Brachyscelus Spence Bate 1861

Genus Brachyscelus Spence Bate, 1861

Brachyscelus Spence Bate, 1861: 7.— Spence Bate 1862: 333.— Claus 1873: 468.— Claus 1876: 519.— 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 /1996: 395/488–489 (incl. key).— Shih & Chen 1995: 170 (key), 178.— Vinogradov 1999: 1193 (incl. key).— Zeidler 2016: 44, 47.

Dairinia (part)— Spence Bate 1862: 309.

Thamyris Spence Bate, 1862: 335.— Claus 1879: 178 (32) (key), 178–182 (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 Spence 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 Spence Bate (1861) readily characterise this genus and its status is well established in the literature.

Type species of synonyms. The type species of Thamyris is T. antipodes Spence 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 Spence Bate (1862) are clearly those of a species of Brachyscelus, most likely B. globiceps.

The type species of Schnehagenia is S. rapax Claus, 1871 by monotypy. Type material could not be found in any major European institution 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.

Diagnosis. Body shape robust or globular. Head round. 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 males orientated more or less parallel to palp. Maxillae 1 consisting of very small, rounded plates. Maxillae 2 extremely reduced in size to almost indistinguishable lobes, 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 distinct pocket to accommodate fold of articles 2 and 3 of second antennae. Gnathopods 1 and 2 sub-chelate; carpal process knife-shaped, armed with prominent teeth. Pereopods 3 and 4 distinctly shorter than P 5 and 6. Pereopod 5 basis slightly more than 3 x as wide as merus, non-locking but may overlap with P6, also coxa 5 and 6 seem to have a medial locking mechanism (see fig. 2); articles 3–7 inserted terminally on basis. Pereopod 6 basis as wide or less than 5 x as wide as merus, not operculate, does not overlap, or lock, with opposing pereopod; articles 3–7 inserted almost sub-terminally on basis. Pereopod 7 reduced in size, basis large; all articles present; dactylus hook-shaped, partly retractile. Uropods all with articulated exopods and endopods; all lanceolate, usually with serrated margins.

Species included. Five in total: Brachyscelus crusculum Spence Bate, 1861; B. globiceps (Claus, 1879); B. macrocephalus Stephensen, 1925; B. rapacoides Stephensen, 1925; and B. rapax (Claus, 1871).

Sexual dimorphism. Apart from obvious differences in the morphology of the antennae, mandibles and gnathopod 1, males tend to be more slender in habit than females, and their 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 & 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, 1996). However, studies of Tasman Sea material (Zeidler 1992, 1998), and an examination of specimens in most major museums, has resulted in the recognition of at least five species.

Brachyscelus bares some similarities to the families Parapronoidae and Amphithyridae, especially in the enlarged basis of pereopods 5 and 6, but there is no evidence of a locking mechanism on the basis of pereopod 6, or that the basis of the opposing sixth pereopods 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 but is readily distinguished from it as detailed above.

Brachyscelus has been recorded from a variety of gelatinous plankton (see remarks for each species) although most species seem to be preferentially associated with medusae. Land (1989) and Land et al. (1995) provide some information on the structure and function of the eyes.

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 (e.g., Stephensen 1925). Species, for which data are available, appear to be epipelagic to shallow-mesopelagic in habit (e.g., Thurston 1976).