Ethusina dilobotus Chen, 1993

Castro, Peter, 2005, Crabs of the subfamily Ethusinae Guinot, 1977 (Crustacea, Decapoda, Brachyura, Dorippidae) of the Indo-West Pacific region, Zoosystema 27 (3), pp. 499-600 : 559-560

Ethusina dilobotus Chen, 1993

Ethusina dilobotus Chen, 1993: 336 (key), 338, fig. 17. — Ng & Ho 2003: 72 , fig. 1.

Ethusina insolita Ng & Ho, 2003: 81 , figs 12, 13.

TYPE MATERIAL. — Holotype of Ethusina dilobotus Chen, 1993 : cl 9.0 mm, cw 8.5 mm, BIOCAL, stn CP 62 ( MNHN-B 22428 ).

Type material of Ethusa insolita Ng & Ho, 2003 : holotype: cl 9.7 mm, cw 9.2 mm, TAIWAN 2000 ( NTOU).

TYPE LOCALITY. — New Caledonia, Norfolk Ridge, 24°19’S, 167°49’E, 1395-1410 m.

MATERIAL EXAMINED. — Taiwan. TAIWAN 2000, stn CP 53, 24°15.7’N, 122°11.6’E, 2947- 2903 m, 3.VIII.2000, 1 holotype of E. insolita Ng & Ho, 2003 ( NTOU).

TAIWAN 2001, stn CD 129, 22°05.89’N, 121°05.21’E, 1271-1275 m, 21.VIII.2001, 1 ( NTOU).

TAIWAN 2002, stn CP 189, 21°39.91’N, 118°20.94’E, 1649- 1629 m, 27.VIII.2002, 2 ( NTOU). — Stn CP 190, 21°35.01’N, 118°15.02’E, 1650-1665 m, 28.VIII.2002, 1 ( NTOU).

New Caledonia. BIOCAL, stn CP 62, 24°19’S, 167°49’E, 1395-1410 m, 2.IX.1985, 1 holotype ( MNHN-B 22428).

DISTRIBUTION. — Known from Taiwan ( Ng & Ho 2003, in part as E. insolita Ng & Ho, 1993 ) and New Caledonia ( Chen 1993). Depth: 1271-2947 m ( Fig. 34 ).

SIZE. — Maximum size: cl 9.7 mm, cw 9.2 mm ( NTOU), unknown.

REMARKS

Ethusina dilobotus is known only from male specimens. Diagnostic is a distinctive transverse ridge on somite 5 of the male abdomen ( Ng & Ho 2003: fig. 13F, as E. insolita Ng & Ho, 2003 ) and G1 each with two flattened terminal lobes ( Chen 1993: fig. 17d-f; Ng & Ho 2003: fig. 13H, as E. insolita ). The eyes are clearly visible at the proximal margin of the orbital sinuses ( Chen 1993: fig. 17a; Ng & Ho 2003: fig. 12, as E. insolita ).

Comparison of the holotypes of both E. dilobotus ( MNHN-B 22428) and E. insolita (NTOU) showed that the two are clearly conspecific. E. insolita was described as a distinct species on account of the presence of transverse ridges across the abdominal somite 5 of the male holotype. These were found in the holotype of E. dilobotus but unfortunately not shown in the illustration of Chen (1993: fig. 17c). The holotype was not examined by Ng & Ho. Other differences (having outer orbital teeth that are directed anteriorly rather than slightly outward, the telson being more elongated, and the G1 “shorter and stouter”) are minor and, as agreed by Ng & Ho (2003: 81), most probably the result of variation in a species known from only two specimens. E. dilobotus is certain to be found outside Taiwan and New Caledonia.

Ng & Chen (2003), however, correctly identified a second, slightly smaller specimen from Taiwan (cl 8.7 mm, cw 8.2 mm, NTOU) as E. dilobotus , even if its median frontal teeth end as distinct acute teeth instead of being sinuous, lobe-like as in the holotype ( Chen 1993: fig. 17a) and the holotype of E. insolita . Similar sinuous median frontal teeth are diagnostic to E. challengeri ( Miers, 1886) ( Fig. 16A ). Very unusual about this specimen is the presence of a very slight notch on the posterior margin of each orbital sinus, a characteristic shared with E. crenulata n. sp. (see below). The morphology of the G1 and the distinct abdominal ridges, however, confirm its identity as E. dilobotus .

Figures

FIG. 34. — Bathymetric distribution of Indo-West Pacific ethusine crabs.

FIG. 16. — Ethusina challengeri (Miers, 1886), cl 12.6 mm, cw 12.1 mm, Mozambique Basin, SAFARI 1, stn 1, trawl CP 01, 2608 m (MNHN-B 8775); A, dorsal surface of carapace; B, ventral view of anterior portion of body; C, right G1, ventral (left) and dorsal (right) views; D, left G2, dorsal view. Scale bars: A, 5 mm; B, D, 2 mm; C, 1 mm.