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.     

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.     

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

The Channichthyidae, one of five Antarctic notothenioid families, includes 16 species and 11 genera. Most live at depths of 200-800 m and are a major component of fish biomass in many shelf areas. Channichthyids are unique among adult fishes in possessing pale white blood containing a few vestigal erythrocytes and no hemoglobin. Here we describe the brains of seven species and special sense organs of eight species of channichthyids. We emphasize Chionodraco hamatus and C. myersi, compare these species to other channichthyids, and relate our findings to what is known about brains and sense organs of red-blooded notothenioids living sympatrically on the Antarctic shelf. Brains of channichthyids generally resemble those of their bathydraconid sister group. Among channichthyids the telencephalon is slightly regressed, resulting in a stalked appearance, but the tectum, corpus cerebellum, and mechanoreceptive areas are well developed. Interspecific variation is present but slight. The most interesting features of channichthyid brains are not in the nervous tissue but in support structures: the vasculature and the subependymal expansions show considerable elaboration. Channichthyids have large accessory nasal sacs and olfactory lamellae are more numerous than in other notothenioids. The eyes are relatively large and laterally oriented with similar duplex (cone and rod) retinae in all eight species. Twin cones are the qualitatively dominant photoreceptor in histological sections and, unlike bathydraconids, there are no species with rod-dominated retinae. Eyes possess the most extensive system of hyaloid arteries known in teleosts. Unlike the radial pattern seen in red-blooded notothenioids and most other teleosts, channichthyid hyaloid arteries arise from four or five main branches and form a closely spaced anastomosing series of parallel channels. Cephalic lateral line canals are membranous and some exhibit extensions (canaliculi), but canals are more ossified than those of deeper-living bathydraconids. We conclude that, with respect to the anatomy and histology of the neural structures, the brain and sensory systems show little that is remarkable compared to other fishes, and exhibit little diversification within the family. Thus, the unusual habitat and a potentially deleterious mutation resulting in a hemoglobinless phenotype are reflected primarily in expansion of the vasculature in the brain and eye partially compensating for the absence of respiratory pigments. Neural morphology gives the impression that channichthyids are a homogeneous and little diversified group.     

Morphology of the brain and sense organs in the snailfish Paraliparis devriesi: Neural convergence and sensory compensation on the Antarctic shelf

The Antarctic snailfish Paraliparis devriesi (Liparidae) is an epibenthic species, inhabiting depths of 500–650 m in McMurdo Sound. Liparids are the most speciose fish family in the Antarctic Region. We examine the gross morphology and histology of the sense organs and brain of P. devriesi and provide a phyletic perspective by comparing this morphology to that of four scorpaeniforms and of sympatric perciform notothenioids. The brain has numerous derived features, including well-developed olfactory lamellae with thick epithelia, large olfactory nerves and bulbs, and large telencephalic lobes. The retina contains only rods and exhibits a high convergence ratio (82:1). Optic nerves are small and nonpleated. The tectum is small. The corpus of the cerebellum is large, whereas the valvula is vestigial. The rhombencephalon and bulbospinal junction are extended and feature expanded vagal and spinal sensory lobes as well as hypertrophied dorsal horns and funiculi in the rostral spinal cord. The lower lobes of the pectoral fins have taste buds and expanded somatosensory innervation. Although the cephalic lateral line and anterior lateral line nerve are well developed, the trunk lateral line and posterior lateral line nerve are reduced. Near-field mechanoreception by trunk neuromasts may have been compromised by the watery, gelatinous subdermal extracellular matrix employed as a buoyancy mechanism. The expanded somatosensory input to the pectoral fin may compensate for the reduction in the trunk lateral line. The brains of P. devriesi and sympatric notothenioids share well-developed olfactory systems, an enlarged preoptic-hypophyseal axis, and subependymal expansions. Although the functional significance is unknown, the latter two features are correlated with habitation of the deep subzero waters of the Antarctic shelf. J. Morphol. 237:213–236, 1998. © 1998 Wiley-Liss, Inc.     

Anatomy and histology of the brain and sense organs of the Antarctic eel cod Muraenolepis microps (Gadiformes; Muraenolepididae)

Brain regions, cranial nerves, and sense organs in Muraenolepis microps, an Antarctic gadiform fish, were examined to determine which features could be attributed to a gadiform ancestry and which to habitation of Antarctic waters. We found that the central nervous system and sense organs are well developed, showing neither substantial regression nor hypertrophy. A detailed drawing of the brain and cranial nerves is provided. The rostral position of the olfactory bulbs and telencephalic size and lobation are common for the order. The optic tectum and corpus cerebelli are smaller than in most other gadiforms. The shape of the corpus cerebelli is not distinctive among gadiforms. The lateral line region is moderately well-developed, but not hypertrophied to the extent seen in deep-sea gadiforms. As is the case in gadids possessing barbels and elongated pelvic rays, Muraenolepis has well-developed facial lobes, although these are smaller and more laterally positioned. The vagal lobes are deeply placed in the rhombencephalon and project into the fourth ventricle. The brain of Muraenolepis resembles that of a phyletically derived gadoid, especially a phycid, more than it resembles the brain of a phyletically basal macrourid. Two histological features of the diencephalon of Muraenolepis appear to be unique among gadiforms: a well-organized thalamic central medial nucleus and subependymal expansions. Muraenolepis has a pure rod retina like many deep-sea species but lacks the superimposed layers of rod outer segments. The histology of the nonvisual sense organs, especially the olfactory and external taste systems, are well-developed in Muraenolepis but not hypertrophied. We relate our findings to what is known about neural morphology in other gadiforms and in phyletically distant notothenioids and liparids that are sympatric with Muraenolepis on the Antarctic shelf. The only feature that reflects an Antarctic existence is the diencephalic subependymal expansions, which within notothenioids mirror the habitation of cold waters and have been found in every Antarctic species examined to date. Although the waters of the Antarctic shelf are cold, dark, and deep, brain and sense organ morphology in Muraenolepis are remarkably free of extreme specialization.     

Anatomy and histology of the brain and sense organs of the antarctic plunderfish Dolloidraco longedorsalis (Perciformes: Notothenioidei: Artedidraconidae), with comments on the brain morphology of other artedidraconids and closely related harpagiferids

In the high-latitude shelf waters of Antarctica, fishes in the perciform suborder Notothenioidei dominate the fish fauna and constitute an adaptive radiation and a species flock. The 25 species of notothenioid plunderfishes, comprising four genera of the family Artedidraconidae, contribute substantially to fish species diversity on the high Antarctic shelf. A mental barbel is an autapomorphy for the family. Dolloidraco longedorsalis is the most abundant artedidraconid at depths over 400 m in these waters. In this article we present the anatomy and histology of the brain and special sense organs of Dolloidraco and compare it to the brains of other artedidraconids, closely related harpagiferids, and more generally to other notothenioids. We provide a detailed drawing of the brain and cranial nerves. The brain of Dolloidraco is simple, without external hypertrophy of sensory or motor regions, but contains several unusual features associated with the ventricular system and CSF, including well-developed circumventricular organs, subependymal expansions, and subarachnoid cisterns; and a ventricle in the corpus cerebellum. The brain of Dolloidraco also contains a lobed chief sensory nucleus of the trigeminal nerve that is correlated across species with barbel length. The eyes are large and contain a small choroid rete, a structure previously thought to be absent from members of this family. We document the histology of the duplex retina, olfactory apparatus, cutaneous taste buds, and barbel musculature and innervation. We discuss the role of pedomorphy in producing simplified brain morphologies. We consider the possibility that Dolloidraco is a somatosensory specialist-an unusual feature among vertebrates-and decide that this is unlikely.     

Diversification of brain morphology in antarctic notothenioid fishes: Basic descriptions and ecological considerations

The Notothenioidei, a perciform suborder of 120 species, dominates the ichthyofauna of the Southern Ocean around Antarctica. Unlike most teleost groups, notothenioids have undergone a corresponding ecological and phyletic diversification and therefore provide an excellent opportunity to study the divergence of the nervous system in an unusual environment. Our goal is to evaluate notothenioid brain variation in light of this diversification. To provide a baseline morphology, we examine the gross morphology and histology of the brain of Trematomus bernacchii, a generalized member of the family Nototheniidae. We then examine the variation in brain gross anatomy (32 species) and histology (10 species) of other notothenioids. Our sample represents about 27% of the species in this group and includes species from each of the six families, as well as species representing diverse ecologies. For comparison we reference the well-studied brains of two species of temperate perciformes (Perca flavescens and Lepomis humilis). Our results show that, in general, notothenioid brains are more similar to the brains of temperate perciforms than to the unusual brains of cave-dwelling and deep-sea fishes. Interspecific variation in gross brain morphology is comparable to that in Old World cyprinids and is illustrated for 17 species. Variation is especially noteworthy in the ecologically and geographically diverse family Nototheniidae. Measurements indicate that sensory regions (olfactory bulbs, eminentia granularis, and crista cerebellaris) exhibit the most pronounced variation in relative surface area. Association areas, including the corpus cerebelli and the telencephalon, exhibit moderate variation in size, shape, and lobation patterns. Regulatory areas of the brain, including the saccus vasculosus and the subependyma of the third ventricle, are also variable. These regions are best developed in species living in the subfreezing water close to the continent. In some species the expanded ependymal lining forms ventricular sacs, not previously described in any other vertebrate. Three species, including two nototheniids (Eleginops maclovinus and Pleuragramma antarcticum) and the only artedidraconid in our sample, have distinctive brains. The unique brain morphology of Pleuragramma is probably related to a sensory (lateral line) specialization for feeding. Within the Nototheniidae, a phyletic effect on cerebellar morphology is evident in the Coriiceps group and in the Pleuragramminae. Neither phyletic position nor ecological factors (water temperature, position in the water column, dietary habits) alone fully expalin the pattern of notothenioid brain diversification. © 1995 Wiley-Liss, Inc.     

Nervous and sensory system correlates of an epibenthic evolutionary radiation in antarctic notothenioid fishes, genus Trematomus (Perciformes; Nototheniidae)

The perciform suborder Notothenioidei consists of 120 species, with 94 confined to the Antarctic Region of the Southern Ocean. On the Antarctic shelf, this phyletic radiation has been accompanied by a substantial morphological and ecological diversification towards a pelagic existence. For example, the primarily benthic genus Trematomus contains an epibenthic radiation that includes T. loennbergii, T. lepidorhinus, and T. eulepidotus. By comparing these epibenthic species with three congeneric benthic species (T. scotti, T. pennellii, and T. bernacchii) we tested three null hypotheses regarding brain variation in Antarctic trematomids: 1) that there is no difference in brain morphology among the six species; 2) that phylogenetic and ecological factors do not influence brain morphology; and 3) that peripheral sensory structures do not influence brain morphology. We rejected each of these hypotheses, leading us to conclude that Trematomus brains vary interspecifically, between benthic and epibenthic species, and with a species' depth distribution. Further, we conclude that brain variation is correlated with differences in peripheral sensory systems and motor activity. Specifically, epibenthic Trematomus have larger percentages of their brain volume devoted to lateral line mechanoreceptive and motor (cerebellar) structures. Species living at greater depths have low ratios of cones:rods in the retina and larger olfactory structures.     

The brain organization of butterflyfishes

Synopsis The encephalization indices of angelfishes (Pomacanthidae) and butterflyfishes (Chaetodontidae) are typical of advanced perciform fishes: both families lie in the upper part of the polygon of teleost indices. The chaetodontids seem to be a little more encephalized than pomacanthids. The general morphology of the brains in both families is very similar: small olfactory bulbs, large optic tectum and a cerebellum which covers the brain structures in front of it like a cap. This morphology is shared by another family of the coral reef biotope, the Acanthuridae. The histological architecture is also typical of advanced teleosts, with a cortex-like pallium, a laminated nucleus geniculatus (= pretectalis superficialis), a complex valvula cerebelli and a corpus glomerulosum with a clear neuropile centre. The quantitative analysis of the main subdivisions of the brain, either from relative volumes or from indices, shows small olfactory bulbs (microsmy) but important telencephalic and diencephalic centres, large tectal centres (vision) and large cerebellum (precise locomotion). Many of these peculiarities are shared by other fishes inhabiting coral reefs. The differences between the two families seem to be primarily correlated with food habits: the angelfishes, which are sponge-feeders and may have an overweight due to the ballast of the sponge-skeleton in their digestive tract, and which do not need either such good vision or such precise locomotion to pick up their prey, could be a little less encephalized than the butterflyfishes.     

Comparison of response distance to prey via the lateral line in the ruffe and yellow perch

The ruffe Gymnocephalus cernuus and the yellow perch Perca flavescens (both Percidae), have very different cephalic lateral line systems. The ruffe, which is nocturnal and frequents turbid water, has a cephalic lateral line with very wide canals, large neuromasts, and membranes covering the canal openings. This anatomy is convergent with that of many deep-sea fishes. The yellow perch has a lateral line composed of neuromasts enclosed in narrow canals freely open to the water. This anatomy is typical of active, diurnal, shallow-water fishes. Laboratory experiments in the dark using infra-red video equipment revealed that the ruffe detects Daphnia magna (Crustacea: Daphnidae) and the mayfly Hexagenia limbata (Insecta: Ephemeridae) at a greater distance than the yellow perch and that it also swims faster whilst searching for prey. The swimming of the ruffe consists of a thrust by the pectoral and caudal fins, followed by a glide, the prey being detected during the glide. It is suggested that the membranes over the openings in the ruffe's lateral line function to eliminate self-generated laminar flow 'noise' from reaching the neuromasts.     

Osteology and ontogeny of the wrymouths,genus Cryptacanthodes (Cottiformes: Zoarcoidei: Cryptacanthodidae)

The four species included in the family Cryptacanthodidae are eel‐like, burrowing fishes distributed in the cold‐temperate coastal waters of the North Pacific and the western North Atlantic. This study describes the osteology and aspects of the ontogenetic skeletal development of two species, Cryptacanthodes maculatus from the western North Atlantic and C. aleutensis from the eastern North Pacific. We discuss the relationships of Cryptacanthodidae among other zoarcoid families. The Cryptacanthodidae have been previously included in the Stichaeidae, but removed and classified as a separate family based on the skull, pectoral radial, and cephalic lateral‐line morphology. Our observations (similarities in gill arch and pectoral girdle morphology; specifically, a thin sheet‐like flange of bone from the posterior margin of the supracleithrum) suggest a close relationship to at least some of the members of the family Stichaeidae. J. Morphol. 276:185–208, 2015. © 2014 Wiley Periodicals, Inc.     

Fish Brains: Evolution and Anvironmental Relationships

Fish brains and sensory organs may vary greatly between species. With an estimated total of 25 000 species, fish represent the largest radiation of vertebrates. From the agnathans to the teleosts, they span an enormous taxonomic range and occupy virtually all aquatic habitats. This diversity offers ample opportunity to relate ecology with brains and sensory systems. In a broadly comparative approach emphasizing teleosts, we surveyed classical and more recent contributions on fish brains in search of evolutionary and ecological conditions of central nervous system diversification. By qualitatively and quantitatively comparing closely related species from different habitats, particularly cyprinids and African cichlids, we scanned for patterns of divergence. We examined convergence by comparing distantly related species from similar habitats, intertidal and deep-sea. In particular, we asked how habitats relate to the relative importance of different sensory faculties. Most fishes are predominantly visually orientated. In addition, lateral line and hearing are highly developed in epi- and mesopelagic species as well as in the Antarctic notothenoids. In bathypelagics, brain size and the lobes for vision and taste are greatly reduced. Towards shallow water and deep-sea benthic habitats, chemosenses increase in importance and vision may be reduced, particularly in turbid environments. Shallow tropical marine and freshwater reefs (African lakes) enhance visual predominance and appear to exert a considerable selection pressure towards increased size of the (non-olfactory)telencephalon. The development of cognitive skills (spatial learning, problem solving) in fish seems to be associated with visual orientation and well-structured habitats.     

The Lepocreadiidae Odhner, 1905 (Digenea) of fishes from the north-east Atlantic: summary paper,with keys and checklists

Results published in seven previous papers on the Lepocreadiidae are summarised and keys are given to the 19 species of lepocreadiid parasite from north-eastern Atlantic fishes. Diagnoses for the family and the two subfamilies represented in the north-eastern Atlantic are presented. Parasite-host and host-parasite lists, arranged in taxonomic order, are given. The NE Atlantic fauna is dominated numerically by members of the subfamily Lepidapedinae, which occur most frequently in gadiform fishes, often in the deep-sea.The three lepocreadiine forms are parasites of cosmopolitan fishes or fish families that predominate in warm waters.     

Evolution and Diversification of Antarctic Notothenioid Fishes

Antarctica supported fossil ichthyofaunas during the Devonian,Jurassic, Cretaceous and Eocene/Oligocene. These faunas arenot ancestral to each other, nor are they related to any componentof the modern fauna. About one hundred species of notothenioidsdominate a modern fauna of over 200 species of bottom fishes.This highly endemic perciform suborder is not representedinthe fossil record of Antarctica. Notothenioids may have evolvedin situ on the margins of the Antarctic continent while graduallyadapting to cooling conditions during the Tertiary. Cladisticstudies indicate that notothenioids are a monophyletic group,but a sister group has not been identified among perciform fishes.With relatively few non-notothenioid fishes in Antarctic waters,notothenioids fill ecological roles normally occupied by taxonomicallydiverse fishes in temperate waters. There are six notothenioidfamilies: Bovichtidae, Nototheniidae, Harpagiferidae, Artedidraconidae,Bathydraconidae and Channichthyidae. Aspects of theirbiologyare briefly considered with emphasis on the Nototheniidae, themost speciose family. Evolutionary diversification within thisfamily allows recognition of species which are pelagic, cryopelagic,benthopelagic and benthic.     

Pressure-adaptive differences in proteolytic inactivation of M4-lactate dehydrogenase homologues from marine fishes

The inactivation by hydrostatic pressure of muscle-type lactate dehydrogenase (M4-LDH, EC 1.1.1.27; L-lactate: NAD+ oxidoreductase) homologues from five shallow-living and six deep-living marine teleost fishes was compared. The pressures which inactivate these enzymes are much higher than the pressures experienced by any of the species. To determine whether hydrostatic pressure effects on protein aggregation state and conformation might influence proteolysis, the inactivation of LDH by the proteases, trypsin (EC 3.4.21.4) and subtilisin (EC 3.4.4.16) was determined at atmospheric pressure and 1,000 atm pressure. At 10 degrees C and atmospheric pressure, the enzymes of the shallow-living fishes are inactivated four times faster by trypsin and three times faster by subtilisin than are the homologues of the deep-living species. At 1,000 atm pressure, the homologues of shallow-occurring fishes were inactivated 28 to 64% more than predicted from the summed effects of denaturation by 1,000 atm pressure and tryptic inactivation at atmospheric pressure. In contrast, the homologues of the deep-sea species were inactivated by trypsin 0 to 21% more than expected. At 1,000 atm, inactivation by subtilisin increased to a similar degree for enzymes from both deep- and shallow-living species. However, at 1,000 atm, the M4-LDH homologues of the deep-sea species lost less activity (55.3%) than did the homologues of the shallow species (86.4%). In comparisons made at 200 atm, a pressure typical of the habitat of the deep-occurring species, tryptic inactivation of the LDH of the shallow-living Sebastes melanops was increased 14%. No pressure inactivation of the enzyme is evident at 200 atm.(ABSTRACT TRUNCATED AT 250 WORDS)     

Divergence of brain and retinal anatomy and histology in pelagic antarctic notothenioid fishes of the sister taxa Dissostichus and Pleuragramma

The neutrally buoyant Antarctic fishes of the sister taxa Dissostichus (D. eleginoides and D. mawsoni) and Pleuragramma antarcticum diverged early in the notothenioid radiation and filled different niches in the pelagic realm of the developing Southern Ocean. To assess the influence of phylogenetic and ecological factors in shaping neural morphology in these taxa, we studied the anatomy and histology of the brains and retinae, and determined the proportional weights of brain regions. With the brain of the non‐Antarctic sister taxon Eleginops maclovinus as plesiomorphic, statistically significant departures in the brains of the two Antarctic taxa include reduction of the corpus cerebelli and expansion of the mesencephalon and medulla. Compared to Eleginops, both species also have a relatively smaller telencephalon, although this is significant only in Dissostichus. There are a number of apomorphic features in the brain of Pleuragramma including reduced olfactory nerves and bulbs, an extremely small corpus cerebelli and an expanded mesencephalon. Although there is not a significant difference in the relative weights of the medulla in the two taxa, the prominence of the eminentia granularis and bulging cap‐like appearance of the crista cerebellaris are distinctive in Pleuragramma. Brain histology of Dissostichus and Pleuragramma reflects typical perciform patterns and the two species of Dissostichus are histologically identical. Lateral compression in Pleuragramma and notable lobation in Dissostichus also contribute to differences between the taxa. Compression in Pleuragramma is attributable to convergence on an anchovy/herring body shape and to the relatively large brain in this small fish. The less prominent pattern of lobation of the telencephalon, inferior lobes and corpus cerebelli in Pleuragramma probably reflects underlying histology, specifically a reduction in cellularity of the neuropil in the nuclei and lobes. The retinal histology of Dissostichus and Pleuragramma encompasses the extremes seen in Antarctic notothenioids. Dissostichus has a thin scotopic retina with few cones and a high degree of summation. The retina of Pleuragramma is thick and cellular with many small single cones and rods and resembles that of Eleginops. Pedomorphy has not influenced brain morphology in these species but Pleuragramma has superficial neuromasts that are pedomorphic. Although Dissostichus and Pleuragramma are sympatric in the water column, their brains and retinae are highly divergent and reflect the influences of both phylogeny and ecological partitioning of the pelagic realm. Compared to Eleginops, the relatively smaller corpus cerebelli but relatively larger medulla probably indicates, respectively, reduced activity levels of notothenioids in subzero temperatures and expansion of the mechanosensory lateral line system as a supplement to vision under conditions of reduced light. Compared to Dissostichus, Pleuragramma has reduced olfactory bulbs and corpus cerebelli and an expanded mesencephalon. The reduction of the corpus to a small round knob is consistent with physiological parameters and video observations suggesting that, although pelagic, it is relatively inactive. Because mesencephalic weights also include the valvula cerebelli, the relatively large value for Pleuragramma may be attributable to its role in integration and sensorimotor coordination of information from the highly cellular duplex retina and to integration of signals from thewell‐developed octavolateralis system. The brain of Dissostichus displays considerable persistent morphology in its overall resemblance to that of Eleginops, especially the large olfactory bulbs and the relatively large caudally projecting corpus, and Dissostichus exhibits olfactory tracking ability and migratory behavior in common with Eleginops. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Two new species of Cryptocephalum n. gen. (Monogenoidea: Dactylogyridae) from the cephalic lateral line of Percichthys trucha (Perciformes: Percichthyidae) in Patagonia, Argentina

Two new species of Monogenoidea were found parasitizing the cephalic lateral line canals of Percichthys trucha (Valenciennes) (Perciformes: Percichthyidae). These species are described as members of a newly proposed genus of Dactylogyridae. Cryptocephalum n. gen. is characterized by the site of infection and the combination of the several features: ventral and dorsal anchor/bar complexes, anchors with strongly elongated shaft and recurved point, shaft and point of dorsal anchors protruding laterally from haptor, hooks with 2 subunits and with pair 5 smaller than the others; gonads overlapping; coiled male copulatory organ with counterclockwise rings, accessory piece formed by 2 distinct parts, and a tubular, sclerotized ventral vagina. C ryptocephalum petreum n. sp. is characterized by having both anchor pairs protruding laterally from haptor, male copulatory organ with a coil of 2-1/2 rings, accessory piece tweezers-shaped, and sclerotized vaginal vestibule. Cryptocephalum spiralis n. sp. has ventral anchors protruding ventrally and dorsal ones protruding laterally, male copulatory organ with a coil of 1-1/2 rings, the antero-dorsal part of the accessory piece saddle-shaped, vaginal vestibule not present, and coiled vagina. This is the first record of Dactylogyridae species parasitizing the cephalic lateral line of fishes.     

Morphology and ontogeny of multiple lateral‐line canals in the rock prickleback,Xiphister mucosus (Cottiformes: Zoarcoidei: Stichaeidae)

The structure and ontogeny of lateral‐line canals in the Rock Prickleback, Xiphister mucosus, were studied using cleared‐and‐stained specimens, and the distribution and morphology of neuromasts within lateral‐line canals were examined using histology. X. mucosus has seven cephalic canals in a pattern that, aside from four branches of the infraorbital canals, is similar to that of most teleostean fishes. Unlike most other teleosts, however, X. mucosus features multiple trunk lateral‐line canals. These include a short median posterior extension of the supratemporal canal and three paired, branching canals located on the dorsolateral, mediolateral, and ventrolateral surfaces. The ventrolateral canal (VLC) includes a loop across the ventral surface of the abdomen. All trunk canals, as well as the branches of the infraorbitals, are supported by small, dermal, ring‐like ossifications that develop independently from scales. Trunk canals develop asynchronously with the mediodorsal and dorsolateral canals (DLC) developing earliest, followed by the VLC, and, finally, by the mediolateral canal (MLC). Only the mediodorsal and DLC connect to the cephalic sensory canals. Fractal analysis shows that the complexity of the trunk lateral‐line canals stabilizes when all trunk canals develop and begin to branch. Histological sections show that neuromasts are present in all cephalic canals and in the DLC and MLC of the trunk. However, no neuromasts were identified in the VLC or its abdominal loop. The VLC cannot, therefore, directly function as a part of the mechanosensory system in X. mucosus. The evolution and functional role of multiple lateral‐line canals are discussed. J. Morphol. 276:1218–1229, 2015. © 2015 Wiley Periodicals, Inc.     

Deep-sea fishes in Canada’s Atlantic: population declines and predicted recovery times

Because of their slow growth rates, late maturity, low fecundity and long potential lifespans, deep-sea fishes are vulnerable to and theoretically slow to recover from overexploitation and bycatch. As industrial fishing moved into the deep sea, population declines were predicted and five species were shown to meet The World Conservation Union (IUCN) criteria for endangered species in Atlantic Canadian waters and two other deep-living species were listed as threatened by the Committee on the Status of Endangered Wildlife in Canada. We used data from scientific surveys to determine population trends in a 17-year time series for an additional 32 deep-sea fishes from the same geographic region. Eight species exhibited significant population declines, five increased, two were data deficient, and 17 showed no significant trends. Thus approximately 38% of the deep-sea bottom-living fishes in that well-investigated region could be at-risk, but definitive assignment to an IUCN category for most species is hampered by a lack of basic biological information, especially species specific generation times. Lack of biological information also limits efforts to determine possible recovery times, especially with respect to calculating intrinsic rates of population growth (r). For two Atlantic grenadiers (where r could be estimated using life-history parameters and standard life table techniques), the time to recovery with no fishing mortality could range from over a decade to over a century. This broad range results from the general uncertainty on life-history characteristics of these deep-sea species. Given the documented declines, the lack of basic data on life-history parameters, and the conservative assumption that recovery rates are likely to be prolonged, we argue that it is imperative to conduct additional studies pertaining to life history characteristics of deep-sea fishes and implement conservation measures in the deep sea immediately.