Brain and sense organ anatomy and histology in hemoglobinless Antarctic icefishes (Perciformes: Notothenioidei: Channichthyidae)

Microplastic ingestion in five demersal, bathydemersal and bathypelagic fish species from the eastern Weddell Sea, Antarctica

Antarctica has traditionally been viewed as a relatively isolated ecosystem. Although still considered pristine, it is increasingly also being affected by microplastic pollution. Reported high sea floor concentrations raise concern that these ecosystems might act as major sink for microplastic pollution. This is significant as species in those remote ecosystems are likely more sensitive to rapid environmental change due to a high level of specialization, and lower tolerance levels. Microplastic ingestion in fish has barely been assessed in high latitude environments. Here we aimed to provide baseline data for the eastern Weddell Sea, which is particularly remote, and suggested for an area of conservation. By analyzing gastrointestinal tracts of 40 specimens from five species, we report an overall microplastic incidence rate of 0.23. This is lower than recent studies have found for other species in the Southern Ocean, and below global means. The highest incidence rate was detected in L. squamifrons (0.67), followed by P. evansii (0.29). The most common polymer was polyethylene recovered as 8 particles (42.1 %) from one specimen, while from the remaining 11 microplastics polyester was most common (36.8 %). This study shows that even in a remote region of the Antarctic Ocean with almost no vessel traffic, fisheries or touristic activity, bathydemersal and bathypelagic fish exhibit microplastic particles in their gastrointestinal tract.

Neuron mapping in the Molly fish optic tectum: An emphasis on the adult neurogenesis process

Teleost fish exhibit the most pronounced and widespread adult neurogenesis. Recently, functional development and the fate of newborn neurons have been reported in the optic tectum (OT) of fish. To determine the role of neurogenesis in the OT, this study used histological, immunohistochemical, and electron microscopic investigations on 18 adult Molly fish specimens (Poecilia sphenops). The OT of the Molly fish was a bilateral lobed structure located in the dorsal part of the mesencephalon. It exhibited a laminated structure made up of alternating fiber and cellular layers, which were organized into six main layers. The stratum opticum (SO) was supplied by optic nerve fibers, in which the neuropil was the main component. Radial bipolar neurons that possessed bundles of microtubules were observed in the stratum fibrosum et griseum superficiale (SFGS). Furthermore, oligodendrocytes with their processes wrapped around the nerve fibers could be observed. The stratum album centrale (SAC) consisted mainly of the axons of the stratum griseum centrale (SGC) and the large tectal, pyriform, and horizontal neurons. The neuronal cells of the SO and large tectal cells of the SAC expressed autophagy-related protein-5 (APG5). Interleukin-1β (IL-1β) was expressed in both neurons and glia cells of SGC. Additionally, inducible nitric oxide synthase (iNOS) was expressed in the neuropil of the SAC synaptic layer and granule cells of the stratum periventriculare (SPV). Also, transforming growth factor beta (TGF-β), SRY-box transcription factor 9 (SOX9), and myostatin were clearly expressed in the proliferative neurons. In all strata, S100 protein and Oligodendrocyte Lineage Transcription Factor 2 (Olig2) were expressed by microglia, oligodendrocytes, and astrocytes. In conclusion, it was possible to identify different varieties of neurons in the optic tectum, each with a distinct role. The existence of astrocytes, proliferative neurons, and stem cells highlights the regenerative capacity of OT. RESEARCH HIGHLIGHTS: The OT of the Molly fish exhibited a laminated structure made up of alternating fiber and cellular layers, which were organized into six main layers. Radial bipolar neurons that possessed bundles of microtubules were observed in the stratum fibrosum et griseum superficiale (SFGS). The stratum album central (SAC) consisted mainly of the axons of the stratum griseum centrale (SGC) and the large tectal, pyriform, and horizontal neurons. Inducible nitric oxide synthase (iNOS) was expressed in the neuropil of the SAC synaptic layer and granule cells of the stratum periventricular (SPV). Also, transforming growth factor beta (TGF-β), SRY-box transcription factor 9 (SOX9), and myostatin were clearly expressed in the proliferative neurons. The existence of astrocytes, proliferative neurons, and stem cells highlights the regenerative capacity of OT.

Population genomics of an icefish reveals mechanisms of glacier-driven adaptive radiation in Antarctic notothenioids

Background

Antarctica harbors the bulk of the species diversity of the dominant teleost fish suborder—Notothenioidei. However, the forces that shape their evolution are still under debate.

Results

We sequenced the genome of an icefish, Chionodraco hamatus, and used population genomics and demographic modelling of sequenced genomes of 52 C. hamatus individuals collected mainly from two East Antarctic regions to investigate the factors driving speciation. Results revealed four icefish populations with clear reproduction separation were established 15 to 50 kya (kilo years ago) during the last glacial maxima (LGM). Selection sweeps in genes involving immune responses, cardiovascular development, and photoperception occurred differentially among the populations and were correlated with population-specific microbial communities and acquisition of distinct morphological features in the icefish taxa. Population and species-specific antifreeze glycoprotein gene expansion and glacial cycle-paced duplication/degeneration of the zona pellucida protein gene families indicated fluctuating thermal environments and periodic influence of glacial cycles on notothenioid divergence.

Conclusions

We revealed a series of genomic evidence indicating differential adaptation of C. hamatus populations and notothenioid species divergence in the extreme and unique marine environment. We conclude that geographic separation and adaptation to heterogeneous pathogen, oxygen, and light conditions of local habitats, periodically shaped by the glacial cycles, were the key drivers propelling species diversity in Antarctica.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01432-x.

The Arrangement of the Peripheral Olfactory System of Pleuragramma antarcticum: A Well-Exploited Small Sensor, an Aided Water Flow, and a Prominent Effort in Primary Signal Elaboration

Simple Summary

How animals perceive their surrounding environment is crucial to their reactions and behavior. Olfaction, among others, is one of the more important senses for wide-range communication and in low-light environments. This study aims to give a morphological description of the peripheral olfactory system of the Antarctic silverfish, which is a key species in the coastal Antarctic ecosystem. The head of the Antarctic silverfish is specialized to assure that the olfactory organ keeps in contact with a large volume of water, even when the fish is not actively swimming. The sensory surface area and the number of neurons in the primary olfactory brain region show that this fish invests energy in the detection and elaboration of olfactory signals. In the cold waters of the Southern Ocean, the Antarctic silverfish is therefore likely to rely considerably on olfaction.

Abstract

The olfactory system is constituted in a consistent way across vertebrates. Nasal structures allow water/air to enter an olfactory cavity, conveying the odorants to a sensory surface. There, the olfactory neurons form, with their axons, a sensory nerve projecting to the telencephalic zone—named the olfactory bulb. This organization comes with many different arrangements, whose meaning is still a matter of debate. A morphological description of the olfactory system of many teleost species is present in the literature; nevertheless, morphological investigations rarely provide a quantitative approach that would help to provide a deeper understanding of the structures where sensory and elaborating events happen. In this study, the peripheral olfactory system of the Antarctic silverfish, which is a keystone species in coastal Antarctica ecosystems, has also been described, employing some quantitative methods. The olfactory chamber of this species is connected to accessory nasal sacs, which probably aid water movements in the chamber; thus, the head of the Antarctic silverfish is specialized to assure that the olfactory organ keeps in contact with a large volume of water—even when the fish is not actively swimming. Each olfactory organ, shaped like an asymmetric rosette, has, in adult fish, a sensory surface area of about 25 mm², while each olfactory bulb contains about 100,000 neurons. The sensory surface area and the number of neurons in the primary olfactory brain region show that this fish invests energy in the detection and elaboration of olfactory signals and allow comparisons among different species. The mouse, for example—which is considered a macrosmatic vertebrate—has a sensory surface area of the same order of magnitude as that of the Antarctic silverfish, but ten times more neurons in the olfactory bulb. Catsharks, on the other hand, have a sensory surface area that is two orders of magnitude higher than that of the Antarctic silverfish, while the number of neurons has the same order of magnitude. The Antarctic silverfish is therefore likely to rely considerably on olfaction.

OUP accepted manuscript

Animals rely on their sensory systems to inform them of ecologically relevant environmental variation. In the Southern Ocean, the thermal environment has remained between −1.9 and 5 °C for 15 Myr, yet we have no knowledge of how an Antarctic marine organism might sense their thermal habitat as we have yet to discover a thermosensitive ion channel that gates (opens/closes) below 10 °C. Here, we investigate the evolutionary dynamics of transient receptor potential (TRP) channels, which are the primary thermosensors in animals, within cryonotothenioid fishes—the dominant fish fauna of the Southern Ocean. We found cryonotothenioids have a similar complement of TRP channels as other teleosts (∼28 genes). Previous work has shown that thermosensitive gating in a given channel is species specific, and multiple channels act together to sense the thermal environment. Therefore, we combined evidence of changes in selective pressure, gene gain/loss dynamics, and the first sensory ganglion transcriptome in this clade to identify the best candidate TRP channels that might have a functional dynamic range relevant for frigid Antarctic temperatures. We concluded that TRPV1a, TRPA1b, and TRPM4 are the likeliest putative thermosensors, and found evidence of diversifying selection at sites across these proteins. We also put forward hypotheses for molecular mechanisms of other cryonotothenioid adaptations, such as reduced skeletal calcium deposition, sensing oxidative stress, and unusual magnesium homeostasis. By completing a comprehensive and unbiased survey of these genes, we lay the groundwork for functional characterization and answering long-standing thermodynamic questions of thermosensitive gating and protein adaptation to low temperatures.

Antarctic teleosts with and without hemoglobin behaviorally mitigate deleterious effects of acute environmental warming

Recent studies forecast that many ectothermic animals, especially aquatic stenotherms, may not be able to thrive or even survive predicted climate change. These projections, however, generally do not call much attention to the role of behavior, an essential thermoregulatory mechanism of many ectotherms. Here we characterize species-specific locomotor and respiratory responses to acute ambient warming in two highly stenothermic Antarctic Notothenioid fishes, one of which (Chaenocephalus aceratus) lacks hemoglobin and appears to be less tolerant to thermal stress as compared to the other (Notothenia coriiceps), which expresses hemoglobin. At the onset of ambient warming, both species perform distinct locomotor maneuvers that appear to include avoidance reactions. In response to unavoidable progressive hyperthermia, fishes demonstrate a range of species-specific maneuvers, all of which appear to provide some mitigation of the deleterious effects of obligatory thermoconformation and to compensate for increasing metabolic demand by enhancing the efficacy of branchial respiration. As temperature continues to rise, Chaenocephalus aceratus supplements these behaviors with intensive pectoral fin fanning which may facilitate cutaneous respiration through its scaleless integument, and Notothenia coriiceps manifests respiratory-locomotor coupling during repetitive startle-like maneuvers which may further augment gill ventilation. The latter behaviors, found only in Notothenia coriiceps, have highly stereotyped appearance resembling Fixed Action Pattern sequences. Altogether, this behavioral flexibility could contribute to the reduction of the detrimental effects of acute thermal stress within a limited thermal range. In an ecologically relevant setting, this may enable efficient thermoregulation of fishes by habitat selection, thus facilitating their resilience in persistent environmental change.

Global gene expression profile under low-temperature conditions in the brain of the grass carp (Ctenopharyngodon idellus)

Grass carp is an important commercial fish widely cultivated in China. A wide range of temperatures, particularly extremely low temperatures, have dramatic effects on the aquaculture of this teleost. However, relatively few studies have characterized the molecular responses of grass carp exposed to acute cooling in natural environment. Here, we investigated the transcriptome profiles of the grass carp brain in response to cooling. Through regulation pattern analyses, we identified 2,513 differentially expressed genes (DEGs) that responded to moderate cold stress (12°C), while 99 DEGs were induced by severe low temperature (4°C).The pathway analyses revealed that the DEGs sensitive to moderate cold were largely enriched in steroid biosynthesis, spliceosome, translation, protein metabolism, phagosome, gap junction and estrogen signaling pathways. Additionally, we discerned genes most likely involved in low temperature tolerance, of which the MAPK signaling pathway was dominantly enriched. Further examination and characterization of the candidate genes may help to elucidate the mechanisms underpinning extreme plasticity to severe cold stress in grass carp.

Morphogenesis of Olfactory Organ of Bushymouth Catfish Ancistrus dolichopterus (Teleostei: Loricariidae) before Switching to Exogenous Feeding

Olfaction plays an important role in a fish's life. Its value may differ at different developmental stages, depending on the feeding style of the species. The goal of the present study was to investigate the olfactory organ of a species that feeds mainly on algae- the bushymouth catfish, Ancistrus dolichopterus-at developmental stages from olfactory placode to the definitive olfactory chamber. For this study, we used light and electron (scanning) microscopy. The topography of the olfactory placode of A. dolichopterus is typical for teleostei. Formation of olfactory pit takes place at the same time as rostral elevation formation. Rostral elevation participates in the formation of the nasal bridge and anterior tubular nostril. It was found out that the anlage of olfactory rosette in A. dolichopterus arises earlier than in most teleostei. However, the number of lamellae does not increase until switching to exogenous feeding. We suppose that the early development of olfactory organ is necessary for intraspecific communication, not just for finding food.

Comparative gross encephalon morphology in Callichthyidae (Teleostei: Ostariophysi: Siluriformes)

ABSTRACT Callichthyidae comprises the subfamilies Callichthyinae and Corydoradinae, both of which are morphologically distinct and monophyletic. Although there is consensus regarding the monophyly of the family, the relationships of about 80% of its species, currently included in the genus Corydoras, remain poorly known. Despite the vast amount of osteological information for Teleostei, knowledge regarding the phylogenetic implications of encephalon anatomy is sparse and represents a poorly explored source of potential characters. The present study aims to describe the encephalon morphology in members of the Callichthyidae in order to propose new characters that may help address phylogenetic questions regarding this group. In addition to representatives of Callichthyidae, specimens belonging to the Nematogenyidae, Trichomycteridae, Scoloplacidae, Astroblepidae and Loricariidae were dissected for comparative purposes. Head dissection revealed information on the structure of the medulla spinalis, rhombencephalon, mesencephalon, diencephalon and telencephalon. The conditions observed on the encephalons examined suggest that representatives of Callichthyidae have great taste perception and processing, while Corydoradinae stand out for visual acuity and Callichthyinae for mechanoreception processing subunits. Our results also indicate that the encephalon has important features for systematic studies of the family bringing greater resolution to current phylogenetic hypotheses.

Gross morphology of the brain of Pseudopimelodus bufonius (Valenciennes, 1840) (Siluriformes: Pseudopimelodidae)

The gross morphology of the brain of the pseudopimelodid Pseudopimelodus bufonius is described and compared with congeners. Observations were made on removed brains after elimination of bones from the top of the skull and severing of the cranial nerves and the spinal cord. Nine morphometric characters associated with the major subdivisions of the brain were identified, seven of which revealed significant differences among the species examined. The corpus cerebelli in all examined species of the genus is the largest structure of the brain. The behavior of the species of Pseudopimelodus is still unknown, but in other teleosts that condition is typically correlated with a higher degree of motor coordination. Relative size proportions of the tectum opticum, eminentia granularis, lobus facialis and lobus vagi, might be related to carnivory and an enhanced capacity for food selection.

Molecular adaptations in haemoglobins of notothenioid fishes

Since haemoglobins of all animal species have the same haem group, differences in their properties, including oxygen affinity, electrophoretic mobility and pH sensitivity, must result from the interaction of the prosthetic group with specific amino-acid residues in the primary structure. For this reason, fish globins have been the subject of extensive studies in recent years, not only for their structural characteristics, but also because they offer the possibility to investigate the evolutionary history of these ancient molecules in marine and freshwater species living in a great variety of environmental conditions. This review summarizes the current knowledge on the structure, function and phylogeny of haemoglobins of notothenioid fishes. On the basis of crystallographic analysis, the evolution of the Root effect is analysed. Adaptation of the oxygen transport system in notothenioids seems to be based on evolutionary changes, involving levels of biological organization higher than the structure of haemoglobin. These include changes in the rate of haemoglobin synthesis or in regulation by allosteric effectors, which affect the amount of oxygen transported in blood. These factors are thought to be more important for short-term response to environmental challenges than previously believed.

The Antarctic hemoglobinless icefish, fifty five years later: a unique cardiocirculatory interplay of disaptation and phenotypic plasticity

The teleostean Channichthyidae (icefish), endemic stenotherms of the Antarctic waters, perennially at or near freezing, represent a unique example of disaptation among adult vertebrates for their loss of functional traits, particularly hemoglobin (Hb) and, in some species, cardiac myoglobin (Mb), once considered to be essential-life oxygen-binding chromoproteins. Conceivably, this stably frigid, oxygen-rich habitat has permitted high tolerance of disaptation, followed by subsequent adaptive recovery based on gene expression reprogramming and compensatory responses, including an alternative cardio-circulatory design, Hb-free blood and Mb-free cardiac muscle. This review revisits the functional significance of the multilevel cardio-circulatory compensations (hypervolemia, near-zero hematocrit and low blood viscosity, large bore capillaries, increased vascularity with great capacitance, cardiomegaly with very large cardiac output, high blood flow with low systemic pressure and systemic resistance) that counteract the challenge of hypoxemic hypoxia by increasing peripheral oxygen transcellular movement for aerobic tissues, including the myocardium. Reconsidered in the context of recent knowledge on both polar cold adaptation and the new questions related to the advent of nitric oxide (NO) biology, these compensations can be interpreted either according to the "loss-without-penalty" alternative, or in the context of an excessive environmental oxygen supply at low cellular cost and oxygen requirement in the cold. Therefore, rather than reflecting oxygen limitation, several traits may indicate structural overcompensation of oxygen supply reductions at cell/tissue levels. At the multilevel cardio-circulatory adjustments, NO is revealing itself as a major integrator, compensating disaptation with functional phenotypic plasticity, as illustrated by the heart paradigm. Beside NOS-dependent NO generation, recent knowledge concerning Hb/Mb interplay with NO and nitrite has revealed unexpected functions in addition to the classical respiratory role of these proteins. In fact, nitrite, a major biologic reservoir of NO, generates it through deohyHb- and deoxyMb-dependent nitrite reduction, thereby regulating hypoxic vasodilation, cellular respiration and signalling. We suggest that both Hb and Mb are involved as nitrite reductases under hypoxic conditions in a number of cardiocirculatory processes. On the whole, this opens new horizons in environmental and evolutionary physiology.

Brain and sense organ anatomy and histology of the Falkland Islands mullet, Eleginops maclovinus (Eleginopidae), the sister group of the Antarctic notothenioid fishes (Perciformes: Notothenioidei)

The perciform notothenioid fish Eleginops maclovinus, representing the monotypic family Eleginopidae, has a non-Antarctic distribution in the Falkland Islands and southern South America. It is the sister group of the five families and 103 species of Antarctic notothenioids that dominate the cold shelf waters of Antarctica. Eleginops is the ideal subject for documenting the ancestral morphology of nervous and sensory systems that have not had historical exposure to the unusual Antarctic thermal and light regimes, and for comparing these systems with those of the phyletically derived Antarctic species. We present a detailed description of the brain and cranial nerves of Eleginops and ask how does the neural and sensory morphology of this non-Antarctic notothenioid differ from that seen in the phyletically derived Antarctic notothenioids? The brain of Eleginops is similar to those of visually oriented temperate and tropical perciforms. The tectum is smaller but it has well-developed olfactory and mechanoreceptive lateral line areas and a large, caudally projecting corpus cerebellum. Eye diameter is about twofold smaller in Eleginops than in many Antarctic species. Eleginops has a duplex (rod and cone) retina with single and occasional twin cones conspicuous centrally. Ocular vascular structures include a large choroid rete mirabile and a small lentiform body; a falciform process and hyaloid arteries are absent. The olfactory rosette is oval with 50-55 lamellae, a large number for notothenioids. The inconspicuous bony canals of the cephalic lateral line system are simple with membranous secondary branches that lack neuromasts. In Antarctic species, the corpus cerebellum is the most variable brain region, ranging in size from large and caudally projecting to small and round. "Stalked" brains showing reduction in the size of the telencephalon, tectum, and corpus cerebellum are present in the deep-living artedidraconid Dolloidraco longedorsalis and in most of the deep-living members of the Bathydraconini. Eye diameter is generally larger in Antarctic species but there is a phylogenetic loss of cellularity in the retina, including cone photoreceptors. Some deep-living Antarctic species have lost most of their cones. Mechanosensation is expanded in some species, most notably the nototheniid Pleuragramma antarcticum, the artedidraconid genera Dolloidraco and Pogonophryne, and the deep living members of the bathydraconid tribe Bathydraconini. Reduction in retinal cellularity, expansion of mechanoreception, and stalking are the most noteworthy departures from the morphology seen in Eleginops. These features reflect a modest depth or deep-sea effect, and they are not uniquely "Antarctic" attributes. Thus, at the level of organ system morphology, perciform brain and sensory systems are suitable for conditions on the Antarctic shelf, with only minor alterations in structure in directions exhibited by other fish groups inhabiting deep water. Notothenioids retain a relative balance among their array of senses that reflects their heritage as inshore perciforms.

Genetic determinants of hyaloid and retinal vasculature in zebrafish

Background

The retinal vasculature is a capillary network of blood vessels that nourishes the inner retina of most mammals. Developmental abnormalities or microvascular complications in the retinal vasculature result in severe human eye diseases that lead to blindness. To exploit the advantages of zebrafish for genetic, developmental and pharmacological studies of retinal vasculature, we characterised the intraocular vasculature in zebrafish.

Results

We show a detailed morphological and developmental analysis of the retinal blood supply in zebrafish. Similar to the transient hyaloid vasculature in mammalian embryos, vessels are first found attached to the zebrafish lens at 2.5 days post fertilisation. These vessels progressively lose contact with the lens and by 30 days post fertilisation adhere to the inner limiting membrane of the juvenile retina. Ultrastructure analysis shows these vessels to exhibit distinctive hallmarks of mammalian retinal vasculature. For example, smooth muscle actin-expressing pericytes are ensheathed by the basal lamina of the blood vessel, and vesicle vacuolar organelles (VVO), subcellular mediators of vessel-retinal nourishment, are present. Finally, we identify 9 genes with cell membrane, extracellular matrix and unknown identity that are necessary for zebrafish hyaloid and retinal vasculature development.

Conclusion

Zebrafish have a retinal blood supply with a characteristic developmental and adult morphology. Abnormalities of these intraocular vessels are easily observed, enabling application of genetic and chemical approaches in zebrafish to identify molecular regulators of hyaloid and retinal vasculature in development and disease.

Morphometry of retinal vasculature in Antarctic fishes is dependent upon the level of hemoglobin in circulation

We quantitatively assessed ocular vascular patterns of six Antarctic notothenioid fishes that vary in their expression of the circulating oxygen-binding protein, hemoglobin (Hb). Digital image analyses revealed marked differences in vessel morphometries among notothenioid species. Hemoglobinless (–Hb) icefishes display mean vessel length densities that are greater (Chaenocephalus aceratus, 5.51±0.32 mm mm–2; Champsocephalus gunnari, 5.15±0.50 mm mm–2) than those observed in red-blooded (+Hb) species(Gymnodraco acuticeps, 5.20±0.46 mm mm–2; Parachaenichthyes charcoti, 4.40±0.30 mm mm–2; Trematomus hansoni, 3.94±0.08 mm mm–2; Notothenia coriiceps, 2.48±0.21 mm mm–2). –Hb fishes also have mean vessel diameters that are ∼1.5 times greater than vessel diameters of +Hb species (–Hb,0.193±0.006 mm; +Hb, 0.125±0.005 mm). Vascular density index(VDI), a stereological index that is affected by both vessel number and length, is greatest in –Hb C. aceratus (3.51±0.20) and lowest in +Hb N. coriiceps (1.58±0.14). Among four +Hb species, there is a direct relationship between red blood cell content and retinal vasculature. Hematocrit (Hct) is inversely correlated to vascular density (r2=0.934) and positively correlated to intervessel distance (r2= 0.898) over a >2.3-fold range of Hct. These results indicate that anatomical capacity to supply blood to the retina increases to compensate for decreases in oxygen-carrying capacity of the blood.

Brain and sense organ anatomy and histology of two species of phyletically basal non-Antarctic thornfishes of the Antarctic suborder Notothenioidei (Perciformes: Bovichtidae)

The predominantly non-Antarctic family Bovichtidae is phyletically basal within the perciform suborder Notothenioidei, the dominant component of the Antarctic fish fauna. In this article we focus on the South Atlantic bovichtids Bovichtus diacanthus, the klipfish from tide pools at Tristan da Cunha, and Cottoperca gobio, the frogmouth from the Patagonian shelf and Falkland Islands. We document the anatomy and histology of the brains, olfactory apparatus, retina, and cephalic lateral line system. We also use the microvascular casting agent Microfil to examine ocular vascular structures. We provide detailed drawings of the brains and cranial nerves of both species. Typical of perciforms, the brains of both species have a well-developed tectum and telencephalon and robust thalamic nuclei. The telencephalon of C. gobio is prominently lobed, with the dorsomedial nucleus more conspicuous than in any other notothenioid. The corpus cerebelli is relatively small and upright and, unlike other notothenioids, has prominent transverse sulci on the dorsal and caudal surfaces. Areas for lateral line mechanoreception (eminentia granularis and crista cerebellaris) are also conspicuous but olfactory, gustatory, and somatosensory areas are less prominent. The anterior lateral line nerve complex is larger than the posterior lateral line nerve in B. diacanthus, and in their cephalic lateral line systems both species possess branched membranous tubules (which do not contain neuromasts) with small pores. These are especially complex in B. diacanthus where they become increasingly branched and more highly pored in progressively larger specimens. Superficial neuromasts are sparse. Both species have duplex (cone and rod) retinae that are 1.25-fold thicker and have nearly 5-fold more photoreceptors and than those of most Antarctic notothenioids. Convergence ratios are also high for bovichtids. Bovichtus diacanthus has a yellow intraocular filter in the dorsal aspect of the cornea. Both species are unique among notothenioids in possessing all three vascular structures present in the generalized teleostean eye: the choroid rete mirabile, the lentiform body (also a rete), and the falciform process. When comparing the phyletically derived Antarctic clade exemplified by the families Artedidraconidae, Bathydraconidae, and Channichthyidae to the phyletically basal bovichtids, we observe phyletic regression and reduction in some regions of the brain and in some sensory modalities that are well displayed in bovichtids. In the phyletically derived families the brain is less cellular and nuclei are smaller and less prominent. In some species reduction in the size of the telencephalon, tectum, and corpus cerebelli imparts a "stalked" appearance to the brain with the neural axis visible between the reduced lobes. There is also a phyletic reduction in the number of ocular vascular structures from three in bovichtids to one or none in artedidraconids, bathydraconids, and channichthyids. There are no morphological features of bovichtid brains and sense organs that presage the divergence of the phyletically derived members of the clade in the Antarctic marine environment with its cold and deep continental shelves. We conclude that this environment does not require sensory or neural morphology or capabilities beyond those provided by the basic perciform body plan.

Demographic history and population structure of the Antarctic silverfish Pleuragramma antarcticum

The Antarctic silverfish Pleuragramma antarcticum (Nototheniidae) is the most abundant pelagic fish inhabiting Antarctic waters. In this study we investigated, through partial sequencing of the D-loop mitochondrial region, samples collected at four different locations in the Southern Ocean, three in the Atlantic and one in the Pacific sector. Sampling was replicated in two different years at two locations. Sequence analysis showed a remarkably high polymorphism, with 110 haplotypes over the 256 investigated specimens, and about 80% of haplotypes occurring only once. Neutrality tests indicated that all samples were not at mutation-drift equilibrium, and suggested a past population expansion. This result was supported by the presence of a star-like topology in the D-loop gene tree, and by results of mismatch distribution. The start of the expansion was dated, using a specifically calibrated clock, between 111 and 126 thousand years ago. This value corresponds to the start of the cooling period that led to the last glaciation peak, and is in close agreement with a recently suggested range expansion for pelagic Antarctic ecosystems. Analysis of molecular variation indicated a small, though highly significant, value of differentiation between samples. This result, together with the lack of association between clades and geographical locations, indicates a weak population structure for the species.

When bad things happen to good fish: the loss of hemoglobin and myoglobin expression in Antarctic icefishes

The Antarctic icefishes (Family Channichthyidae) provide excellent examples of unique traits that can arise in a chronically cold and isolated environment. Their loss of hemoglobin (Hb) expression, and in some cases, loss of myoglobin (Mb) expression, has taught us much about the function of these proteins. Although absences of the proteins are fixed traits in icefishes, the losses do not appear to be of adaptive value. Contrary to some suggestions, loss of Hb has led to higher energetic costs for circulating blood, and losses of Mb have reduced cardiac performance. Moreover, losses of Hb and Mb have resulted in extensive modifications to the cardiovascular system to ensure adequate oxygen delivery to working muscles. Recent studies suggest that losses of Hb and Mb, and their associated nitric oxide (NO)-oxygenase activities, may have accelerated the development and evolution of these cardiovascular modifications. The high levels of NO that should occur in the absence of Hb and Mb have been shown in other animal groups to lead to an increase in tissue vascularization, an increase in the lumenal diameter of blood vessels, and an increase in mitochondrial densities. These characteristics are all hallmark traits of Antarctic icefishes. Homeostatic feedback mechanisms thus may have accelerated evolution of the pronounced cardiovascular traits of Antarctic icefishes.

Brain and sensory organ morphology in Antarctic eelpouts (Perciformes: Zoarcidae: Lycodinae)

Eelpouts of the family Zoarcidae comprise a monophyletic group of marine fishes with a worldwide distribution. Centers of high zoarcid diversity occur in the North Atlantic and North Pacific, with important radiations into the Arctic, along southern South America, and into the Southern Ocean around Antarctica. Along with snailfishes (Liparidae), zoarcids form an important component of the non-notothenioid fauna in the subzero shelf waters of Antarctica. We document the anatomy and histology of the brains, cranial nerves, olfactory apparatus, cephalic lateral lines, taste buds, and retinas of three Antarctic zoarcid species, living at depths of 310-939 m, representing three of the nine genera from this region. The primary emphasis is on Ophthalmolycus amberensis, and we provide a detailed drawing of the brain and cranial nerves of this species. Although this brain reflects general perciform neural morphology, it exhibits a reduction of the (optic) tecta and the eminentia granulares and crista cerebellares of the lateral line system. Interspecific differences among the three species are slight. The olfactory rosette consists of three to four lamellae and the nasal sac, contrary to the claim of Fanta et al. ([2001] Antarct Rec, Natl Inst Polar Res, Tokyo 45:27-42), is not in communication with the cephalic lateral line system. Primary olfactory neurons are abundant and converge on branches of the olfactory nerve. Numerous taste buds are located in the lips. All three species lack an ocular choroid rete and have relatively thin retinas with a low cell density and a single bank of rods as the only type of photoreceptor. Neural diversification among Antarctic zoarcids has not involved the evolution of sensory specialists; brain and sensory organ morphologies do not approach the condition seen in primary deep-sea fishes, or even that of some sympatric non-perciform secondary deep-sea fishes, including liparids and muraenolepidids (eel cods). There may be phylogenetic constraints on brain morphology in perciforms such that we do not see extreme specialization in sensory and neural systems for deep habitats. We suggest that the brains and sensory organs of Antarctic zoarcids reflect habitation of 500-2,000-m depths and likely reflect morphologies seen in zoarcids living on continental slopes elsewhere in the world. This balance among the sensory modalities makes zoarcids relatively generalized among secondary deep-sea fishes and may be one of the reasons this opportunistic and adaptable group has been successful in colonizing a variety of emergent and ephemeral habitats.