Platyscelus Bate, 1861

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 : 101-104

Genus Platyscelus Bate, 1861

( Figs 49–50 )

Typhis Risso, 1816: 122 .— Guérin 1825: 755.— Milne-Edwards 1830: 395.— Milne-Edwards 1838: 285.— Lucas 1840: 239.— Milne-Edwards 1840: 71 (key), 94–96.— Lucas 1846: 57.— Dana 1852: 316.— Dana 1853: 1008.

Dithyrus Dana, 1852: 316 .— Dana 1853: 1008 –1010.

Platyscelus Bate, 1861: 4 .— Bate 1862: 329.— Thomson 1879: 244.— Stebbing 1888: 1462.— Spandl 1924a: 35.— Chevreux & Fage 1925: 419 (incl. key).— Schellenberg 1927: 646 –647.— Spandl 1927: 227 (key), 227–228.— Hurley 1955: 187 (key), 189.— Bowman & Gruner 1973: 55 (incl. key).— Zeidler 1978: 39 (incl. key).— Vinogradov et al. 1982: 439 (key), 439– 440.— Shih & Chen 1995: 225 (key), 226.— Vinogradov 1999: 1200 (key), 1201.

Eutyphis Claus, 1879: 4 (key), 5.— Claus 1880: 558.— Carus 1885: 424.— Claus 1887: 31 (key), 31-35.— Gerstaecker 1886: 482.— Pirlot 1929: 156.

Eutyphes — Bovallius 1887: 45.

Type species. Typhis ovoides Risso, 1816 by monotypy. Type material could not be found at the MNHN or any other major European museum (see acknowledgments). Despite Risso’s limited description and figure, Platyscelus ovoides is a well-established species in the literature. The type locality is the Mediterranean Sea, near Nice, France .

Type species of synonyms. Bate (1861) instituted Platyscelus because Typhis is preoccupied by a genus of molluscs ( Monfort 1810). Thus, the type species is as above.

The type species of Dithyrus is D. faba Dana, 1853 , by subsequent designation. Type material could not be located in any major North American museum and is considered lost (see Evans 1967). Despite the loss of type material it is clear from Dana’s description and figures that D. faba is a species of Platyscelus , most likely P. armatus . The type locality is the north-west Atlantic , off the Canary Islands, from stomach of Bonito, 27 September 1838 .

The type species of Platyscelus is P. serratus Bate, 1861 by monotypy. Type material could not be found at the NHM or MNHN and is considered lost. The type locality is “unknown. Taken by M. Morrisse of Havre”. However, it is clear from the description and figures of Bate that his species is synonymous with P. ovoides . Thus, there is no problem with accepting Typhis ovoides Risso, 1816 as the type species of the genus. Claus (1879) also designates T. ovoides as the type species of his genus Eutyphis .

Diagnosis. Head round. Eyes occupying most of head surface; grouped in two fields 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 3-articulate peduncle; callynophore narrowly rectangular, with two smaller articles inserted terminally. 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 to pereonite 2; 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, much shorter than preceding one; terminal article pointing anteriorly. Antennae 2 of females 5-articulate, with very small terminal article. 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 elongate plates with few bifid, robust setae, distally on medial margin. Maxillae 2 consisting of curved, pointed plates, with rounded medial bulge. Maxilliped with inner lobes completely fused; medial margin of outer lobes without fringe of setae or membranous fringe. Coxae all separate from pereonites. Gnathopods 1 & 2 chelate; carpal process knife-shaped, armed with prominent teeth. Pereopods 3 & 4 distinctly shorter than pereopods 5 & 6. Pereopod 5; basis very broad, more-or-less oval-shaped, almost almost twice as long as broad; articles 3–7 inserted sub-terminally on basis. Pereopod 6; basis very broad, bean-shaped, with rounded or relatively straight distal margin, maximum width about 0.4 x maximum length, with relatively small fissure; articles 3–7 inserted subterminally on basis; merus with antero-distal corner slightly extended, overlapping carpus medially. Pereopod 7 reduced in size with large, elongate basis; with only 1–3 terminal articles. Uropods 1 & 2 with articulated exopoda and endopoda. Uropod 3; endopod fused with peduncle. Rami of all uropoda more or less lanceolate, usually with serrated margins. Gills all with folds.

Species. Platyscelus ovoides ( Risso, 1816) ; P. armatus ( Claus, 1879) ; P. crustulatus ( Claus, 1879) and P. serratulus 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 distinguished by the distinct serrations found on the carpus and propodus of the gnathopoda. Also, in males, the last two articles of the second antennae are usually much shorter than half the length of the preceding one. However, it differs most significantly from all the other genera in that coxa 7 is not fused with the pereonite.

Records of associations with gelatinous plankton are few. Platyscelus ovoides has been recorded with the medusa Aequoria sp. ( Risso 1816), and P. serratulus with the siphonophore Agalma elegans ( Laval 1980) . An unidentified species of Platyscelus has also been found with the medusa Pelagia noctiluca ( Laval 1980) .

Very little is known about the biology of species. Stephensen (1925) and Thurston (1976) provide some limited biogeographical information. Most species seem to prefer tropical waters, and from the available data, appear to be epipelagic in habit.

This genus is desperately in need of taxonomic revision. Only four species are currently recognised ( Vinogradov et al. 1982).

Figures

FIGURE 49. Platyscelus ovoides (Risso, 1816), female (17.5 mm) and male (15.2 mm), Tasman Sea, SAMA C 4224. A – C, habitus, ventral view when rolled up, and anterior view of head of female. Scale bars = 2.0 mm (A – C), 1.0 mm (G 1 & 2), 0.2 mm (remainder).

FIGURE 50. Platyscelus ovoides (Risso, 1816), female (17.5 mm) and male (15.2 mm), Tasman Sea, SAMA C 4224. A, Mx 1 - m, lateral view; B, Mx 2 - m, medial view. Scale bars = 1.0 mm (P 3 – 7, Us), 0.2 mm (mouthparts).