Abstract
A new deep-water catshark, Bythaelurus stewarti, is described based on 121 examined specimens caught on the Error Seamount (Mount Error Guyot) in the northwestern Indian Ocean. The new species differs from all congeners in the restricted distribution, a higher spiral valve turn count and in the morphology of the dermal denticles. It is distinguished from its morphologically and geographically closest congener, B. hispidus (Alcock), by the larger size (maximum size 44 vs. 39 cm TL, maturity size of males 35–39 vs. 21–28 cm TL), darker fresh coloration and dark grayish-brown mottling of the ventral head (vs. ventral head typically uniformly yellowish or whitish). Furthermore, it has a strongly different morphology of dermal denticles, in particular smaller and less elongate branchial, trunk and lateral caudal denticles that are set much less densely and have a surface that is very strongly and fully structured by reticulations (vs. structured by reticulations only in basal fourth). In addition, the new species differs from B. hispidus in having more slender claspers that are gradually narrowing to the bluntly pointed tip without knob-like apex (vs. claspers broader and with distinct knob-like apex), more spiral valve turns (11–12 vs. 8–10) and numerous statistical differences in morphometrics. A review of and a key to the species of Bythaelurus are given.
Highlights
The family Scyliorhinidae sensu lato, comprising all catsharks of the order Carcharhiniformes, is the largest family of sharks with currently 158 described and valid species
It further differs from all congeners in a higher spiral valve turn count (11–12 vs. 6–10) and in the morphology of branchial, trunk and lateral caudal-fin dermal denticles, which are loosely-spaced and not overlapping even in adult specimens of the new species, whereas they are closely-set and overlapping in all other Bythaelurus species
The most obvious difference of the new species compared to B. hispidus is the size: in B. stewarti n. sp., adult males range between 389 and 435 mm total length (TL) and juvenile males between 162 and 348 mm TL
Summary
The family Scyliorhinidae sensu lato, comprising all catsharks of the order Carcharhiniformes, is the largest family of sharks with currently 158 described and valid species (number updated from Weigmann [1,2]). The family Scyliorhinidae sensu lato has been shown to be paraphyletic and the subfamily Pentanchinae is currently considered a valid family, sister to the family Scyliorhinidae sensu stricto. Both families are distinguished by the absence vs presence of a supraorbital crest [3]. Bythaelurus species are small to medium-sized catsharks (maximum total lengths from 30 cm to 76 cm) that live in deep water on continental and insular slopes in temperate and tropical latitudes of the Indian and Pacific oceans, between depths of 200 and 1443 m [1,4,5]. Intrageneric differences in general morphology and body shape, the presence or absence of oral papillae, the presence or absence of a crest of enlarged dermal denticles on the anterior dorsal caudal-fin margin, and, genetics and reproductive modes have been found [4,5]
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