Dissostichus Mawsoni Research Articles

The buoyancy-based biotope axis of the evolutionary radiation of Antarctic cryonotothenioid fishes

In the absence of any prior comprehensive analysis, I evaluate divergence along the biotope axis in the habitat stage of the evolutionary radiation of Antarctic cryonotothenioids. I utilize the available percentage buoyancy (%B) measurements as habitat proxies for recognition of the pelagic, semipelagic, demersal, and benthic biotopes that include, respectively, 5%, 10%, 73%, and 12% of the 59 species and 1749 specimens in the study. The majority of species retain the ancestral demersal biotope of Eleginops maclovinus, and this probably enhances ecological plasticity. Divergence into the pelagic biotope is the most distinctive organismal feature of the radiation and, although only 5% of species are pelagic, this biotope is not depauperate in global comparisons. Pelagic or potentially pelagic species are Dissostichus mawsoni, D. eleginoides, Pleuragramma antarctica, Aethotaxis mitopteryx, and Gvozdarus svetovidovi. Small ontogenetic changes in %B with growth are typical; however, this is extensive in D. mawsoni, a species with the potential to transition through benthic to pelagic biotopes over ontogeny. Occupation of the pelagic biotope by large D. mawsoni may be impermanent as it is lipid-dependent, a contingency reliant on the availability of P. antarctica as prey. In unusual conditions, the specialized sacs of P. antarctica can also yield their lipid for metabolism with possible loss of buoyancy. Pelagic species are inordinately important in the food web. In the southwestern Ross Sea a guild of large mammalian and avian predators, which includes D. mawsoni, is reliant on lipid-rich, energy-dense cryonotothenioid prey. This includes asymmetrical intraguild predation on D. mawsoni, with P. antarctica as a basal resource for the guild.

High mercury levels in Antarctic toothfish Dissostichus mawsoni from the Southwest Pacific sector of the Southern Ocean

Mercury is a bioaccumulating toxic pollutant which can reach humans through the consumption of contaminated food (e.g. marine fish). Although the Southern Ocean is often portrayed as a pristine ecosystem, its fishery products are not immune to mercury contamination. We analysed mercury concentration (organic and inorganic forms – T-Hg) in the muscle of Antarctic toothfish, Dissostichus mawsoni, a long-lived top predator which supports a highly profitable fishery. Our samples were collected in three fishing areas (one seamount and two on the continental slope) in the Southwest Pacific Sector of the Southern Ocean during the 2016/2017 fishing season. Mercury levels and the size range of fish varied between fishing areas, with the highest levels (0.68 ± 0.45 mg kg−1 wwt) occurring on the Amundsen Sea seamount where catches were dominated by larger, older fish. The most parsimonious model of mercury concentration included both age and habitat (seamount vs continental slope) as explanatory variables. Mean mercury levels for each fishing area were higher than those in all previous studies of D. mawsoni, with mean values for the Amundsen Sea seamount exceeding the 0.5 mg kg−1 food safety threshold for the first time. It might therefore be appropriate to add D. mawsoni to the list of taxa, such as swordfish and sharks, which are known to exceed this threshold. This apparent increase in mercury levels suggests a recent contamination event which affected the Southwest Pacific sector, including both the Amundsen and Dumont D'Urville seas.

Ontogenetic changes in habitat and trophic ecology of the giant Antarctic octopus Megaleledone setebos inferred from stable isotope analyses in beaks

The giant Antarctic Octopus Megaleledone setebos is the largest Southern Ocean octopod whose ecology is poorly known. Here, we study ontogenetic shifts of habitat and trophic ecology of M. setebos throughout its life cycle by stable isotopic analysis of δ13C and δ15N on its beaks collected from the diet of Antarctic toothfish in Amundsen and Ross Seas (Antarctica). Values of δ13C (from − 24.3 to − 19.4‰) differed between beaks of individuals from different capture locations, thus reflecting the ability of M. setebos living in different habitats. Despite sequential sampling along beaks showed a small (< 2.3‰), but significant variation in lower beak’s δ13C values, a relation with δ15N values suggests that such differences are related to changes in the diet with M. setebos inhabiting the same area its entire life. Values of δ15N differed between beaks of individuals from different capture locations, suggesting that different habitats of M. setebos are associated with different diets. Serial sampling along the beaks (from + 4.2 to + 10.7‰) suggests an ontogenetic change of, at least, one trophic level from juvenile to adult. We also report a capture of two large intact specimens from Dumont D’Urville Sea (Antarctica): a male with 1150 mm of total length and 18,300 g of mass and a female with 1030 mm of total length and 10,061 g of mass. The beaks of these both specimens, confirmed to be of M. setebos through genetic analysis, were also used to confirm the identification of M. setebos collected from Antarctic toothfish stomachs.

Structural and functional properties of Antarctic fish cytoglobins-1: Cold-reactivity in multi-ligand reactions

While the functions of the recently discovered cytoglobin, ubiquitously expressed in vertebrate tissues, remain uncertain, Antarctic fish provide unparalleled models to study novel protein traits that may arise from cold adaptation. We report here the spectral, ligand-binding and enzymatic properties (peroxynitrite isomerization, nitrite-reductase activity) of cytoglobin-1 from two Antarctic fish, Chaenocephalus aceratus and Dissostichus mawsoni, and present the crystal structure of D. mawsoni cytoglobin-1. The Antarctic cytoglobins-1 display high O2 affinity, scarcely compatible with an O2-supply role, a slow rate constant for nitrite-reductase activity, and do not catalyze peroxynitrite isomerization. Compared with mesophilic orthologues, the cold-adapted cytoglobins favor binding of exogenous ligands to the hexa-coordinated bis-histidyl species, a trait related to their higher rate constant for distal-His/heme-Fe dissociation relative to human cytoglobin. At the light of a remarkable 3D-structure conservation, the observed differences in ligand-binding kinetics may reflect Antarctic fish cytoglobin-1 specific features in the dynamics of the heme distal region and of protein matrix cavities, suggesting adaptation to functional requirements posed by the cold environment. Taken together, the biochemical and biophysical data presented suggest that in Antarctic fish, as in humans, cytoglobin-1 unlikely plays a role in O2 transport, rather it may be involved in processes such as NO detoxification.

The diet of Weddell seals (Leptonychotes weddellii) in Terra Nova Bay using stable isotope analysis

Stable isotope analyses were used to investigate the diet of Weddell seals in Terra Nova Bay (Ross Sea) and the potential variation of their foraging behaviour with age, sex and body mass. For this purpose, skin samples were collected from adult breeding seals and pups, together with muscle samples of their potential prey. Our results showed variation in foraging behavior between age classes, with pups reporting lower δ13C values than adults, while no significant differences in δ15N were recorded. In addition, contrary to expectations, a mixing model analysis showed that adult seals foraged mainly on shallow benthic prey, such as Trematomus spp. (34.1%) and Dissostichus mawsoni (21.1%), rather than on pelagic fish, such as Pleuragramma antarcticum (9.8%). Overall, with this paper we provide novel diet information on a seal colony not previously sampled, adding new insight into the feeding ecology of a top Antarctic predator.

First results of oceanographic works on Ukrainian longline vessels in Antarctica (CCAMLR zone) in 2017/2018 season

Aim of works is implementation of duties, taken by Ukraine on XXXVI Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) Session and study of habitation of main fishing objects in Antarctica waters. Methods of works were the use of DST CTD recorder produced by STAR OGGICompany (Iceland) and the analysis of the obtained data. Results of works include 36 longline and 11 other stations (on a line, on streamer, on Juday plankton net) executed in Amundsen, Ross and Weddell Seas during the period from December 2017 to April 2018. Vertical changeability of temperature, temporal changeability of benthic temperature were analyzed; spatial changeability of temperatures in separate Seas was found out; 4 basic water masses were distinguished, influence of bottom temperature to the catches of Antarctic toothfish was analyzed. Conclusion was made about the increase in catches while lowering the bottom temperature in Weddell Sea.

Spatial distribution and population structure of juvenile Antarctic toothfish (Dissostichus mawsoni) in the South Shetland Islands

As most of biological and ecological features of the Antarctic toothfish have been provided almost exclusively by the recent fishery exploitation in the Ross Sea targeting adult fishes, the early life history of this species is fragmentary and still needs to be fully addressed. Aiming to fill this gap, we provide new insights on the spatial distribution and population structure of juvenile D. mawsoni caught during bottom trawl surveys on the shelves around the South Shetland Islands (CCAMLR Subarea 48.1). Juveniles were relatively evenly distributed on the northern shelf of the South Shetlands and all around Elephant Island, with a few specimens caught in the Bransfield Strait. Although the specimens were caught in all depth strata surveyed, they were most abundant between 100 and 300 m depth, with an evident shift towards deeper waters with ontogeny. The sampled population consisted of 150 juveniles measuring between 22 and 78 cm TL, equally distributed between sexes, most of them with gonads at an immature stage of maturity. Both sexes showed a positive allometric body growth, indicating healthy body condition. Based on counts of annual growth increments from sagittal otolith sections, age estimates ranged between 2 and 7 years in males and between 2 and 8 years in females, respectively. Annual growth rate was estimated from the age-length keys, spanning from 5 to 10 cm TL. Integrating population structure with early life history of juvenile D. mawsoni, we provide some indication on connectivity between the investigated nursery area and potential neighbouring spawning areas.

Characterization of Four Liver-Expressed Antimicrobial Peptides from Antarctic Fish and Their Antibacterial Activity

Liver-expressed antimicrobial peptides (LEAPs) are cysteine-containing cationic peptides. LEAP-1 and LEAP-2 are eight- and four-cysteine containing antimicrobial peptides found in animals, respectively. LEAP-1 is widely known as antibacterial peptide involved in the innate immunity of fish, but the roles of LEAP-1 and LEAP-2 in Antarctic fish species are unknown. In the present study, we synthesized and characterized novel LEAPs with four and eight cysteine residues, derived from Antarctic notothenioid (Dissostichus mawsoni) and Antarctic eelpout (Lycodichthys dearborni). Circular dichroism spectroscopy of these peptides showed a typical β-sheet conformation. The LEAPs were found to be bactericidal against gram-positive as well as gram-negative bacteria. In the SYTOX green uptake assay, LEAPs did not trigger any significant increase in fluorescence. However, LEAPs competitively bound to DNA and replaced the ethidium bromide (EB) dye. To determine the effect of temperature on the activity of LEAPs, we evaluated the antibacterial activity against Listeria monocytogenes at 5, 15, 25, and 35 °C. The results showed that the antibacterial activity of LEAPs increased with a decrease in temperature, which may indicate that the Antarctic fish LEAP are evolutionarily adapted. Taken together, our results suggest that novel Antarctic LEAPs are bactericidal peptides with the likely mode of action being DNA binding and may be evolved to adapt to cold temperature.

Comparing spatial distribution modelling of fisheries data with single-area or spatially-explicit integrated population models, a case study of toothfish in the Ross Sea region

Most models used for assessing the status of fish stocks and providing quantitative advice for fisheries management are not spatially explicit despite the well-known spatial heterogeneity of fish populations. Statistical spatial distribution modelling, which can derive spatially-resolved biomass from catch rates, is a method increasingly used to infer trends in population biomass, particularly where integrated population models are not available. Although the spatial distribution modelling tool VAST was developed to analyze survey data, such methods have also been used with fisheries-dependent catch and effort data, particularly when survey data are not available. We developed a statistical spatial distribution model for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region of the Southern Ocean using the VAST (Vector Autoregressive Spatio-Temporal model) approach. We based the VAST model on catch-rate (catch per unit effort) data from the long-line fishery and compared the biomass time series obtained to that of existing single-area and spatially-explicit integrated population models which have also been developed for this population. The time series obtained from the statistical spatial distribution modelling approach was highly variable between years, inconsistent with constraints imposed on population dynamics by biological parameters, and substantially different from biomass trends obtained from the integrated models. Although it has been used successfully in other analyses, in this instance, the spatial distribution modelling could not overcome fine-scale spatial and vessel-based variability in fishery catch rates to estimate the underlying abundance of toothfish at the scale of the Ross Sea region measurable with the more-informed integrated models.

Distribution of Antarctic toothfish Dissostichus mawsoni along East Antarctica: Environmental drivers and management implications

As Antarctic fish species experience increasing anthropogenic pressures there is a growing need to characterise the structure and function of their populations and understand how they may respond to changes in their environment. We integrated fishery-dependent and environmental data from years 2003–2017 to investigate the distribution of Antarctic toothfish Dissostichus mawsoni along a c. 3000 nm expanse of the shelf and slope along East Antarctica (30–150 °E). Spatially-explicit generalised additive mixed models were used to characterise the environmental drivers of relative abundance, mean individual weight, maturity-stage composition and sex ratio. Antarctic toothfish were not randomly distributed across the region, and spatial variations were characterised by complex relationships involving topography, temperature, salinity and sea ice. In particular, catch rates were highest at depths between 1000–1700 m. Mean weight and the proportion of fish that were mature both increased with depth and decreased with bottom temperature. Model predictions were also used to develop hypotheses relating to population function, including the location of nursery and spawning areas. This characterisation of the population can facilitate evaluation of fishing impacts in East Antarctica, and improve our understanding of the role of toothfish in Antarctic and Southern Ocean ecosystems.

Winter spawning of Antarctic toothfish Dissostichus mawsoni in the Ross Sea region

AbstractA survey of Antarctic toothfish (Dissostichus mawsoni) was conducted in the northern Ross Sea region during the winter of 2016 to document the timing and location of spawning activity, to collect biological information about reproductive status during the spawning season and to look for temporal signals in biological data from D. mawsoni that may indicate a spawning migration of mature toothfish from the continental slope region to the northern Ross Sea region. The 58 day survey showed that spawning of D. mawsoni began on some seamounts by early July. No changes were detected between winter and summer in length, age, sex ratio or condition factor distributions for D. mawsoni in the northern Ross Sea as hypothesized following a spawning migration from the slope to the northern Ross Sea region. These results suggest that the distribution of D. mawsoni in the Ross Sea is mainly accomplished through ontogenetic migration and not annual return spawning migrations.

Monitoring Antarctic toothfish in McMurdo Sound to evaluate the Ross Sea region Marine Protected Area

AbstractWe developed a random, stratified, vertical longline survey in McMurdo Sound, Antarctica, to compare the local age and size composition, diet and reproductive status of Antarctic toothfish (Dissostichus mawsoni) with those observed from a vessel-based survey of the southern Ross Sea shelf that includes a McMurdo Sound stratum. Results indicated that southern McMurdo Sound toothfish were larger and older than those a short distance away in northern McMurdo Sound. These data, in addition to recoveries of tagged fish, suggest that the large toothfish in McMurdo Sound may have limited mixing with the rest of the population. The potential effects of climate change and fishing in northern areas on toothfish abundance in McMurdo Sound will depend on the mechanism of toothfish recruitment to McMurdo Sound. Understanding the ecological relationships between McMurdo Sound toothfish and the larger population is required to predict these impacts. Furthermore, because toothfish predators (type C killer whalesOrcinus orca, Weddell sealsLeptonychotes weddellii) are abundant in the south-west margins of the Ross Sea, it is important to monitor toothfish in McMurdo Sound as part of the monitoring programme for the Ross Sea region Marine Protected Area.

Distribution and biology of the colossal squid, Mesonychoteuthis hamiltoni: New data from depredation in toothfish fisheries and sperm whale stomach contents

We provide new insights into the biology and distribution of the colossal squid (Mesonychoteuthis hamiltoni), by analysing historical data collected during the commercial whaling years (1967–1974) together with recent information collated by CCAMLR observers in 2012–2017. The maximum abundance of colossal squid has been observed in the Indian Ocean sector of the Antarctic (Cooperation Sea), and the lowest in the Ross Sea (the area of highest abundance of its predator and competitor, the Antarctic toothfish, Dissostichus mawsoni). The colossal squid appears to feed primarily on mesopelagic fish. Spawning likely occurs in summer; ovulation is synchronous and mature egg size is ∼3 mm. Adult (large and maturing) M. hamiltoni are the most abundant in sperm whale diets at surface temperatures of −0.9 to 0 °C, which may indicate the locations of spawning grounds.

Olfaction in the Antarctic toothfish Dissostichus mawsoni: clues from the morphology and histology of the olfactory rosette and bulb

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 mm2. 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 Gαi2-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.

Uncovering the Role of Surface Residues and Buffer Composition in Liquid-Liquid Phase Separation of Eye Lens Crystallins from an Antarctic Toothfish

The Antarctic toothfish, Dissostichus mawsoni, lives in the Southern Ocean around Antarctica, requiring adaptations to deal with the challenges of living in a subfreezing environment. One such adaptation is that the D. mawsoni eye lens is completely resistant to liquid-liquid phase separation (LLPS) of the eye lens proteins, also known as cold cataract. Despite this observation, a sub-fraction of the D. mawsoni eye lens proteins was found to undergo LLPS above the body temperature of the toothfish. Six γ-crystallins were identified from this fraction and characterized for their LLPS behavior and general features of stability. No obvious relationship could be discerned between the critical temperatures at which these γ-crystallins undergo LLPS and the various measures of stability. Additionally, mutagenesis of predicted surface exposed residues was performed to study their role in the LLPS behavior. Mutagenesis revealed that replacing exposed arginine for lysine results in a decrease of the phase transition temperature, while replacing lysine with arginine had the opposite effect, but similar magnitude. These finding highlights the importance of arginine in increasing attractive protein-protein interactions even when the site specific charge is largely unchanged. Furthermore, we have shown that the components of the buffer solution can further modulate the phase transition temperature of these crystallins.

The genomic basis for colonizing the freezing Southern Ocean revealed by Antarctic toothfish and Patagonian robalo genomes.

BackgroundThe Southern Ocean is the coldest ocean on Earth but a hot spot of evolution. The bottom-dwelling Eocene ancestor of Antarctic notothenioid fishes survived polar marine glaciation and underwent adaptive radiation, forming >120 species that fill all water column niches today. Genome-wide changes enabling physiological adaptations and the rapid expansion of the Antarctic notothenioids remain poorly understood.ResultsWe sequenced and compared 2 notothenioid genomes—the cold-adapted and neutrally buoyant Antarctic toothfish Dissostichus mawsoni and the basal Patagonian robalo Eleginops maclovinus, representing the temperate ancestor. We detected >200 protein gene families that had expanded and thousands of genes that had evolved faster in the toothfish, with diverse cold-relevant functions including stress response, lipid metabolism, protein homeostasis, and freeze resistance. Besides antifreeze glycoprotein, an eggshell protein had functionally diversified to aid in cellular freezing resistance. Genomic and transcriptomic comparisons revealed proliferation of selcys–transfer RNA genes and broad transcriptional upregulation across anti-oxidative selenoproteins, signifying their prominent role in mitigating oxidative stress in the oxygen-rich Southern Ocean. We found expansion of transposable elements, temporally correlated to Antarctic notothenioid diversification. Additionally, the toothfish exhibited remarkable shifts in genetic programs towards enhanced fat cell differentiation and lipid storage, and promotion of chondrogenesis while inhibiting osteogenesis in bone development, collectively contributing to the achievement of neutral buoyancy and pelagicism.ConclusionsOur study revealed a comprehensive landscape of evolutionary changes essential for Antarctic notothenioid cold adaptation and ecological expansion. The 2 genomes are valuable resources for further exploration of mechanisms underlying the spectacular notothenioid radiation in the coldest marine environment.

Controlling Liquid–Liquid Phase Separation of Cold-Adapted Crystallin Proteins from the Antarctic Toothfish

Liquid–liquid phase separation (LLPS) of proteins is important to a variety of biological processes both functional and deleterious, including the formation of membraneless organelles, molecular condensations that sequester or release molecules in response to stimuli, and the early stages of disease-related protein aggregation. In the protein-rich, crowded environment of the eye lens, LLPS manifests as cold cataract. We characterize the LLPS behavior of six structural γ-crystallins from the eye lens of the Antarctic toothfish Dissostichus mawsoni, whose intact lenses resist cold cataract in subzero waters. Phase separation of these proteins is not strongly correlated with thermal stability, aggregation propensity, or cross-species chaperone protection from heat denaturation. Instead, LLPS is driven by protein–protein interactions involving charged residues. The critical temperature of the phase transition can be tuned over a wide temperature range by selective substitution of surface residues, suggesting general principles for controlling this phenomenon, even in compactly folded proteins.

이빨고기(Dissostichus mawsoni)의 근육에서 분리한 Psychrobacter spp.의 특성

Toothfish Dissostichus mawsoni (Notothenniidae Family, Perciformes Order) is one of the endangered species and usually inhabits the deep sea cold water. This study was performed to identify the characteristics of bacterial microorganisms from the muscle of the toothfish. The Antarctic toothfish captured using longline fisheries in 2016 were transported to the laboratory in frozen state. Bacterial colonies from muscle were identified using sequences analyzes of 16S rRNA gene. Then, sequenced genes were compared by an alignment matching analysis. Biochemical features were examined by Gram staining, oxidase and catalase tests. Moreover, API 20E and API ZYM were also used. Bacteria like Psychrobacter spp.(79.4%), Staphylococcus spp.(1.3%), and Micrococcus spp.(1.3%), Arthrobacter spp.(1.9%), Brachybacterium spp.(11.3%), Salinibacterium sp.(5%) were identified from the muscle and predominant microorganisms were found to be Psychrobacter spp.. The geneus Psychrobacter is classified in the family Moraxellaceae and the class Gammaproteobacteria. The Psychrobacter species exhibited pale yellow color, rod-shape, Gram negative staining, and oxidase positive reactions. The Psychrobacter spp. had tolerence at the 0% to 8%(w/v) NaCl and the optimal growth was 10℃ to 25℃. Further studies seem to be needed to understand microbial characteristics better on Psychrobacter spp. from toothfish muscles.

First in situ estimates of acoustic target strength of Antarctic toothfish (Dissostichus mawsoni)

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 m2 (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.

GC bias lead to increased small amino acids and random coils of proteins in cold-water fishes

BackgroundTemperature adaptation of biological molecules is fundamental in evolutionary studies but remains unsolved. Fishes living in cold water are adapted to low temperatures through adaptive modification of their biological molecules, which enables their functioning in extreme cold. To study nucleotide and amino acid preference in cold-water fishes, we investigated the substitution asymmetry of codons and amino acids in protein-coding DNA sequences between cold-water fishes and tropical fishes., The former includes two Antarctic fishes, Dissostichus mawsoni (Antarctic toothfish), Gymnodraco acuticeps (Antarctic dragonfish), and two temperate fishes, Gadus morhua (Atlantic cod) and Gasterosteus aculeatus (stickleback), and the latter includes three tropical fishes, including Danio rerio (zebrafish), Oreochromis niloticus (Nile tilapia) and Xiphophorus maculatus (Platyfish).ResultsCold-water fishes showed preference for Guanines and cytosines (GCs) in both synonymous and nonsynonymous codon substitution when compared with tropical fishes. Amino acids coded by GC-rich codons are favored in the temperate fishes, while those coded by AT-rich codons are disfavored. Similar trends were discovered in Antarctic fishes but were statistically weaker. The preference of GC rich codons in nonsynonymous substitution tends to increase ratio of small amino acid in proteins, which was demonstrated by biased small amino acid substitutions in the cold-water species when compared with the tropical species, especially in the temperate species. Prediction and comparison of secondary structure of the proteomes showed that frequency of random coils are significantly larger in the cold-water fish proteomes than those of the tropical fishes.ConclusionsOur results suggested that natural selection in cold temperature might favor biased GC content in the coding DNA sequences, which lead to increased frequency of small amino acids and consequently increased random coils in the proteomes of cold-water fishes.