Brachyscelus 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 : 44-47

Genus Brachyscelus Bate, 1861

( Figs 16–17 )

Brachyscelus Bate, 1861: 7 .— Bate 1862: 333.— Stebbing 1888: 1543.— Chevreux & Fage 1925: 427 (incl. key).— Schellenberg 1927: 648 (key), 648–649.— Pirlot 1929: 139.— Hurley 1955: 181.— Bowman & Gruner 1973: 46 (key), 47.— Zeidler 1978: 26 (key), 28.— Vinogradov et al. 1982: 395 (incl. key).— Shih & Chen 1995: 170 (key), 178.— Vinogradov 1999: 1193 (incl. key).

Daira Dana, 1853: 981 (part).

Dairinia Dana, 1853: 1442 (part).— Bate 1862: 309.

Thamyris Bate, 1862: 335 .— Claus 1879: 32 (key), 32–36.— Claus 1880: 588.— Carus 1885: 426.— Gerstaecker 1886: 485.— Claus 1887: 55 (key), 56–59.— Bovallius 1887b: 574.

Schnehagenia Claus 1871: 157 .

Type species. Brachyscelus crusculum Bate, 1861 by monotypy. Type material could not be found at the NHM or MNHN and is considered lost. However, the description and figures of Bate (1861) readily characterise this genus and its status is well established in the literature. The type locality is unknown; not recorded.

Type species of synonyms. The type species of Thamyris is T. antipodes Bate, 1862 by monotypy. Type material could not be located at the NHM or MNHN and is considered lost. However, the description and figures of Bate (1862) are clearly those of a species of Brachyscelus , most likely B. crusculum . The type locality is the southwest Pacific, south of New Zealand [58°S 172°W], H.M.S. Herald, Mr. Rayner.

The type species of Schnehagenia is S. rapax Claus, 1871 by monotypy. Type material could not be located at the MFN or ZMH and is considered lost. It is not clear why Claus (1871) did not recognise that his species belonged to Brachyscelus . The figures and description of Claus (1887) are clearly those of a species of Brachyscelus . The type locality is the South Atlantic, near the Cape of Good Hope, Captain Schnehagen.

Some of the species assigned to Daira or Dairinia by Dana (1853) are clearly species of Brachyscelus .

Diagnosis. Body shape robust or globular. Head rounded. Rostrum only present in mature males of some species; short and rounded. 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 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 articles folded back on each other; extending anteriorly under head and posteriorly into antennal pocket of G1; basal article elongate, sub-equal in length to following article; terminal article shorter than preceding one, pointing anteriorly. Mandibular incisor relatively broad, with several teeth, with small distal lobe medially; in male orientated more or less parallel to palp. Maxillae 1 consisting of very small, rounded plates. Maxillae 2 extremely reduced in size, or most likely absent. Maxilliped with inner lobes completely fused; medial margin of outer lobes with membranous fringe. Gnathopod 1 of males with basis with antennal pocket; antenna 2 with fold of articles 2 & 3 and 4 & 5 held in pocket. Gnathopods 1 & 2 sub-chelate; carpal process knife-shaped, armed with prominent teeth only. Pereopods 3 & 4 distinctly shorter than pereopods 5 & 6. Pereopod 5; basis slightly more than 3 x as wide as merus, non-locking but may overlap with P6; articles 3–7 inserted terminally to basis. Pereopod 6; basis as wide or less than 5 x as wide as merus, but not operculate, does not overlap, or lock, with opposing pereopod; articles 3–7 inserted terminally, or almost sub-terminally, to basis. Pereopod 7 reduced in size with large basis; all articles present; dactylus normal. Uropoda all with articulated exopoda and endopoda; all lanceolate, usually with serrated margins.

Species. Brachyscelus crusculum Bate, 1861 ; B. rapax ( Claus, 1871) ; B. globiceps ( Claus, 1879) ; B. macrocephalus Stephensen, 1925 and B. rapacoides Stephensen, 1925 .

Sexual dimorphism. Apart from obvious differences in the morphology of the antennae, mandibles and gnathopod 1, males are more slender in habit, and the head is more elongate, with a distinct point or beak in the majority of species. In some species the head is rounded in males, as in females, but is relatively smaller.

Remarks. This genus is in a state of considerable taxonomic confusion and a revision is long overdue. According to Madin and Harbison (1977) at least 17 species of Brachyscelus have been described but, of these, only four are recognised as valid by Vinogradov et al. (1982). However, studies of Tasman Sea material ( Zeidler 1992, 1998), and limited examination of specimens in most major museums, have resulted in the recognition of at least five species.

Brachyscelus bares some similarities to the families Eupronoidae and Amphithyridae, especially in the enlarged basis of pereopods 5 & 6, but there is no evidence of a locking mechanism on the basis of pereopod 6, or that the basis of opposing sixth pereopoda overlap. However, the ischium loosely overlaps the merus on both sixth pereopods and this may, in some way, enable the opposing pereopods to be held together. It also seems likely that all species are able to curl up into a ball, as the coxa of pereopod 5 articulates proximally with the coxa/basis of pereopod 6, and the pleonites have a lateral ridge which, when the pleon is curled, lines up with the posterior margin of the basis of pereopod 6.

This genus also resembles Eupronoe in the character of the male second antennae being held in a pocket on the basis of gnathopod 1, which has already been discussed under Eupronoidae.

Brachyscelus has been recorded from a variety of gelatinous plankton although most species seem to be preferentially associated with medusae. Brachyscelus crusculum has been recorded in association with salps ( Stephensen 1923, 1925; Young & Anderson 1987) and medusae ( Pirlot 1939), specifically the salps Cyclosalpa affinis , Iasis zonaria , Pegea socia , Salpa maxima , Thalia democratica (Harbison et al. 1977) and Salpa fusiformis ( Laval 1980) , and the medusa Aequorea coerulescens ( Gasca & Haddock 2004) , as well as Leptomedusae (Harbison et al. 1977). It has also been observed with the heteropod, Pterotrachea sp. (Harbison et al. 1977) and P. hippocampus ( Gasca & Haddock 2004) . Brachyscelus rapacoides has been recorded with Hydromedusae, and the medusae Aequoria sp., Orchististoma sp. and Leuckartiara sp., as well as with the pteropod Cavolina longirostris (Harbison et al. 1977) . Also, Gasca et al. (2014) record an unidentifiable juvenile from the narcomedusa Solmisus incisa .

Brachyscelus is widely distributed in tropical and warm-temperate regions of the world’s oceans, and some species, such as B. crusculum , can be relatively abundant. Species, for which data are available, appear to be epipelagic to shallow-mesopelagic in habit (e.g. Thurston 1976).

Figures

FIGURE 16. Brachyscelus crusculum Bate, 1861, male (7.1 mm—A), Coral Sea, SAMA C 3691; female (19.5 mm) and male (25.0 mm), Tasman Sea, SAMA C 5242. A, habitus of male, C 3691. Mouthparts, A 1, A 2 from male, C 5242. Scale bars = 1.0 mm (A), 0.5 mm (A 2 - m), 0.2 mm (A 1, Mxp), 0.1 mm (Md, Mx 1).

FIGURE 17. Brachyscelus crusculum Bate, 1861, male (7.1 mm), Coral Sea, SAMA C 3691. Scale bar = 0.5 mm.