Zoogeography and relationships of Australasian skates (Chondrichthyes: Rajidae)

Aim To investigate distributional patterns and derivation of skates in the Australasian realm. Location Australasia. Methods Genus‐group skate taxa were defined for this region for the first time and new systematic information, as well as bathymetric and geographical data, used to identify distribution patterns. Results The extant skate fauna of Australasia (Australia, New Zealand, New Caledonia and adjacent subAntarctic dependencies) is highly diverse and endemic with sixty‐two species from twelve currently recognized, nominal genus‐group taxa. These include the hardnose skate (rajin) groups Anacanthobatis, Amblyraja, Dipturus, Okamejei, Rajella and Leucoraja, and softnose skate (arhynchobatin) genera Arhynchobatis, Bathyraja, Insentiraja, Irolita, Pavoraja and Notoraja. Additional new and currently unrecognized nominal taxa of both specific and supraspecific ranks also occur in the region. The subfamily Arhynchobatinae is particularly speciose in Australasia, and the New Zealand/New Caledonian fauna is dominated by undescribed supraspecific taxa and species. The Australian fauna, although well represented by arhynchobatins, is dominated by Dipturus‐like skates and shows little overlap in species composition with the fauna of New Zealand and New Caledonia. Similarly, these faunas exhibit no overlap with the polar faunas of the Australian subAntarctic dependencies (Heard and Macdonald Islands) to the south. Skates appear to be absent from the Macquarie Ridge at the southern margin of the New Zealand Plateau. Their absence off New Guinea probably reflects inadequate sampling and the subsequent poor knowledge of that region's deepwater fish fauna. Main conclusions Skates appear to have existed in the eastern, Australasian sector of Gondwana before fragmentation in the late Cretaceous. The extant fauna appears to be derived from elements of Gondwanan origin, dispersal from the eastern and western Tethys Sea, and intraregional vicariance speciation.     

Sexual dimorphism of external morphological characters in some deepwater skates (Rajidae,Rajiformes, Chondrichthyes) of the North Atlantic

Our studies of the external morphology of five species of deepwater skates (Amblyraja jenseni, Bathyraja pallida, B. richardsoni, Rajella bigelowi, and R. kukujevi) showed that their males and females differ significantly in 9 characters in the first species, 14 characters in the second, 11 characters in the third and fourth, and 10 characters in the fifth. Sexual differences occur most frequently in the length of the first gill slit (in four species out of five), the length of the nasal curtain, the mouth width, and the length of the third gill slit (in three species out of five), the width and length of the disc, the length of the first and second dorsal fin bases, the tail height at the tip of the ventral fin, the length of the fifth gill slit, the distance between the first and fifth gill slits, the interspiracular distance, the preanal length, distance from the tip of the snout to the maximum disc width, and the interorbital distance (in two species out of five).     

The origin of snakes (Serpentes) as seen through eye anatomy

Snakes evolved from lizards but have dramatically different eyes. These differences are cited widely as compelling evidence that snakes had fossorial and nocturnal ancestors. Their eyes, however, also exhibit similarities to those of aquatic vertebrates. We used a comparative analysis of ophthalmic data among vertebrate taxa to evaluate alternative hypotheses concerning the ecological origin of the distinctive features of the eyes of snakes. In parsimony and phenetic analyses, eye and orbital characters retrieved groupings more consistent with ecological adaptation rather than accepted phylogenetic relationships. Fossorial lizards and mammals cluster together, whereas snakes are widely separated from these taxa and instead cluster with primitively aquatic vertebrates. This indicates that the eyes of snakes most closely resemble those of aquatic vertebrates, and suggests that the early evolution of snakes occurred in aquatic environments.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81 , 469–482.     

Observations on the electric organ discharge of two skate species (Chondrichthyes: Rajidae) and its relationship to behaviour

Synopsis The electric organ discharge (EOD) of the little skate,Raja erinacea and winter skate,R. ocellata was recorded both from isolated individuals and from small groups using methods that allowed for the identification of individuals producing EODs. Pulse duration, train lengh, frequency, and pulse patterns are characterized and correlated with behaviour. The two species,R. erinacea andR. ocellata, were found to have characteristically different EOD pulse durations of 70 ms and 217 ms respectively. Isolated skates rarely discharged whereas groups of skates were found to discharge regularly. The EOD was evoked by tactile prodding, physical contact with other skates and electrical stimulation. Skates also discharged reflexively in response to an artificially induced head-positive DC stimulus, sine wave and monopolar square pulses. During approach and contact, skates responded to each other with interacting EOD displays. EOD interaction and pulse duration differences between other species suggest a possible intra-specific communication function of the EOD inRaja.     

Biodiversity and systematics of skates (Chondrichthyes: Rajiformes: Rajoidei)

Skates (Rajiformes: Rajoidei) are a highly diverse fish group, comprising more valid species than any other group of cartilaginous fishes. The high degree of endemism exhibited by the skates is somewhat enigmatic given their relatively conserved body morphology and apparent restrictive habitat, e.g. soft bottom substrates. Skates are primarily marine benthic dwellers found from the intertidal down to depths in excess of 3,000 m. They are most diverse at higher latitudes and in deepwater, but are replaced in shallower, warm temperate to tropical waters by stingrays (Myliobatodei). The number of valid skate species has increased exponentially, with more species having been described since 1950 (n = 126) than had been described in the previous 200 years (n = 119). Much of the renaissance in skate systematics has largely been through the efforts of a few individuals who through author–coauthor collaboration have accounted for 78 of the 131 species described since 1948 and for nine of 13 genera named since 1950. Furthermore, detailed regional surveys and accounts of skate biodiversity have also contributed to a better understanding of the diversity of the skates. A checklist of the living valid skate species is presented.     

Electric organs in skates: Variation and phylogenetic significance (Chondrichthyes: Rajoidei)

A total of 63 species of skates (Chondrichthyes: Rajoidei) were surveyed, along with three species of the outgroup (Chondrichtyes: Rhinobatoidei) for electric organs along the sides of the tail. All skate specimens examined possessed what appeared to be functional electric organs, and the three species of the outgroup lacked evidence of electric organs. The electric organs were tail-positive and arranged into horizontal columns divided by transverse septa. The electrocytes varied considerably within and among supraspecific taxa (subgenera and genera), but they could be broadly classified into cup-shape, modified cup-shape, intermediate-shape, and disc-shape cells, provided that the distinction was partially based on position of the electrocytes within their connective tissue chambers. The survey, in part, corroborates a phylogenetic hypothesis of skates and in some respects further resolves the hypothesis. The supraspecific taxa Atlantoraja and Rioraja have similar derived-type electrocytes, as do the five supraspecific taxa of Rajini, and Cruriraja and Anacanthobatis, and to a lesser extent the supraspecific taxa Arhynchobatis, Psammobatis, and Sympterygia, and the supraspecific taxa Notoraja, Pavoraja, and Pseudoraja, corroborating the hypothesis. The supraspecific taxa Amblyraja, Rajella, Leucoraja, Breviraja, and Dactylobatus were unresolved in the phylogenetic hypothesis, but the electrocyte survey suggested that Leucoraja, Breviraja, and Dactylobatus were derived with respect to Amblyraja and Rajella. © 1994 Wiley-Liss, Inc.     

Abnormal hermaphroditism in the multispine skate Bathyraja multispinis (Chondrichthyes, Rajidae)

A hermaphroditic individual of the multispine skate, Bathyraja multispinis , caught on the Argentinean continental shelf, is described. This is the first record of hermaphroditism in the genus Bathyraja and one of the very few hermaphroditic specimens reported in the Rajidae.     

Morphology and phylogenetic significance of the pectoral articular region in elasmobranchs (Chondrichthyes)

The morphology of the articular region of the scapulocoracoid and the basal cartilages of the pectoral fin endoskeleton of elasmobranchs is reviewed in detail. Examination of this specific morphology in more than 140 species of elasmobranchs (of which 40 are reported here) revealed characters that may have a bearing on the higher‐level phylogeny of the group. Ten distinct characters of the scapular articular region of elasmobranchs are described, varying in terms of the number of distinct articular sites as well as their specific morphology (e.g. whether the articular surface is composed of condyles and/or facets). Previous interpretations of the articular region in morphological phylogenies are also reviewed, revealing much more morphological variation than formerly reported. These prior characters played an important role in supporting the Hypnosqualea, and may still be derived for this clade. The variation and distribution of the new characters discussed provide new insights for the evolution of the pectoral endoskeleton in chondrichthyans. They also highlight the continued importance of morphological characters for phylogenetic studies, and reinforce the necessity of in‐depth anatomical reviews of certain characters employed in previous higher‐level phylogenetic studies of elasmobranchs. © 2015 The Linnean Society of London     

Body plan convergence in the evolution of skates and rays (Chondrichthyes: Batoidea)

Skates, rays and allies (Batoidea) comprise more than half of the species diversity and much of the morphological disparity among chondrichthyan fishes, the sister group to all other jawed vertebrates. While batoids are morphologically well characterized and have an excellent fossil record, there is currently no consensus on the interrelationships of family-level taxa. Here we construct a resolved, robust and time-calibrated batoid phylogeny using mitochondrial genomes, nuclear genes, and fossils, sampling densely across taxa. Data partitioning schemes, biases in the sequence data, and the relative informativeness of each fossil are explored. The molecular phylogeny is largely congruent with morphology crownward in the tree, but the branching orders of major batoid groups are mostly novel. Body plan convergence appears to be widespread in batoids. A depressed, rounded pectoral disk supported to the snout tip by fin radials, common to skates and stingrays, is indicated to have been derived independently by each group, while the long, spiny rostrum of sawfishes similarly appears to be convergent with that of sawsharks, which are not batoids. The major extant batoid lineages are inferred to have arisen relatively rapidly from the Late Triassic into the Jurassic, with long stems followed by subsequent radiations in each group around the Cretaceous/Tertiary boundary. The fossil record indicates that batoids were affected with disproportionate severity by the end-Cretaceous extinction event.     

Standardized diet compositions and trophic levels of skates (Chondrichthyes: Rajiformes: Rajoidei)

Skates by virtue of their abundance and widespread occurrence appear to play an influential role in the food webs of demersal marine communities. However, few quantitative dietary studies have been conducted on this elasmobranch group. Therefore, to better understand the ecological role of skates, standardized diet compositions and trophic level (TL) values were calculated from quantitative studies, and compared within and among skate and shark taxa. Prey items were grouped into 11 general categories to facilitate standardized diet composition and TL calculations. Trophic level values were calculated for 60 skate species with TL estimates ranging from 3.48 to 4.22 (mean TL = 3.80 ± 0.02 SE). Standardized diet composition results revealed that decapods and fishes were the main prey taxa of most skate species followed by amphipods and polychaetes. Correspondingly, cluster analysis of diet composition data revealed four major trophic guilds, each dominated by one of these prey groups. Fish and decapod guilds were dominant comprising 39 of 48 species analyzed. Analysis of skate families revealed that the Arhynchobatidae and Rajidae had similar TL values of 3.86 and 3.79 (t-test, P = 0.27), respectively. The Anacanthobatidae were represented by a single species, Cruriraja parcomaculata, with a TL of 3.53. Statistical comparison of TL values calculated for five genera (Bathyraja, Leucoraja, Raja, Rajella, Rhinoraja) revealed a significant difference between Bathyraja and Rajella (t-test, P = 0.03). A positive correlation was observed between TL and total length (L _T) with larger skates (e.g. >100 cm L _T) tending to have a higher calculated TL value (>3.9). Skates were found to occupy TLs similar to those of several co-occurring demersal shark families including the Scyliorhinidae, Squatinidae, and Triakidae. Results from this study support recent assertions that skates utilize similar resources to those of other upper trophic-level marine predators, e.g. seabirds, marine mammals, and sharks. These preliminary findings will hopefully encourage future research into the trophic relationships and ecological impact of these interesting and important demersal predators.     

First record of dicephalia in a bull shark Carcharhinus leucas (Chondrichthyes: Carcharhinidae) foetus from the Gulf of Mexico,U.S.A.

The first recorded incidence of dicephalia in a bull shark Carcharhinus leucas is reported from a foetus collected by a fisherman in the Gulf of Mexico near Florida, U.S.A. External examination, Radiography and magnetic resonance imaging revealed a case of monosomic dicephalia where the axial skeleton and internal organs were found to divide into parallel systems anterior to the pectoral girdle resulting in two well‐developed heads.     

Ecomorphology of the digestive tract of the brazilian electric ray Narcine brasiliensis (Olfers, 1831) (Torpediniformes: Narcinidae)

Elasmobranchii shows particular characteristics that make them susceptible to the impacts caused by fishing pressure, especially bottom trawling, which mostly affect rays, as they have demersal habits. Based on food availability and feeding ability, Polychaeta and Sipunculiforme are the bases of Brazilian electric ray Narcine brasiliensis diet. Morphologically, the digestion starts in the oesophagus with acid mucines action. The existence of acid and neutral mucines in the posterior intestine prevents lesions in the mucosa and favours the passage of faeces and shell residues. Thus, this study shows the interaction between their habitat, the organisms and their morphophysiology.     

Meiotic chromatin diminution in a vertebrate,the holocephalan fish Hydrolagus collie (Chondrichthyes,Holocephali)

A histochemical, microdensitometric, and electron microscopic study of testes of the ratfish Hydrolagus colliei shows that an instance of the rare phenomenon of germ line chromatin diminution occurs in this vertebrate species. In primary spermatocytes at metaphase I a spherical mass of heterochromatin accumulates at one side of the metaphase plate. At anaphase I the heterochromatic mass is left in the equatorial cytoplasm and is passed into one of the two secondary spermatocytes formed during cytokinesis. As nuclear membranes are being restored, a double membrane envelope is also formed around the heterochromatic mass, which is then termed the ‘chromatin diminution body’ (CDB). At second meiotic division the CDB is included in the cytoplasm of one of the four spermatids and retained there, apparently unchanged, until mid-spermiogenesis. At that time the CDB becomes adherent to the spermatid plasma membrane and is pinched off from the spermatid by a process of apocrine exocytosis, taking a layer of spermatid plasma membrane along with it. Simultaneously this tri-membrane CDB is taken into the adjacent Sertoli cell by endocytosis, thereby acquiring a fourth membrane layer, a part of the Sertoli cell plasma membrane. The CDBs are subsequently phagocytized, possibly first fusing with dense, multilaminate bodies in the Sertoli cell cytoplasm. The CDB chromatin mass is strongly positive with the Feulgen method for DNA and the alkaline fast green method for histones. Microdensitometric analysis shows that the discarded chromatin amounts to about 10% of the diploid nuclear content and that it appears to be part of the normal diploid complement rather than DNA amplified during meiosis.     

A new species of electric knifefish,genus Compsaraia (Gymnotiformes: Apteronotidae) from the Amazon River,with extreme sexual dimorphism in snout and jaw length

Abstract

A new species of the neotropical electric fish genus Compsaraia is described from the western Amazon of Peru and Brazil. Compsaraia samueli is distinguishable from all other apteronotids by sexual dimorphism in which mature males exhibit extreme elongation and slenderness of the snout and jaws. Compsaraia samueli is readily distinguishable from its only congener, C. compsa, by more caudal‐fin rays (17–18 vs. 13–16), a shorter caudal peduncle (mean length 9% vs. 34% body length to end of anal fin), a less tapering body shape in lateral profile (mean ratio of body depth at origins of anal fin and dorsal organ 93% vs. 75%), and a smaller maximum adult body size (230 vs. 305 mm). The genus Compsaraia is readily separated from other apteronotids by a pale antorbital stripe and a pale L‐shaped patch over the supra‐temporal canal. The phylogenetic position of C. samueliis estimated by inclusion in a previously published data matrixof osteological and other morphological characters. Comparisons of the cranial bones in apteronotids shows the derived morphology of C. samueli to be a composite of three developmentally and phylogenetically discrete characters: (1) positive allometric growth before sexual maturity in both sexes of the pre‐orbital region of the neurocranium, (2) positive allometric growth of the (oral) jaws, and (3) secondary sexual dimorphism of snout and jaw morphology. The genus Compsaraia represents one of at least three phylogenetically independent cases of snout elongation and one of at least four cases of jaw elongation within the Apteronotidae. Compsaraia samueli also represents one of at least four cases of secondary sexual dimorphism in snout and jaw length within the Apteronotidae. The phylogenetic distribution of snout and jaw characters within the Apteronotidae suggests the influence of both sexual and trophic functional influences on the evolution of head morphology.     

A reclassification of the anomopod families Macrothricidae and Chydoridae,with the creation of a new suborder,the Radopoda (Crustacea: Branchiopoda)

An investigation, using optical microscopy and SEM, of the trunk limbs of the Anomopoda has revealed a large number of characters, previously underused or unused in taxonomy and comparative morphology. All these characters, which are nicely paralleled by some more conventional traits (head shield and pores, postabdomen, antennae ...), show one clear tendency across all groups studied: a state of complexity at one extreme, and a state of often incisive simplification at the other extreme, with a number of transitional stages in between. The complex character state, which itself is a simplification of the leg structure of the Ctenopoda and other, large Branchiopoda, is here considered to represent a primitive condition. The simplified state is considered advanced. Based on this assumption, we list a number of unifying characters (mainly structural aspects of P1 and P2, but also the gnathobase of P3 and P4) for all macrothricid and chydorid-like anomopods, which we unite in the new suborder Radopoda. Non-radopod Anomopoda are not reclassified. We then derive a cascade of (mainly trunk-limb based) characters to work out a hypothesis on the evolution of the Radopoda. The chydorid line (basically the former family Chydoridae) is classified as a superfamily (the Eurycercoidea), with three families; the macrothricid line is capped by the superfamily Macrothricoidea, with four families. Of these seven families, four are upgraded from subfamily status, the Chydoridae are left status quo, the Macrothricidae are redefined, and the Neothricidae are a new family. The Macrothricidae are further subdivided in two subfamilies, of which the Macrothricinae appear reasonably homogeneous (monophyletic), while the non-Macrothricinae require further study. Some of these (e.g. Guernella) have almost completely lost their P5, a situation parallel to that of the P6 in the Eurycercidae, Acantholeberidae, and Ophryoxidae.     

Microstructural morphology in early dermal denticles of hammerhead sharks (Elasmobranchii: Sphyrnidae) and related taxa

Mello, W.C., de Carvalho, J.J., Brito, P.M.M. 2011. Microstructural morphology in early dermal denticles of hammerhead sharks (Elasmobranchii: Sphyrnidae) and related taxa. —Acta Zoologica (Stockholm) 00 : 1–7. This study uses scanning electron microscopies to investigate and describe the microstructural diversity of dermal denticles in the family Sphyrnidae, which comprises all living hammerhead shark species, comparing them to other related taxa (i.e. Carcharhinus dussumieri, Carcharhinus plumbeus, Carcharhinus acronotus, Rhizoprionodon acutus, Negaprion brevirostris and Hemigaleus microstoma). The results reveal that sphyrnids present noticeable microstructures in the dermal denticles, distinguishing them from the other related species investigated. Additionally, scale patterns are the same in three distinct body regions (i.e. cephalic, branchial and dorsal fin). Species of Sphyrnidae that reach bigger total lengths and that are widely distributed (i.e. Sphyrna lewini and Sphyrna mokarran) presented more, smaller and nearly hexagonal microstructures that do not cover the entire scale surface, unlike species reaching smaller sizes and restricted to coastal habits (i.e. Sphyrna tiburo, Sphyrna tudes, Sphyrna media and Eusphyra blochii). The sphyrnid scales are similar to R. acutus and C. dussumieri rather than to the other species, but it is not possible to identify the sphyrnid species only by scale features. It is clear that a similar morphology of scales is not necessarily related to similar life habits, and that they are candidates to provide new characters in phylogenetical studies among sphyrnids.