Parascelus 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 : 117-120

Genus Parascelus Claus, 1879

( Figs 58–59 )

Parascelus Claus, 1879: 17 (key), 18.— Carus 1885: 425.— Gerstaecker 1886: 483 –484.— Claus 1887: 43 (key), 45–46.— Stebbing 1888: 1496.— Spandl 1924a: 42.— Chevreux & Fage 1925: 423.— Spandl 1927: 251 (key), 262.— Pirlot 1929: 155.— Hurley 1955: 183.— Vinogradov et al. 1982: 464 (key), 470–471.— Vinogradov 1999: 1199 (key), 1200.

Type species. Parascelus edwardsii Claus, 1879 by page priority. Type material could not be located at the MFN or ZMH and is considered lost. However, the descriptions and figures of Claus (1879, 1887) readily distinguish this genus. 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 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 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; article 4 slightly longer than half of article 3; terminal article length about two-thirds article 4, pointing anteriorly. Antennae 2 of females 5-articulate, with very small terminal article. Mandibular incisor styliform, with reduced number of teeth; in male orientated more or less parallel to palp. Maxillae 1 consisting of styliform lobes, with few robust setae distally on medial margin. Maxillae 2 consisting of slightly curved, styliform lobes, with small, rounded medial bulge. Maxilliped with inner lobes completely fused; 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 almost 0.6 x 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, 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; all articles present; dactylus normal. Uropoda all with articulated exopoda and endopoda; all more-or-less lanceolate, usually with serrated margins.

Species. Parascelus edwardsii Claus, 1879 and P. pa r v u s Claus, 1879.

Sexual dimorphism. This genus exhibits considerable minor sexual dimorphism, which may have contributed to some taxonomic confusion in the past. In males gnathopod 1 is slightly shorter than gnathopod 2; the basis of gnathopod 1 is slightly shorter than the following articles combined; the basis of pereopod 6 is more narrowed distally; the basis of pereopod 7 is crescent shaped, and the exopod of uropods 2 & 3 is slightly shorter than half of the length of the endopod. In females gnathopod 1 is relatively shorter than in males, and the basis is slightly longer than the following articles combined; gnathopod 2 is more slender than in males; the basis of pereopod 7 is very long and narrow, and the exopod of uropods 2 & 3 is slightly longer than half of the length of the endopod.

Remarks. This genus is very similar to Thyropus and in the past has been synonymised with it (e.g. Bowman & Gruner 1973). However, as suggested by Vinogradov et al. (1982), Parascelus and Thyropus should be recognised as separate genera as a number of characters readily distinguish them. Parascelus differs from Thyropus mainly in that the basis of pereopod 6 lacks a fissure. Other differences are as follows. The eye facets are arranged in single (not double) fields on either side of the head; antennae 1 of males have a two-articulate peduncle (onearticulate in Thyropus ); the first maxillae are armed with about six teeth distally, whereas in Thyropus there are four groups of stout setae, and the maxilliped is more slender, with the inner lobes completely fused.

Very little is known about the biology of this genus, and it is difficult to extract information from the literature, because of the taxonomic difficulties in determining species, and its confusion with Thyropus . It seems to be preferentially associated with siphonophores. Parascelus edwardsii has been recorded from the siphonophores Bathyphysa sibogae ( Biggs & Harbison 1976, Harbison et al. 1977), Agalma okeni , Diphyes dispar , Forskalia tholoides (Harbison et al. 1977) , F. edwardsi ( Laval 1980) and Athorybia rosacea ( Gasca & Haddock 2004) .

Parascelus is relatively uncommon, but is widely distributed in tropical and warm-temperate waters. From the available data it seems to be epipelagic in habit with a preference for the upper 200 m layer.

Harbison et al. (1977) provisionally recognise three species (all as Thyropus ) one of which, P. parvus Claus, 1879 , Vinogradov et al. (1982) regard a synonym of P. edw ardsii . However, P. parvus , as illustrated by Claus (1887), has both gnathopoda with serrations on the postero-distal corner of the carpus, a feature not found in any other described species of Parascelus . This species may therefore be valid. The other two species (also recognised by Vinogradov et al.) have recently been shown to represent the male and female of the same species, P. edwardsii ( Zeidler 1998) . Thus, this genus is currently restricted to two species, with the status of P. pa r v u s being doubtful.

Figures

FIGURE 58. Parascelus edwardsii Claus, 1879, male (4.2 mm), Coral Sea, SAMA C 3708 (A); female (5.2 mm) and male (4.7 mm), Tasman Sea, SAMA C 5417 & C 5414. A, habitus of male, C 3708. Scale bars = 0.5 mm (A), 0.1 mm (A 1 & 2), 0.05 mm (mouthparts).

FIGURE 59. Parascelus edwardsii Claus, 1879, male (4.2 mm), Coral Sea, SAMA C 3708; male (4.7 mm), Tasman Sea, SAMA C 5414 (Mxp). Scale bars = 0.1 mm (Mxp), 0.2 mm (remainder).