Bythaelurus
Creators: name: Weigmann, Simon
name: Ebert, David A.
name: Clerkin, Paul J.
name: Stehmann, Matthias F. W.
name: Naylor, Gavin J. P.
Publisher: name: Zenodo
Year: 2016
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Elasmobranchii
Carcharhiniformes
Scyliorhinidae
Bythaelurus

description: Review of Bythaelurus species
The 12 (including B. bachi n. sp.) currently valid species of Bythaelurus are found in water deeper than 200 m in the Indian and Pacific Oceans (Weigmann 2016). Geographically, the western Indian Ocean appears to be a hotspot of Bythaelurus species diversity with seven of the 12 currently valid species occurring in this area. Six of the seven species, i.e. B. alcockii, B. bachi n. sp., B. clevai, B. lutarius, B. naylori and B. tenuicephalus, are found exclusively in this area, whereas the seventh species, B. hispidus, is also known from the eastern Indian Ocean (Weigmann 2016).
The type species of Bythaelurus is B. canescens, which is known from the southeastern Pacific Ocean from off Peru and Chile to the Straits of Magellan (Ebert et al. 2013). It is a common bycatch in demersal trawl and longline fisheries in central and southern Chile (Concha et al. 2010). The species is plain dark brown or blackish above and below, has distinct labial furrows, with lowers noticeably longer than uppers, an anal-fin base length less than 1.5 times second dorsal-fin base length, matures at 52–59 cm TL and reaches a maximum size of 73 cm TL. The second species reported from the southeastern Pacific Ocean is B. giddingsi McCosker, Long & Baldwin, 2012, which may be endemic to the Galapagos Islands (McCosker et al. 2012). It is distinguishable from all congeners by having very few tooth rows per jaw (20–26 vs. 53–111) and its striking coloration: chocolate brown dorsally with pale spots, the largest being about equal in size to eye diameter above midline, smaller below; posterior margin of dorsal, pectoral, and pelvic fins pale; ventral surface pale. Additionally, it has a dorsal caudal-fin margin with prominent crest of comb-like dermal denticles. From the western Pacific Ocean, two species have been recorded, B. dawsoni and B. immaculatus (Chu & Meng, 1982). Bythaelurus dawsoni is apparently endemic to the waters around New Zealand (Francis 2006). It is light brown to gray on dorsal and lateral surfaces with a line of white spots on sides of small individuals and whitish ventrally, has fin tips with broad white areas and dark bands on the caudal fin. The labial furrows are distinct, with lowers noticeably longer than uppers, the anal-fin base length is less than 1.5 times second dorsal-fin base length, the size at maturity is 32–38 cm TL and the maximum size is 42 cm TL. Bythaelurus immaculatus is known only from the three type specimens, caught in the South China Sea (White & Last 2013). This species has a plain dark yellowish brown coloration, reduced labial furrows, with uppers and lowers about equal in length, an anal-fin base length less than 1.5 times second dorsal-fin base length, a prevent length 1.3 times in tail length, a preorbital snout length subequal to eye length, a length of lateral trunk denticles less than twice their width and reaches a maximum size of 76 cm TL (White & Last 2013), representing the largest known species of Bythaelurus (Weigmann 2016). For the eastern Indian Ocean, two species have been reported: B. incanus Last & Stevens, 2008 and B. hispidus. Bythaelurus incanus is known only from the holotype, a juvenile male collected off the Ashmore Terrace, western Australia. This species has a mostly plain grayish brown coloration with a few white blotches on belly, reduced labial furrows, with uppers and lowers about equal in length, an anal-fin base length less than 1.5 times second dorsal-fin base length, a pre-vent length exceeding tail length, a preorbital snout length 1.3 times eye length and a length of lateral trunk denticles more than twice their width (Last & Stevens 2008). Bythaelurus hispidus is known from the eastern and western Indian Ocean with records from off Kenya (three SAIAB specimens listed under Comparative material), Socotra Islands (uncatalogued specimens at ZMH), Yemen (al Sakaff & Esseen 1999), Oman (L. Jawad, pers. comm., 2013), southern India (Nair & Appukuttan 1973; Nair & Lal Mohan 1973; Appukuttan & Nair 1988; Raje et al. 2002; Akhilesh et al. 2013), Andaman Islands (Alcock 1891; Springer & D’Aubrey 1972; Springer 1979; Séret 1987; Kaschner et al. 2015), and off Myanmar (T. Krajangdara & P.N. Psomadakis, pers. comm., 2015). This species has 5–6 indistinct, dark blotches on trunk and tail, an anal-fin base more than 1.5 times second dorsal-fin base length, rather slender (base width ~1.5% TL) adult claspers with knob-like apex, a maturity size of 22–24 cm TL and a maximum size of 32 cm TL. In addition to B. hispidus, six further species of Bythaelurus are described from the western Indian Ocean, i.e. B. alcockii, B. bachi n. sp., B. clevai, B. lutarius, B. naylori and B. tenuicephalus. Bythaelurus alcockii was described from the Arabian Sea without exact locality data and is known only from the holotype, which has been lost (Compagno 1984b; K.V. Akhilesh, pers. comm., 2014). For B. alcockii, a blackish coloration with hoary gray surface and some fins white-tipped posteriorly, as well as teeth with cusps and lateral cusps of subequal length were described (Alcock 1899). However, its validity is questionable (e.g. Springer 1979; Compagno 1984b, 1988, 1999, 2005; Compagno et al. 2005; Last & Stevens 2008; Ebert et al. 2013; Kaschner et al. 2015; Weigmann 2016). It was originally described as Halaelurus alcockii and preliminarily placed in the subgenus Bythaelurus by Compagno (1988) but earlier Compagno (1984b) had also stated that the species might instead belong to the genus Apristurus. As the only known specimen has been lost, it is currently impossible to resolve this issue. Bythaelurus bachi n. sp. is known only from the southern Madagascar Ridge. The new species is distinguished from all congeners by the plain beige to light gray-brown coloration, high diversity in dermal denticle morphology and presence of composite oral papillae. It has a maturity size of 36–40 cm TL and reaches a maximum size of 45 cm TL. Bythaelurus clevai is apparently endemic to the waters around Madagascar (Séret TABLE 3. Maximum sizes, geographic and depth distributions, reproductive modes, as well as vertebral, tooth row and spiral valve counts of Bythaelurus species.
Species Maximum Geographic Depth Reproductive Tooth row counts Vertebral counts Spiral valve total length distribution distribution mode counts
upper jaw lower jaw monospondylous diplospondylous total caudal total
precaudal precaudal
Bythaelurus alcockii (Garman, 1913) small WIO 1134–1262 m unknown
specimen
Bythaelurus canescens (Günther, 1878) 73 cm SEP 237–1260 m single 80–101 2 ~60–111 2 39–43>20 43 1 82 1 40 1 117–132 24 7–8 5
oviparity
Footer: Superscript numbers in meristics indicate numbers of specimens on which the values are based. * In Table 2 of McCosker et al. (2012), individual values for diplospondylous precaudal count indicate a maximum of 44, but range indicates 43 as maximum count. Data sources: data of Bythaelurus bachi from the present study, sources for all other species: maximum total length and geographic distribution from Weigmann (2016) except for maximum total length of B. hispidus based on comparative specimen SAIAB 13741; depth data from Weigmann (2016) except for maximum depth of B. canescens from Meléndez & Meneses (1989); reproductive modes from Francis (2006), Concha et al. (2010), Akhilesh et al. (2013) and Ebert & Clerkin (2015); counts from Springer (1971), Springer & D'Aubrey (1972), Bass et al. (1975), Springer (1979), Séret (1987), Compagno (1988), Last & Stevens (2008), McCosker et al. (2012), Ebert & Clerkin (2015) and Kaschner et al. (2015), as well as unpublished counts of comparative specimens of B. canescens and B. dawsoni. Abbreviations: EIO = eastern Indian Ocean, NWP = northwestern Pacific Ocean, SEP = southeastern Pacific Ocean, SWP = southwestern Pacific Ocean, WIO = western Indian Ocean.
1987). This species has a grayish coloration with a pattern of dark brown saddle-like markings on back and tail, with variegated dark brown blotches on flanks, is grayish with brown speckles inside of the mouth, has a whitish ventral surface, matures at 28–36 cm TL and reaches a maximum size of 40 cm TL. Bythaelurus lutarius so far has been confirmed only from off Mozambique. The records of B. lutarius from off Somalia (Springer & D’Aubrey 1972; Bass et al. 1975; Springer 1979) are based on B. tenuicephalus and an undescribed Bythaelurus species in the ZMB collection. Bythaelurus lutarius has a largely plain coloration with dusky areas near the fins, an anal-fin base more than 1.5 times second dorsal-fin base length, a maturity size of 28–31 cm TL and reaches a maximum size of 39 cm TL. Bythaelurus naylori is known only from the Southwest Indian Ridge. It has a medium to dark brown coloration with light fin edges and a distinct dark dusky-colored snout, a dorsal caudal-fin margin with prominent crest of comb-like dermal denticles, distinct labial furrows, with lowers noticeably longer than uppers, an anal-fin base length equal to or less than 1.5 times second dorsal-fin base length, matures at 38–48 cm TL and reaches a maximum size of 55 cm TL. Bythaelurus tenuicephalus is known only from off Tanzania and Mozambique. This species differs from all congeners in the narrow head without distinct lateral indention anterior to outer nostrils. Furthermore, the adult claspers are rather broad (base width ~2% TL) without knob-like apex, the species matures at ~ 28 cm TL and reaches a maximum size of ~ 30 cm TL.
The 12 currently valid species can be grouped into two general morphotypes: one consists of species with slender bodies, i.e. Bythaelurus clevai, B. hispidus, B. lutarius and B. tenuicephalus, the other includes species with stocky bodies, at least in large specimens, i.e. B. bachi, B. canescens, B. dawsoni, B. giddingsi, B. immaculatus, B. incanus and B. naylori. The body shape of Bythaelurus alcockii is unknown.
Another possible grouping arises from the presence or absence of oral papillae: species with numerous oral papillae are B. bachi, B. canescens, B. clevai, B. dawsoni, B. giddingsi, B. hispidus,? B. immaculatus, B. incanus and B. tenuicephalus, species without (or with rudimentary) oral papillae are B. lutarius and B. naylori. The presence of oral papillae in B. immaculatus is unknown but it is assumed that the species has oral papillae based on its apparently close morphological relationship to B. canescens and B. incanus. White & Last (2013) did not examine the holotype of B. immaculatus for the presence of oral papillae (W.T. White, pers. comm., 2015). Furthermore, none of the type specimens could be found upon recent requests (H.-C. Ho & X.-Y. Kong, pers. comm., 2015 & 2016) so the whereabouts of the type specimens and the presence of oral papillae remain unclear. The presence of oral papillae in Bythaelurus alcockii is unknown.
A third possible grouping arises from the reproductive modes of Bythaelurus species that was reviewed by Francis (2006). He noted that some species, e.g. B. canescens and B. dawsoni, are oviparous, whereas others are viviparous. A detailed comparison of reproductive modes, as well as maximum sizes, geographic and depth distributions, vertebral, tooth row and spiral valve counts of the 12 species of Bythaelurus can be found in Table 3.
Generally, the taxonomy and biology of Bythaelurus species are poorly known. So far, studies on the biology and distribution are restricted to few species, i.e. B. canescens, B. dawsoni, B. hispidus and B. lutarius, and partially based only on a small number of specimens examined (Nair & Appukuttan 1973; Bass et al. 1975; Springer 1979; Meléndez & Meneses 1989; Francis 2006; Valenzuela et al. 2008; Acuña & Villarroel 2010; Concha et al. 2010; Akhilesh et al. 2013; Lopez et al. 2013). Therefore, more specimens are needed of several species, especially of those from the Indian Ocean and B. immaculatus. In order to further improve the knowledge of Bythaelurus species in this area, a comprehensive study on the taxonomy and distribution of B. hispidus and descriptions of two further undescribed species of the genus from the western Indian Ocean are currently in preparation.
description: Published as part of Weigmann, Simon, Ebert, David A., Clerkin, Paul J., Stehmann, Matthias F. W. & Naylor, Gavin J. P., 2016, Bythaelurus bachi n. sp., a new deep-water catshark (Carcharhiniformes, Scyliorhinidae) from the southwestern Indian Ocean, with a review of Bythaelurus species and a key to their identification, pp. 401-432 in Zootaxa 4208 (5) on pages 424-427, DOI: 10.11646/zootaxa.4208.5.1, http://zenodo.org/record/211686