Spurdog

Abstract: Segregation of the sexes within a species is a widespread behavioural phenomenon in both terrestrial and aquatic animals. In the marine realm, sexual segregation is exhibited by many taxa including whales, seals, seabirds and fish. Of the latter group, sharks may be particularly appropriate model animals to test theories on the mechanisms underlying sexual segregation, because sexual segregation is a general characteristic of shark populations, with both sexually dimorphic and monomorphic species being well represented among the approximately 400 extant species (Springer, 1967; Compagno, 1999). The reproductive modes of sharks are diverse ranging from egg-laying (oviparity) to placental live-bearing (viviparity) (Wourms & Demski, 1993). Among sexually dimorphic, viviparous shark species it is generally the female that is larger than the male, whilst in some oviparous species males are larger than females. Sexually monomorphic species also occur. Therefore, sharks possess a number of characteristics that make them an interesting alternative to terrestrial animal models for investigating the causes of sexual segregation. In this chapter the prevalence and nature of sexual segregation in sharks is described and the relationship with reproductive modes is explored. Hypotheses suggested to account for sexual segregation in sharks are examined with respect to new field and laboratory behaviour studies of males and females of a monomorphic species, the lesser spotted dogfish (Scyliorhinus canicula). The chapter concludes by drawing

Abstract: Squalus rancureliFourmanoir, 1979 and S. melanurusFourmanoir, 1979 are deep-sea sharks described from off New Caledonia and Vanuatu. Morphological separation between them is limited to caudal fin coloration and a few morphometric characters. These two nominal species were described as new in more than one taxonomic account, thus requiring nomenclatural elucidation. Taxonomic complexity behind S. rancureli and S. melanurus has been disregarded and underestimated in previous studies, and indicated to rely on propagation of erroneous authorship and negligence of requirements from the International Code on Zoological Nomenclature for describing new taxa. Analysis of type material and topotypic specimens revealed congruency of external morphology, external measurements and meristic data, suggesting conspecificity. Squalus rancureli is herein considered a junior synonym of S. melanurus, and authorship of these nominal species is clarified. Differential diagnosis and redescription of S. melanurus and an updated diagnosis of S. bucephalus Last, Seret & Pogonoski, 2007 are also provided based on new material. Comparative examinations with other regional congeners support their validity and recognition of a third species provisionally identified as Squalus cf. montalbani. Squalus melanurus is easily distinguished from S. bucephalus Last, Seret & Pogonoski, 2007 by bearing a black caudal-fin lower lobe, snout conspicuously elongate, dermal denticles tricuspidate, and smaller number of vertebrae (vs whitish caudal-fin lower lobe, snout conspicuously short, dermal denticles unicuspidate, higher number of vertebrae).

Abstract: Sources of variation in dietary composition were examined in the piked spurdog (Squalus megalops). The species is an opportunistic predator that consumes a wide range of prey items. When importance of prey was measured by weight or occurrence, S. megalops preyed largely on molluscs and teleosts. However, when number of prey was considered, the main items were crustaceans. A bootstrap analysis showed that considerable variability can be expected in the importance of prey items in the species' overall diet. Regional, seasonal, and ontogenetic differences in dietary composition were found, but there were no differences between mature and immature sharks or between males and females. The spatial and temporal variation in diet exhibited by S. megalops and the intrinsic natural variability of the dietary composition of this opportunistic predator suggest that studies that infer predator-prey interactions from overall diet are likely to miss information on the ecological relationships among species and thus account for only part of these interactions.

Abstract: Age and growth estimates of Squalus megalops were derived from the first dorsal fin spine of 452 sharks, ranging from 274 to 622 mm total length. Age bias plots and indices of precision indicated that the ageing method was precise and unbiased. Edge analysis of the enameled surface of whole spines and similarities in the banding pattern deposited in the enameled surface of spines and in spine sections supported the hypothesis of annual band formation. Multiple versions of two growth models were fitted to length-at-age data, from which a two-phase von Bertalanffy model produced the best fit. For males, the change in growth rate corresponded with size-at-maturity, whereas for females, the change was slightly before size-at-maturity. Regardless of the growth model used, growth rate of females (0.034 to 0.098 years −1 ) was very low, making S. megalops highly susceptible to overexploitation by fisheries.