Euscelus Claus, 1879

Zeidler, Wolfgang, 2016, A review of the families and genera of the superfamily PLATYSCELOIDEA Bowman & Gruner, 1973 (Crustacea: Amphipoda: Hyperiidea), together with keys to the families, genera and species, Zootaxa 4192 (1), pp. 1-136 : 126-130

Genus Euscelus Claus, 1879

( Figs 64–66 )

Euscelus Claus, 1879: 17 (key), 22.— Claus 1880: 588.— Gerstaecker 1886: 484.— Claus 1887: 42 (key), 43.— Spandl 1927: 250 (key), 251.— Bowman & Gruner 1973: 57 (incl. key).— Vinogradov et al. 1982: 464 (key), 465.— Shih & Chen 1995: 254 (key).— Vinogradov 1999: 1199 (key).

Type species. Euscelus robustus Claus, 1879 by monotypy. Type material could not be found at the MFN or ZMH and is considered lost. However , Euscelus is a very distinctive genus, adequately characterised by the descriptions and figures of Claus (1879, 1887). The type locality is the tropical south-western Indian Ocean, near Zanzibar .

Diagnosis. Head round. Eyes occupying only part of lateral head surface; grouped in one field on each side of head. Antennae 1 of males with 1-articulate peduncle; flagellum with large, crescent-shaped callynophore, with aesthetascs arranged in 1.5-field brush medially, with three smaller articles inserted on antero-dorsal corner. Antennae 1 of females with 2-articulate peduncle; callynophore as broad as flagellum, with two smaller articles inserted medially on antero-distal corner. Antennae 2 of males 5-articulate; strongly zig-zagged, with most articles folded back on each other, extending anteriorly under head and posteriorly between the gnathopoda and pereopoda to pereonite 3; basal article distinctly inflated, about half or less the length of following article; articles 2 & 3 subequal in length; article 4 shorter than preceding one; terminal article shorter than preceding one, pointing anteriorly. Antennae 2 of females 3-articulate. Mandibular incisor styliform, with reduced number of teeth; in male orientated more or less parallel to palp. Maxillae 1 consisting of elongate plates, with few robust setae distally. Maxillae 2 absent. Maxilliped with inner lobes completely fused; medial margin of outer lobes with membranous fringe. Gnathopods 1 & 2 chelate; carpal process with 4–5 teeth on antero-distal corner; propodus curved to form pincer with carpus. Pereopods 3 & 4 distinctly shorter than pereopods 5 & 6. Pereopod 5; basis very broad, anterior and distal margins straight and posterior margin convex, maximum width almost 0.6 x length; articles 3–7 inserted subterminally on basis. Pereopod 6; basis very broad, slightly narrowed distally, with rounded distal margin, maximum width about half length, without fissure; articles 3–7 inserted sub-terminally on basis; merus with antero-distal corner extended, distinctly overlapping carpus medially. Pereopod 7 reduced in size with large basis; all articles present; dactylus hook-shaped, retractile. Uropod 1 with articulated exopoda and endopoda. Uropods 2 & 3; endopod fused with peduncle. Rami of all uropoda more-or-less lanceolate, usually with serrated margins.

Species. Euscelus robustus Claus, 1879 .

Sexual dimorphism. Females of E. robustus where unknown prior to this review. Although the available material is limited, males have relatively larger eyes, and the head is more produced than in females. Also, in common with other members of the family Parascelidae , the basis of pereopod 7 of males is more inflated.

Remarks. The morphology of the gnathopoda and pereopod 6, and the relatively small eyes readily distinguish this genus. Its similarity to Schizoscelus has already been discussed under that genus.

Euscelus robustus is an extremely rare species and has only been recorded twice in the literature. Initially from the Indian Ocean ( Zanzibar) by Claus (1879), and secondly from the Atlantic Ocean ( Azores ) by Spandl (1927). Both authors only record males. The record of E. steueri Spandl (1924b) is referable to Hemityphis tenuimanus , which has been confirmed by examination of the type ( NMW 18337).

Recently, while examining the collections of the ZMUC, more specimens of E. robustus were located, from the Dana expeditions of 1928–30. This material represents the only specimens available for study, apart from the NMW material ( Spandl 1927), which is on microscope slides. Most of this material is of females, so it is possible to illustrate the female for the first time ( Figs 64–66 ). The mouthparts are also illustrated for the first time ( Fig. 64 ). Considering the rarity of this material, the following details of the Dana material is provided.

Male (6.7 mm): Dana stn. 3556 VI, North Pacific, just north-east of the Galapagos Islands (02º52’N 87º38’W), 300 mw, 4 September 1928.

Male (6.7 mm): Dana stn. 3556 VIII, as above, 50 mw.

Female (6.5 mm): Dana stn. 3626 VIII, South Pacific, just north-west of the Kermadec Islands (27º00’S 177º41’W), 1500 mw, 31 December 1928.

Nineteen females (6.9–9.2 mm): Dana stn. 3689 V, South China Sea (07º13.5’N 111º49’E), 1000 mw, 9 April 1929.

Female (9.4 mm): Dana stn. 4003 VIII, North Atlantic, off south-western Africa (08º26’N 15º11’W), 600 mw, 9 March 1930.

There are no records of Euscelus in association with gelatinous plankton.

Figures

FIGURE 64. Euscelus robustus Claus, 1879, female (8.7 mm), South China Sea, ZMUC; male (6.7 mm), tropical Pacific, near Galapogos Islands, ZMUC. A, habitus and dorsal view of head of female; B, head of male. Scale bars = 0.5 mm (A, B, Us), 0.1 mm (Md, Mx 1), 0.2 mm (remainder).

FIGURE 65. Euscelus robustus Claus, 1879, female (8.7 mm), South China Sea, ZMUC; male (6.7 mm), tropical Pacific, near Galapogos Islands, ZMUC. Scale bars = 0.2 mm.

FIGURE 66. Euscelus robustus Claus, 1879, female (8.7 mm), South China Sea, ZMUC. Scale bar = 0.5 mm.