Schizoscelus 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 : 123-126

Genus Schizoscelus Claus, 1879

( Figs 62–63 )

Schizoscelus Claus 1879: 17 (key), 20.— Gerstaecker 1886: 484.— Claus 1887: 42 (key), 43–44.— Stebbing 1888: 1503.— Spandl 1927: 250 (key), 255.— Bowman & Gruner 1973: 57 (key), 58.— Vinogradov et al. 1982: 465 (key), 466.— Shih & Chen 1995: 254 (key), 262.— Vinogradov 1999: 1199 (key), 1200.

Type species. Schizoscelus ornatus Claus, 1879 by monotypy. Type material could not be located at the MFN or ZMH and is considered lost. However, Schizoscelus is a very distinctive genus, adequately characterised by the descriptions and figures of Claus (1879, 1887). No specific type locality is given, just the “Atlantic”.

Diagnosis. Head round. Eyes occupying most of head surface; grouped in one field on each side of head. Antennae 1 of males with 2-articulate peduncle; flagellum with large, crescent-shaped callynophore, with aesthetascs arranged in two-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 postero-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 4; basal article distinctly inflated, about half or less the length of following article; articles 3–5 progressively slightly shorter than preceding one; terminal article pointing anteriorly. Antennae 2 of females 4- articulate. Mandibular incisor styliform, with reduced number of teeth; in male orientated more or less parallel to palp. Maxillae 1 consisting of elongate, rectangular plates with few robust setae on distal margin. Maxillae 2 absent. Maxilliped with inner lobes completely fused; medial margin of outer lobes with membranous fringe.

Gnathopod 1 simple. Gnathopod 2 chelate; carpal process knife-shaped, armed with microscopic teeth or setae. Pereopods 3 & 4 distinctly shorter than pereopods 5 & 6. Pereopod 5; basis very broad, sub-rectangular in shape, maximum width about half length; articles 3–7 inserted sub-terminally on basis. Pereopod 6; basis very broad, slightly narrowed distally, with relatively straight distal margin, maximum width about half length, with very long 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 hookshaped, 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. Schizoscelus ornatus Claus, 1879 .

Sexual dimorphism. Apart from minor differences in the relative lengths of the pereopoda and pereopod articles, females have a more rounded head, and in males the basis of pereopod 7 is crescent shaped, and relatively wider, as found in other genera of the family.

Remarks. This genus is readily distinguished by the morphology of the gnathopoda and pereopod 6.

Schizoscelus is most similar to Euscelus , particularly in the morphology of the maxillae, antennae, gnathopod 2 and the urosome, and in that the basis of pereopod 6 has an exceptionally deep telsonic groove, and pereopod 7 has a hook-shaped, retractable dactylus. The close relationship of these two genera is strongly supported by the phylogenetic analysis (Fig. 2). Schizoscelus differs from Euscelus mainly in that the eyes are relatively larger, gnathopod 1 is simple, and the basis of pereopod 6 has an unusually long fissure.

Schizoscelus ornatus seems to be preferentially associated with the siphonophore Bathyphysa sibogae ( Biggs & Harbison 1976) . It has not been recorded from any other gelatinous plankton, and Biggs and Harbison found that it occurred with five of the eleven colonies of B. sibogae that they collected. Field and aquarium observations conducted by Biggs and Harbison (1976) “indicate that S. ornatus moves about freely on the pneumatophore and the smaller gastrozooids but avoids the gastrozooids with tentacles. If the amphipod’s freedom of movement is restricted, as when it is enclosed in a jar with its host, it can be captured and quickly ingested”.

It is a relatively rare species and consequently very little is known about its biology. It is known from widely separated records in tropical and sub-tropical regions ( Dick 1970). Most records are from surface tows ( Chevreux 1900, Dick 1970, Pirlot 1939, Stephensen 1925), but Thurston (1976) found a juvenile female in a night haul from 300 m.

Figures

FIGURE 62. Schizoscelus ornatus Claus, 1879, male (3.5 mm), tropical Pacific, off Costa Rica, SAMA C 5826; female (4.0 mm), Gulf of Mexico, USNM 181811. A, habitus of male; B, A 1 of female, dorsal view; C, Md palp of male. Scale bars = 0.5 mm (A), 0.2 mm (A 2 - m, Us), 0.05 mm (Mx 1), 0.1 mm (remainder).

FIGURE 63. Schizoscelus ornatus Claus, 1879, male (3.5 mm), tropical Pacific, off Costa Rica, SAMA C 5826. Scale bar = 0.2 mm.