Thyropus Dana 1852

Genus Thyropus Dana, 1852

(Figs 60–61)

Thyropus Dana, 1852: 316.— Dana 1853: 1008, 1012, 1013.— Bate 1862: 326 (refers to Platyscelus & Hemityphis).— Stebbing 1888: 1492.— Spandl 1927: 251 (key), 258.— Bowman & Gruner 1973: 57 (key), 57 (part).— Zeidler 1978: 37 (part).— Vinogradov et al. 1982: 464 (key), 468.— Shih & Chen 1995: 254 (part).— Vinogradov 1999: 1199 (key), 1200. Tanyscelus Claus, 1879: 17 (incl. key).— Gerstaecker 1886: 483.— Claus 1887: 43 (key), 45.

Type species. Thyropus diaphanus Dana, 1853 by subsequent designation. Type material could not be located at any major North American museum and is considered lost (see Evans 1967). Dana’s (1853) description and figures are inadequate to determine the specific status, but clearly refer to a species of Platyscelidae or Parascelidae. His figure of the male second antennae does not correspond to any genus of Platyscelidae and is most like Parascelus. However, his description of the eyes, “pigments of eye four in number”, readily distinguish the genus Thyropus as defined here. The type locality is the tropical South Atlantic [04°25’S 21°30’W], 7 November 1838.

The diagnosis of Thyropus, presented here, is based on T. sphaeroma (Claus, 1879), as the specific status of T. diaphanus is uncertain, and T. sphaeroma is the next oldest available species.

Type species of synonyms. The type species of Tanyscelus is T. sphaeroma Claus, 1879 by monotypy. There are two lots of specimens in the ZMH which may represent type material; two males labelled “sansibar Tanyscelus ” (K8757), and about 20 specimens labelled “ Ombaistr ” (K8755). The latter locality is Ombai Strait, just north of Timor. This corresponds with the type localities given by Claus (1879), “ Zanzibar and Ombaistrasse ”, and the age of the material appears to correspond to the time of Claus. All of the specimens are in poor condition but are clearly T. sphaeroma, as described and figured by Claus (1879, 1887). Tanyscelus is clearly a synonym of Thyropus, based on the morphology of the eyes, second male antennae, and the basis of pereopod 6, and Claus (1879) even suggested that his species may be the same as that of Dana (1853).

Diagnosis. Head round. Eyes occupying most of head surface; grouped in two fields 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 3-articulate peduncle; callynophore narrowly rectangular, with two smaller articles inserted terminally. 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 2–4 sub-equal in length; terminal article length about half preceding one, pointing anteriorly. Antennae 2 of females 5-articulate. Mandibular incisor styliform, with reduced number of teeth; in male orientated more or less parallel to palp. Maxillae 1 consisting of styliform lobes, with robust setae in four groups distally on medial margin. Maxillae 2 consisting of curved, rounded, elongate plates, with terminal denticles, and with rounded medial bulge. Maxilliped with inner lobes incompletely fused, slightly separate terminally; medial margin of outer lobes with membranous fringe. Gnathopods 1 & 2 simple. Pereopods 3 & 4 distinctly shorter than pereopods 5 & 6. Pereopod 5; basis very broad, oval-shaped, maximum width about half length; articles 3–7 inserted sub-terminally on basis. Pereopod 6; basis very broad, distinctly narrowed for distal half, length about twice maximum proximal width, with 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; all articles present; dactylus normal. Uropoda all with articulated exopoda and endopoda; all more-or-less lanceolate, usually with serrated margins.

Species. Thyropus sphaeroma (Claus, 1879) and T. similis (Stephensen, 1925).

Sexual dimorphism. Sexual dimorphism in Thyropus is similar to that found in Parascelus.

Remarks. The similarity of this genus to Parascelus has already been discussed under that genus.

Thyropus, like Parascelus, seems to be preferentially associated with siphonophores. Thyropus sphaeroma has been recorded from Stephanophyes superba (Harbison et al. 1977), and T. similis from Agalma okeni, Athorybia sp., A. rosacea (Harbison et al. 1977), and A. lucida (Biggs 1978). Harbison et al. (1977) also record Thyropus sp. from the following siphonophores, but some of these records may be of Parascelus sp., as these authors synonymised the two genera; Agalma okeni, Abyla sp., Athorybia sp., A. rosacea, Diphyes dispar, Forskalia sp., F. edwardsi and Stephanophyes superba.

Very little is known about the biology of this genus. It is relatively uncommon, but widely distributed in the tropical and warm-temperate regions of the world’s oceans. Although most records are from surface hauls, Thurston (1976) found T. sphaeroma down to 450 m.

Vinogradov et al. (1982) consider this genus to be monotypic but Parascelus similis Stephensen, 1925 is recognised as a valid species of Thyropus, in addition to T. sphaeroma (Zeidler 1998).