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Abstract Rising ocean temperatures pose significant threats to marine ectotherms. Sensitivity to temperature change varies across life stages, with embryos often being less tolerant to thermal perturbation than adults. Antarctic notothenioid fishes evolved to occupy a narrow, cold thermal regime (−2 to +2°C) as the high-latitude Southern Ocean (SO) cooled to its present icy temperatures, and they are particularly vulnerable to small temperature changes, which makes them ideal sentinel species for assessing climate change impacts. Here, we detail how predicted warming of the SO may affect embryonic development in the Antarctic bullhead notothen,Notothenia coriiceps. Experimental embryos were incubated at +4°C, a temperature projected for the high-latitude SO within the next 100–200 years under high emission climate models, whereas control embryos were incubated at present-day ambient temperature, ∼0°C. Elevated temperature caused a high incidence of embryonic morphological abnormalities, including body axis kinking/curvature and reduced body size. Experimental embryos also developed more rapidly, such that they hatched 68 days earlier than controls (87 vs. 155 days post-fertilization). Accelerated development disrupted the evolved timing of seasonal hatching, shifting larval emergence into the polar winter when food availability is scarce. Transcriptomic analyses revealed molecular signatures of hypoxia and disrupted protein-folding in near-hatching embryos, indicative of severe cellular stress. Predictive modeling suggested that temperature-induced developmental disruptions would narrow seasonal reproductive windows, thereby threatening population viability under future climate scenarios. Together, our findings underscore the vulnerability of Antarctic fish embryos to higher water temperature and highlight the urgent need to understand the consequences of disruption of this important trophic component on ecosystem stability in the SO. Significance StatementAntarctic fishes evolved cold-adapted phenotypes suited to the stable thermal conditions of the Southern Ocean, yet are threatened by rising temperatures. The impact of rising temperatures on early life stages in Antarctic fishes is not well understood; our findings show that projected warming may induce premature hatching, developmental abnormalities, and molecular stress responses in embryos, potentially reducing recruitment and leading to population instability and trophic-level ecosystem disruptions. These results underscore the urgency of assessing climate-driven vulnerabilities across life stages of Antarctic marine organisms to refine population projections and enhance conservation strategies amid ongoing environmental change. 

Abstract Pathogens affecting Antarctic fishes remain mostly unknown and are largely limited to the description of macroparasites such as leeches and endoparasitic worms. Fish, however, occupy a crucial role in the functioning of the Antarctic ecosystem and deterioration of their health can alter the entire Antarctic food chain. In recent years, several studies have identified novel viruses and unicellular parasites affecting the health of notothenioid fishes. Among those, the unicellular parasitic family Xcellidae has received attention following the discovery of an unprecedented disease outbreak in a fjord on the Western Antarctic Peninsula. This pathological situation was caused by a novel X-cell genusNotoxcellia. Soon thereafter, an additional X-cell genus,Cryoxcellia, was described infecting the Bald NotothenTrematomus borchgrevinkiin the Ross Sea. These studies raised awareness and drew observers’ and researchers’ attention to pathologies in Antarctic fishes. Here, we report that during a 2023 Ross Sea shelf survey, a specimen of the Scaly NotothenTrematomus loennbergiidisplaying skin lesions reminiscent ofNotoxcelliainfection had been ingested by an Antarctic ToothfishDissostichus mawsoniand was recovered from its stomach. Molecular analyses confirmed the presence ofNotoxcelliasp. X-cell parasites in the fish’s lesions. This new case of X-cell disease suggests thatNotoxcelliaspp. may have a circumpolar distribution and stresses the need for monitoring Antarctic fish health similar to surveillance protocols for Antarctic birds and marine mammals. 

Abstract The rapid diversification of notothenioid fishes in Antarctic waters is a prime example of the process of adaptive radiation. Within around 10 million years, Antarctic notothenioids have diversified into over 100 species with a broad range of lifestyles and ecological adaptations. However, the exact number of species within this radiation has long been unclear. Particularly challenging is the taxonomy of the genusChannichthys, for which between one and nine species have been recognized by different authors. The putative species of this genus are known from a limited number of specimens, of which most were sampled decades ago. Here, we investigated the mitochondrial genomes of museum specimens representing the four species Unicorn Icefish (C. rhinoceratus), Red Icefish (C. rugosus), Sailfish Pike (C. velifer), and Charcoal Icefish (C. panticapaei), complemented by morphological analyses. All analyzed specimens were collected in the 1960s and 1970s and fixed in formaldehyde, and their DNA has been heavily degraded. Applying ancient-DNA protocols for DNA extraction and single-stranded library preparation, we were able to obtain sufficient endogenous DNA to reconstruct the mitochondrial genomes of one specimen per species. These mitochondrial genome sequences were nearly identical for the three specimens assigned to Unicorn Icefish, Red Icefish, and Sailfish Pike, while greater divergence was observed for the Charcoal Icefish specimens. We discuss possible explanations of the contrast between these molecular results and the recognizable morphological variation found among the four species, and recommend that at least the Charcoal Icefish be included in the list of valid notothenioid species. 

Abstract Expression of multiple hemoglobin isoforms with differing physiochemical properties likely helps species adapt to different environmental and physiological conditions. Antarctic notothenioid fishes inhabit the icy Southern Ocean and display fewer hemoglobin isoforms, each with less affinity for oxygen than temperate relatives. Reduced hemoglobin multiplicity was proposed to result from relaxed selective pressure in the cold, thermally stable, and highly oxygenated Antarctic waters. These conditions also permitted the survival and diversification of white-blooded icefishes, the only vertebrates living without hemoglobin. To understand hemoglobin evolution during adaptation to freezing water, we analyzed hemoglobin genes from 36 notothenioid genome assemblies. Results showed that adaptation to frigid conditions shaped hemoglobin gene evolution by episodic diversifying selection concomitant with cold adaptation and by pervasive evolution in Antarctic notothenioids compared to temperate relatives, likely a continuing adaptation to Antarctic conditions. Analysis of hemoglobin gene expression in adult hematopoietic organs in various temperate and Antarctic species further revealed a switch in hemoglobin gene expression underlying hemoglobin multiplicity reduction in Antarctic fish, leading to a single hemoglobin isoform in adult plunderfishes and dragonfishes, the sister groups to icefishes. The predicted high hemoglobin multiplicity in Antarctic fish embryos based on transcriptomic data, however, raises questions about the molecular bases and physiological implications of diverse hemoglobin isoforms in embryos compared to adults. This analysis supports the hypothesis that the last common icefish ancestor was vulnerable to detrimental mutations affecting the single ancestral expressed alpha- and beta-globin gene pair, potentially predisposing their subsequent loss. 

Abstract Long‐read sequencing is driving a new reality for genome science in which highly contiguous assemblies can be produced efficiently with modest resources. Genome assemblies from long‐read sequences are particularly exciting for understanding the evolution of complex genomic regions that are often difficult to assemble. In this study, we utilized long‐read sequencing data to generate a high‐quality genome assembly for an Antarctic eelpout,Ophthalmolycus amberensis, the first for the globally distributed family Zoarcidae. We used this assembly to understand howO. amberensishas adapted to the harsh Southern Ocean and compared it to another group of Antarctic fishes: the notothenioids. We showed that selection has largely acted on different targets in eelpouts relative to notothenioids. However, we did find some overlap; in both groups, genes involved in membrane structure, thermal tolerance and vision have evidence of positive selection. We found evidence for historical shifts of transposable element activity inO. amberensisand other polar fishes, perhaps reflecting a response to environmental change. We were specifically interested in the evolution of two complex genomic loci known to underlie key adaptations to polar seas: haemoglobin and antifreeze proteins (AFPs). We observed unique evolution of the haemoglobin MN cluster in eelpouts and related fishes in the suborder Zoarcoidei relative to other Perciformes. For AFPs, we identified the first species in the suborder with no evidence ofafpIIIsequences (Cebidichthys violaceus) in the genomic region where they are found in all other Zoarcoidei, potentially reflecting a lineage‐specific loss of this cluster. Beyond polar fishes, our results highlight the power of long‐read sequencing to understand genome evolution. 

The Southern Ocean that surrounds Antarctica is one of the coldest on earth and hovers freezing temperatures year-round. What fish can live in such a frigid environment? In this short scientific graphic novel, What cool fish live in icy Antarctica?, Isabel Lopez, John Postlethwait and Thomas Desvignes present the history of Antarctica and how it became the icy continent. These glacial conditions prevented most fish species from living there, but one group, the notothenioids, evolved a way to survive and even thrive in the ice-cold Antarctic waters. They diversified from the surface to the dark depths of the ocean, from being just a few centimeters long to extremely large, and in many other surprising ways. And among them, the white-blooded icefishes are also certainly some of the strangest fish on the planet! But today, Antarctica is changing fast, raising concerns about the survival of these unique fish. 

Bathydraconidae (Notothenioidei) are a group of benthic fishes endemic to the Southern Ocean. Because of their recent evolutionary radiation and limited sampling efforts due to their occurrence in remote regions, their diversity is likely underestimated. Akarotaxis nudiceps, currently the only recognized member of its genus, is an especially poorly known bathydraconid. Although A. nudiceps has a circumpolar distribution on the Antarctic continental shelf, its deep habitat and rarity limit knowledge of its life history and biology. Using a combination of morphological and genetic analyses, we identified an undescribed species of this genus, herein named Akarotaxis gouldae sp. nov. (Banded Dragonfish). The separation of this species was initially identified from archived larval specimens, highlighting the importance of early life stage taxonomy and natural history collections. All currently known adult and larval A. gouldae sp. nov. specimens have been collected from a restricted ~400 km coastal section of the western Antarctic Peninsula, although this is possibly due to sampling bias. This region is targeted by the epipelagic Antarctic krill fishery, which could potentially capture larval fishes as bycatch. Due to the extremely low fecundity of A. gouldae sp. nov. and near-surface occurrence of larvae, we suggest the growing Antarctic krill fishery could negatively impact this speces.  

In mammals and in birds, the sex of an individual is determined by its genes. A sex determining gene on a sex chromosome influence the development of ovaries or testes. But sex in fishes is much more diverse! What controls sex development in fish? Sex genes like in mammals and birds? Or do other types of sex determination systems exist in fish? In this short scientific graphic novel, Sophia Breslin, John Postlethwait, and Thomas Desvignes introduce you to the control of sex determination in fish: from the genetic regulation by sex determining genes and sex chromosomes to various cases of hermaphrodism and the influence of the environment, revealing the myriad of different sex determination systems found in fishes.