Hexanchus Rafinesque, 1810a

Ebert, David A., White, William T., Ho, Hsuan-Ching, Last, Peter R., Nakaya, Kazuhiro, Séret, Bernard, Straube, Nicolas, Naylor, Gavin J. P. & De Carvalho, Marcelo R., 2013, An annotated checklist of the chondrichthyans of Taiwan, Zootaxa 3752 (1), pp. 279-386 : 282-283

Genus Hexanchus Rafinesque, 1810a

Sixgill Sharks / ÁẪḛé

Hexanchus Rafinesque, 1810a: 14 . Type species: " Squalus griseus Lacépède ", by original designation, a junior synonym of Squalus griseus Bonnaterre, 1788 .

Hexanchus griseus ( Bonnaterre, 1788)

Bluntnose Sixgill Shark/ DzÁẪḛ

Squalus griseus Bonnaterre, 1788: 9 . Types unknown according to Boeseman in Hureau & Monod (1973). Type locality: "La Méditerranée [= Mediterranean Sea]".

Local synonymy: Hexanchus griseus : (?)Nakamura, 1936: 7, fig. 1, pl. 1 [fig. & plate possibly of H. nakamurai ]; Shen, 1984a: 50; Chen & Yu, 1986: 108; Ebert, 1990: 45; Chen & Joung, 1993: 34, pl. 1.4 (captioned as H. griseus , but actually H. nakamurai ); Shao et al., 2008: 237; Shen & Wu, 2011: 77, fig. [figure captioned as H. griseus is actually a H. nakamurai ]. Hexanchus griseus nakamurai: Chen, 1963: 6 .

Taiwan voucher material: TFRI uncat., 938 mm TL, female; National Taiwan Ocean University uncat., 2000 mm SL, female. One of us ( DAE, May 1988) examined an individual at the Da-xi fish market that was estimated to be about 4 m TL, but had been partially processed prior to examination. The very large size, broad blunt head, and presence of six large, comb-shaped, anterolateral lower teeth, confirm the identification of this species as H. griseus . The dried jaws of another specimen apparently landed in Keelung fish market, estimated in excess of 2 m TL, was also examined, but no further information was available on this latter individual .

Remarks: Hexanchus griseus does not appear to be nearly as common at the fish markets around Taiwan as the bigeye sixgill, H. nakamurai ( Ebert, 1990, pers. obs.). Records of H. griseus in the Taiwanese literature are probably unreliable as most illustrations and photographs appear to be of H. nakamurai . This includes the original records of this species by Nakamura (1936), which is actually H. nakamurai based on the figure. Subsequent figures and photographs captioned as H. griseus , including Chen & Joung (1993), appear to be H. nakamurai . Specimens of H. griseus caught in Taiwanese waters should be retained as voucher specimens and placed in a museum collection.

Conservation status: Near Threatened.

Hexanchus nakamurai Teng, 1962

Bigeye Sixgill Shark / ǺḊÁẪḛ

Hexanchus griseus nakamurai Teng, 1962: 30 , fig. 5. Holotype: TFRI 2515, 750 mm TL male, probably juvenile, Keelung, Taiwan [lost] . Neotype: NMMB-P 15835 , 1565 mm TL mature male, Cheng-gong , Taiwan, 22°58’ N, 120°08’ E, 29 Mar 2011, collected at fish market by H.-C.. Ho .

Local synonymy: Hexanchus griseus :?Nakamura, 1936: 7, fig. 1, pl. 1; Hexanchus griseus nakamurai: Teng, 1962: 30 , fig. 5; Chen, 1963: 6; Chen & Yu, 1986: 108. Hexanchus vitulus: Chen & Joung, 1993: 34 , pl. 1.5 (captioned as H. griseus ), 1.5. Hexanchus nakamurai: Ebert, 1990: 54 ; Shao et al., 2008: 237; Ho & Shao, 2011: 9, 18; Shen & Wu, 2011: 77, fig. Hexanchus griseus: Shen & Wu, 2011: 77 , fig. [figure is of H. nakamurai ].

Taiwan voucher material: Neotype: NMMB-P 15835 , 1565 mm TL mature male, Cheng-gong , Taiwan, 22°58’N, 120°08’E, 29 Mar 2011, coll. H.-C. Ho (neotype designated in Ebert et al., this volume) . Other specimens (8): NMMB-P15782 , mature female, 1683 mm TL, Cheng-gong , Taiwan, 25 Dec 2010, coll. H.-C. Ho ; CAS 235382 , juvenile female 958 mm TL, Da-xi , 25 May 2005 ; DAE 881504 , juvenile male 482 mm TL, Su-ao , 15 Apr 1988 ; TFRI uncat., juvenile female 860 mm TL, 22°26'N, 120°30'E, Dong-gang , 23 Apr 1988 ; TFRI uncat., female 1155 mm TL, TFRI uncat., adult male 1526 mm TL, TFRI uncat., female 1230 mm TL, 22°36'N, 120°17'E, Hsiao-liuchiu , 28 Apr 1988 ; NMF 0295 , juvenile male 845 mm TL, 25°00'N, 121°50'E, Keelung, 7 Apr 1988 .

Remarks: Teng (1962) described Hexanchus griseus nakamurai as a subspecies of H. griseus noting several distinctive morphological characteristics, including a concave posterior margin of the pectoral fin, a longer caudal peduncle, a long interspace between the pelvic and anal fins, and the origin of the dorsal fin located more anterior to the anal fin. He further commented that what Nakamura (1936) called H. griseus (fig. 1, pl. 1) was actually H. nakamurai ( Teng, 1962) . Chen (1963) considered H. g. nakamurai a junior synonym of H. griseus , but provided no figures and only a brief description of a 989 mm TL specimen figured by Nakamura (1936: fig. 1), which is actually a H. nakamurai .

Ebert (1990) compared morphological and meristic data of H. nakamurai from Taiwan and other regions, and to the holotype and paratype of H. vitulus , and concluded that these two species were identical, with H. nakamurai having priority. Taniuchi & Tachihawa (1991) also synonymised these two species, but did not provide any material basis for their conclusion. However, in a recent molecular study Naylor et al. (2012) found western Indian Ocean H. nakamurai to differ from a H. vitulus specimen from the Bahamas. Additional material, morphological and molecular, from the western North Pacific and western North Atlantic, and elsewhere may help elucidate the status of this species. The holotype of this species appears to have been lost despite extensive searches for it by the authors dating back to 1988. Therefore, Ebert et al. (2013) designated a neotype for this species. Hexanchus nakamurai appears to be far more common in fish markets around Taiwan, especially where deep-sea fisheries take place, than H. griseus , which appears to be relatively rare in Taiwanese waters.

Conservation status: Data Deficient.