Tetrathyrus Claus 1879
Creators: name: Zeidler, Wolfgang
Publisher: name: Zenodo
Year: 2016
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Amphipoda
Platyscelidae
Tetrathyrus

description: Genus Tetrathyrus Claus 1879
(Figs 56–57)
Tetrathyrus Claus, 1879: 4 (key), 14.— Claus 1880: 588.— Gerstaecker 1886: 483.— Claus 1887: 31 (key), 40.— Stebbing 1888: 1480.— Chevreux & Fage 1925: 419 (key), 422.— Spandl 1927: 227 (key), 240.— Barnard 1940: 541 (key).— Hurley 1955: 187 (key).— Bowman & Gruner 1973: 55 (key), 56.— Zeidler 1978: 39 (key), 43.— Vinogradov et al. 1982: 439 (key), 454.— Shih & Chen 1995: 225 (key), 248–249.— Vinogradov 1999: 1200 (key), 1202.
Type species. Tetrathyrus forcipatus Claus, 1879 by monotypy. Type material could not be found at the MFN or ZMH and is considered lost. However, the descriptions and figures of Claus (1879, 1887) readily characterise this genus. The type locality is the South Atlantic, near the Cape of Good Hope.
Diagnosis. Head round. 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 two-field brush medially; with three smaller articles inserted on antero-dorsal corner. Antennae 1 of females with 2-articulate peduncle; callynophore narrowly rectangular, with two smaller articles inserted terminally. Antennae 2 absent in females. Antennae 2 of males 5-articulate; strongly zig-zagged, with all articles folded back on each other, extending anteriorly under head and posteriorly between the gnathopoda and pereopoda to pereonite 5; basal article distinctly inflated, about half or less the length of following article; articles 2 & 3 sub-equal in length; terminal two articles of similar length, slightly shorter than preceding one; terminal article pointing anteriorly. Mandibular incisor relatively broad, straight with several teeth, with small distal lobe medially; in male orientated more or less parallel to palp. Maxillae 1 consisting of styliform lobes, with few tiny setae terminally on medial margin. Maxillae 2 consisting of rounded, plate-like lobes, with rounded, medial bulge, with few setae terminally. Maxilliped with inner lobes incompletely fused, slightly separate terminally; medial margin of outer lobes with membranous fringe. Coxae 1–6 separate from pereonites; coxa 7 mostly fused with pereonite. Gnathopods 1 & 2 sub-chelate; distal margin of propodus distinctly excavate, forming subchela with dactylus. Pereopods 3 & 4 sub-equal in length to pereopods 5 & 6. Pereopod 5; basis very broad, with relatively straight anterior margin and convex posterior margin, about twice as broad as long; articles 3–7 inserted sub-terminally on basis. Pereopod 6; basis very broad, quadrate, with oblique distal margin, maximum width about half maximum length, without fissure; articles 3–7 inserted sub-terminally on basis; merus with antero-distal corner slightly extended, overlapping carpus medially. Pereopod 7 reduced in size with large basis; with only 1–3 terminal articles. Uropod 1 with articulated exopoda and endopoda. Uropods 2 & 3; endopod articulated, or fused, with peduncle. Rami of all uropoda more-or-less lanceolate, usually with serrated margins. Gills all without folds.
Species. Tetrathyrus forcipatus Claus, 1879 and T. arafurae Stebbing, 1888.
Sexual dimorphism. The sexes are remarkably similar in general morphology, and except for the antennae and mandibles, there is no obvious sexual dimorphism.
Remarks. This genus is readily distinguished by the morphology of the gnathopoda and pereopod 6.
The inclusion of Tetrathyrus in the family Platyscelidae is not entirely satisfactory as it has a number of characters that are not consistent with the other genera that currently make up the family as follows. The eye facets are arranged in a single group on either side of the head (not double). The first maxillae are relatively simple, without teeth or robust setae. The inner lobe of the maxilliped is only fused for the proximal two-thirds. The callynophore of the first antennae of females is bulged proximally, instead of distally. The second antennae of females are absent. The basis of pereopod 6 lacks a fissure, and the telsonic groove is relatively narrow. And the endopod of uropod 2 & 3 is sometimes fused with the peduncle. Otherwise the characters of Tetrathyrus are consistent with its congeners, and it has all the characters that enable individuals to roll up into a ball, as detailed previously. Thus, Tetrathyrus is best kept in the family Platyscelidae until a thorough taxonomic revision is undertaken.
Although Tetrathyrus is relatively common in tropical and sub-tropical waters worldwide, very little is known regarding the biology of species. Stephensen (1925) provides limited biogeographical information, and judging from previous records, Tetrathyrus seems to be epipelagic in habit, preferring near-surface waters.
The only records of associations with gelatinous plankton are by Harbison et al. (1977) who record T. forcipatus from the siphonophores Agalma clausi and Nanomia bijuga and Gasca et al. (2014) who record it as an associate of the pteropod mollusc Corolla spectabilis.
Pirlot (1930) considers this genus to be monotypic and synonymises the six nominal species with T. forcipatus. Vinogradov et al. (1982) also recognise T. arafurae Stebbing, 1888 as a valid species, based mainly on that species having uropods 2 and 3 with endopods not fused with the peduncle. However, studies of T. forcipatus (Zeidler 1978) revealed that the fusion of the endopod, with the peduncle of uropod 2, is variable and thus an unreliable character, although no specimen had the endopod of uropod 3 free. Thus, the genus is in need of further revision in order to determine the number of valid species. The following key, derived from Vinogradov et al. (1982) distinguishes the two currently recognised species.
description: Published as part of 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, pp. 1-136 in Zootaxa 4192 (1) on pages 112-115, DOI: 10.11646/zootaxa.4192.1.1, http://zenodo.org/record/166420