Amphithyropsis, Zeidler, Wolfgang, 2016

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 : 39-43

Genus Amphithyropsis gen. nov.

( Figs 13–15 )

Type species. Amphithyropsis pulchellus ( Barnard, 1930) comb. nov., by present designation; originally described as a species of Tetrathyrus . The holotype male is in the NHM (1930.8.1.861). The type locality is off Three Kings Islands, New Zealand, Terra Nova stn. 109, 3 m., 5 August 1911.

Diagnosis. Body shape robust or globular. Head rounded, slightly truncate ventrally. Eyes occupying most of 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 two smaller articles inserted on antero-dorsal corner. Antennae 1 of females without peduncle; callynophore narrowly rectangular; with one smaller article inserted terminally. Antennae 2 absent in females. 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 2; basal article distinctly inflated, about half or less the length of following article; last two articles sub-equal in length to preceding one; terminal article pointing anteriorly. Mandibular incisor reduced to few teeth; in male orientated more or less parallel to palp. Maxillae 1 much reduced in size, consisting of plate-like lobes, with four rounded teeth distally on medial margin. Maxillae 2 very reduced in size, consisting of curved lobes with rounded medial bulge. Maxilliped with inner lobes completely fused; margin of outer lobes without fringe of setae or membranous fringe. Gnathopods 1 & 2 simple; propodus with thumb-like postero-distal corner. Pereopods 3 & 4 distinctly shorter than pereopods 5 & 6. Pereopod 5; basis about 3 x as wide as merus, with proximal ball and socket locking mechanism with P6; articles 3–7 inserted terminally to basis. Pereopod 6; basis very broad, but not operculate, more than 5 x as wide as merus, with relatively large fissure, without telsonic groove and locking mechanism, but can overlap with opposing pereopod; articles 3–7 inserted sub-terminally on basis; merus with antero-distal corner extended, distinctly overlapping carpus medially. Pereopod 7 reduced in size with basis not particularly enlarged, with only 1–3 terminal articles. Uropods 1 & 2 with articulated exopoda and endopoda. Uropod 3; endopod fused with peduncle. Rami of all uropoda lanceolate, usually with serrated margins.

Species. Amphithyropsis pulchellus ( Barnard, 1930) .

Sexual dimorphism. Apart from the morphology of the antennae and the mandibles, males differ from females in having a rounded head, and the rounded thumb-like process of the propodus of the gnathopods is slightly longer, extending just beyond the base of the dactylus.

Remarks. This new genus has been established to accommodate Amphithyropsis pulchellus ( Barnard, 1930) because this species does not belong to Amphithyrus or Paralycaea as defined here. It does not belong to Amphithyrus because the first antennae of females consist of only two articles, as found in Paralycaea ; the shape of the basis of pereopod 6 is more like Paralycaea , and the gnathopoda are not complexly chelate. It does not belong to Paralycaea because the basis of pereopod 6 is like that of Amphithyrus , although closer in shape to that of Paralycaea , and the gnathopoda are not simple, with gnathopod 1 approaching that of Amphithyrus similis . Thus, this genus is intermediate between Amphithyrus and Paralycaea .

Recently the unique type of Tetrathyrus pulchellus was examined and it was found to be the same as Paralycaea platycephala Zeidler, 1998 . Barnard’s (1930) description of this species is inadequate, and he only illustrated pereopods 6 and 7. He placed it in Tetrathyrus , probably on the basis of the thumb-like projection of the propodus of the gnathopoda. However, the basis of pereopod 6 has a fissure (not illustrated or referred to by Barnard), which is absent in Tetrathyrus . The peculiar feature of the carpus of pereopod 6, being bent upwards, led me to suspect that Barnard’s species may be the same as mine. Apart from Hurley (1955), who repeats Barnard’s (1930) record, this species has not been recorded in the literature since its description. This is not surprising as it is clearly not a species of Tetrathyrus , or even a species of Platyscelidae . Curiously Hurley (1955) illustrates this species from New Zealand waters, but identified it as Paralycaea gracilis .

The genus has been recorded from off Three Kings Islands, New Zealand ( Barnard 1930); north of Stewart Island, New Zealand ( Hurley 1955), south-east of Cape Howe, Victoria, Australia ( Zeidler 1998) and off the eastern coast of Tasmania (SAMA collections). There are also a number of specimens from the south-east coast of South Africa (SAM collections), which are indistinguishable from the Tasman Sea material. Thus, this genus is currently monotypic.

Figures

FIGURE 13. Amphithyropsis pulchellus (Barnard, 1930), female (3.8 mm), Tasman Sea, SAMA C 5187. A – C, dorsal view of head, habitus and cuticular sculpture. Scale bars = 0.5 mm (A, B), 0.2 mm (G 1 & 2, P 7, A 1), 0.05 mm (mouthparts).

FIGURE 14. Amphithyropsis pulchellus (Barnard, 1930), female (3.8 mm), Tasman Sea, SAMA C 5187. Scale bar = 0.2 mm.

FIGURE 15. Amphithyropsis pulchellus (Barnard, 1930), male (3.0 mm), Indian Ocean, off South Africa, SAMA C 5843. Scale bars = 0.1 mm (G 1 & 2, A 1 & 2), 0.05 mm (mouthparts).