the area. However, some findings may support the hypothesis of the possible establishment of the species in the Mediterranean Sea. For example, the stomach of the specimen was nearly full, with rests of one crustacean decapod, one fish, and some cephalopods, which are commonly reported prey items for the species. Macroscopic observation and histological analysis of the gonads revealed that the specimen was a male in an advanced stage of gonadal development. Finally, the coherence of habitat type and prey items with that of extra-Mediterranean populations coupled with gonadal maturation consistent with the observations on other specimens caught in the Mediterranean.

Antarctic toothfish (Dissostichus mawsoni) is a large nototheniid endemic in Antarctic waters. They are a top predator, and an important commercially fished species with a circumpolar distribution mostly south of the Antarctic convergence. Fisheries acoustics is a potential tool to estimate toothfish abundance and distribution, but previous studies have been limited by lack of information on the acoustic target strength (TS). In this paper we present the first in situ estimates of TS for Antarctic toothfish. Data were collected by deploying acoustic equipment through the sea ice in conjunction with vertical line fishing and baited underwater video (BUV) observations. Estimated mean TS from 250 tracked single targets detected in situ in Terra Nova Bay was -37.8 dB re 1 m(2) (95% confidence interval -38.2 to -37.5 dB). Estimates of toothfish length from BUV images of 42 individuals ranged from 92 to 201 cm total length (TL) with mean length 134 cm. Estimates from 15 Antarctic toothfish (104-153 cm TL, average 131 cm) captured using vertical lines in McMurdo Sound gave a lower mean TS of -40.2 dB (range of individual fish means - 38.3 to -43.7 dB). Although estimates from hooked fish have a number of potential biases, due to unnatural orientation and acoustic interference from fishing gear and adjacent hooked fish, these results supported the conclusion from our in situ estimates that toothfish have higher TS than previously thought. In situ acoustic observations showed most toothfish within 100 m of the seabed.

Antarctic silverfish (Pleuragramma antarctica Boulengcr) are a keystone species in the Ross Sea. Silverfish eggs and larvae are abundant during spring amongst the sub-surface platelet ice in Terra Nova Bay. It is not known whether the eggs are spawned elsewhere and accumulate under the ice or whether there is mass migration of silverfish to coastal spawning sites in winter. To test the latter hypothesis, an upward-looking 67 kHz echo sounder was moored in Terra Nova Bay to observe potential silverfish migration. The echo sounder was deployed at 380 m in a seabed depth of 550 m and ran for 210 days from 15 May until 11 December 2015. Acoustic reflections consistent with silverfish were observed at depths of 230-380 m during 9-22 September. This timing is consistent with the presence of eggs typically observed in October. Adult silverfish were also detected with an echo sounder and camera deployed through the ice in McMurdo Sound on 10 November 2015. Juvenile silverfish, but not adults, were observed through the ice in Terra Nova Bay during 11-16 November 2017. This paper provides a proof of concept, showing that innovative use of acoustics may help fill important observation gaps in the life history of silverfish.

The Arctic is a unique and fragile ecosystem that needs to be preserved and protected. Despite its remoteness, plastic pollution has been documented in this region. In the coming years, it is likely to worsen since, with climate changes and the opening of new shipping routes, the human presence is going to increase in the whole area. Here, we investigated the presence of microplastics (MPs) in subsurface water and in two mid-trophic level Arctic fishes collected off Northeast Greenland: the demersal bigeye sculpin, Triglops nybelini, and the pelagic polar cod, Boreogadus saida. Plastics debris were found in the water samples at a concentration of 2.4 items/m(3) +/- 0.8 SD which is higher than in most seas at lower latitudes. Both fish species had eaten MPs with different proportion among the species, 34% for T. nybelini (n = 71) and 18% for B. saida (n = 85). The significant difference in the occurrence of MPs between the two species is likely a consequence of their feeding behavior and habitat. Polyethylene was the main plastic polymer for water samples (41%, n = 17) and polyester (34%, n = 156) for fish samples as analyzed by Fourier Transformed Infrared (FT-IR) spectroscopy. Our data underscore that the Arctic regions are turning into a hotspot for plastic pollution, and this calls urgently for precautionary measures. (C) 2018 Elsevier Ltd. All rights reserved.

Several papers regard the anatomy of the peripheral olfactory organ, the olfactory rosette, in the class of Chondrichtyes. The complex shape of this organ and the differences among species give clues to functional, evolutionary, and ecological observations; data on a larger number of species are needed in order to have a more complete insight. The rosette is made up of a central support and of numerous lamellae, which are lined by the sensory epithelium. The size, shape and number of these lamellae, which are highly variable among species, are noteworthy because they affect the sensory surface area, the water dynamic within the olfactory chamber, and the organization of the olfactory bulb. In the literature about Chondrichthyes, the definition of olfactory lamella is confused, because authors use the same words for different structures. The average number of lamellae is distinctive for each species, and the meaning of this difference is not completely understood and, in this frame, a not unambiguous definition of lamella leads to difficulties in comparing data from different publications and analyzing them together. We aim to give here an unambiguous definition of olfactory lamella, that should be each single fold of tissue extending from the raphe, and of lamellar number of a given species, that should be the average number of lamellae in one olfactory organ. Anat Rec, 2017. © 2017 Wiley Periodicals, Inc. Anat Rec, 300:2039-2045, 2017. © 2017 Wiley Periodicals, Inc.

Gross morphology and histology of the olfactory rosette of Greenland shark Somniosus microcephalus (Bloch & Schneider, 1801), suggest a well-developed olfactory capability for this top predator and scavenger native to Arctic waters. In the present paper, observations on the olfactory rosette are reported together with the gross morphology and histology of the olfactory bulb whereby the histological analyses revealed some peculiar traits. The olfactory bulb is macroscopically divided into two sub-bulbs and a central undivided part that contain the layers typical of olfactory bulb histology. The two visible sub-bulbs are actually bundles of fila olfactoria. In addition, the olfactory lateral ventricle is peculiarly branched, which may increase the ependymal surface or, alternatively, decrease the distance between the tissues of the olfactory bulb and the ventricle itself. The ependymal surface is known to be a proliferative zone in the olfactory bulb of fishes. In Greenland shark, the olfactory epithelium showed frequent mitosis and apoptosis highlighting the importance of this site in cell renewal. This issue should be pursued further to gain a deeper understanding of the sensory biology of Greenland shark and of elasmobranchs in general.

The Antarctic silverfish Pleuragramma antarctica is the most abundant pelagic fish inhabiting the Ross Sea. Given its ecological relevance in the local food web, it is considered a keystone species in the Antarctic coastal ecosystems. Many aspects of its biology have been elucidated, but knowledge of important parts of its life cycle, including reproduction, is still poor. Here we use macroscopic and histological approaches to describe the reproductive features of the Antarctic silverfish based on fish sampled in late summer in the Ross Sea. Both males and females were at an early developmental stage, consistent with what has been reported for the same species from other Antarctic sectors and with spawning occurring in late winter. Widespread follicular atresia has been detected in the fish examined. The analysis of its intensity and prevalence suggests that skipped spawning (not all adults spawn every year) is likely to be a reproductive strategy of the Antarctic silverfish.

The Greenland shark (Somniosus microcephalus) is the largest predatory fish in Arctic waters. Knowledge of the fundamental biology and ecological role of the Greenland shark is limited, and the sensory biology of the Greenland shark has been poorly studied. Given the potential relevant contribution of chemoreception to the sensory capability of the Greenland shark to forage and navigate in low-light environments, we examined the architecture of the peripheral olfactory organ (the olfactory rosette) through morphological, histological and immunohistochemical assays. We found that each olfactory rosette consists of a small number of lamellae (22) associated with a relatively high surface area of the olfactory epithelium. The general organization of the epithelium is similar to that described for other elasmobranchs. However, details that have emerged concerning the cell type composition (absence of crypt neurons, presence of unusually large cells along the olfactory fiber bundles) deserve further investigation. Overall, the structure of the olfactory rosette suggests a well-developed olfactory capability for the Greenland shark coherent with a bentho-pelagic lifestyle.

Background: Over the past 40 million years water temperatures have dramatically dropped in the Southern Ocean, which has led to the local extinction of most nearshore fish lineages. The evolution of antifreeze glycoproteins in notothenioids, however, enabled these ancestrally benthic fishes to survive and adapt as temperatures reached the freezing point of seawater (-1.86 °C). Antarctic notothenioids now represent the primary teleost lineage in the Southern Ocean and are of fundamental importance to the local ecosystem. The radiation of notothenioids has been fostered by the evolution of "secondary pelagicism", the invasion of pelagic habitats, as the group diversified to fill newly available foraging niches in the water column. While elaborate craniofacial modifications have accompanied this adaptive radiation, little is known about how these morphological changes have contributed to the evolutionary success of notothenioids. Results: We used a 3D-morphometrics approach to investigate patterns of morphological variation in the craniofacial skeleton among notothenioids, and show that variation in head shape is best explained by divergent selection with respect to foraging niche. We document further an accelerated rate of morphological evolution in the icefish family Channichthyidae, and show that their rapid diversification was accompanied by the evolution of relatively high levels of morphological integration. Whereas most studies suggest that extensive integration should constrain phenotypic evolution, icefish stand out as a rare example of increased integration possibly facilitating evolutionary potential. Finally, we show that the unique feeding apparatus in notothenioids in general, and icefish in particular, can be traced to shifts in early developmental patterning mechanisms and ongoing growth of the pharyngeal skeleton. Conclusion: Our work suggests that ecological opportunity is a major factor driving craniofacial variation in this group. Further, the observation that closely related lineages can differ dramatically in integration suggests that this trait can evolve quickly. We propose that the evolution of high levels of phenotypic integration in icefishes may be considered a key innovation that facilitated their morphological evolution and subsequent ecological expansion.

In fish, the determination of sex can be controlled by genetic factors, environmental factors or a combination of both. The presence of heteromorphic sex-related chromosomes is widely acknowledged as strongly indicative of genetic control of sex determination (GSD) acting over other sex control systems. Heteromorphic sex-related chromosomes have been observed in a minority of teleosts (approximately 4 %). However, when looking at the fishes of the suborder Notothenioidei the frequency of sex-related chromosomes increases substantially, reaching 26.67 % of the cytogenetically studied species. Noteworthy, sex chromosomes were observed only in cold-adapted species which live in the Antarctic coastal waters, whereas morphologically differentiated sex chromosomes were never observed in the temperate non-Antarctic notothenioid families. Recent evidence suggests that the sex-linked chromosomes across the Antarctic notothenioid families may not share a common origin, but likely originated more than once during notothenioid evolutionary history, thus implying the presence of selection pressures operating toward fixation of GSD system. On the whole, the cytogenetic evidences suggest the Antarctic-specific fixation of differentiated heteromorphic sex-related chromosomes and of a prominent GSD across Antarctic notothenioids that may be an additional manifestation of notothenioid evolution in thermally stable cold environment.

Fluorescence in situ hybridization (FISH) allows nucleic acid sequences to be visualized inside fi xed cells or chromosomes. The method, based on the principle that nucleic acids can denature and renature, allows the detection of a known nucleotide sequence (probe), in a cell/chromosome, thanks to its ability to hybridize the complementary nucleic-acid sequence (target) preserved in situ. FISH probes specifi c for fi sh are not commercially available and must be homemade. Before the hybridization experiment, the DNA probe has to be directly or indirectly labelled with a fl uorochrome, so that the hybridization product can be revealed and visualized as a fl uorescent signal. In direct labelling, the fl uorochrome is directly bound to the DNA probe. In indirect labelling, a hapten (most frequently biotin or digoxigenin) is incorporated in the probe and immunochemically detected by a fl uorophore-tagged antibody. The labelled probe and the target sequence are denatured and then combined, so that their complementary DNA sequences are annealed. When the probe and target DNA belong to the same species, the FISH is referred as homologous, whereas it is heterologous when they are derived from different species.