BRACHYSCELIDAE Stephensen, 1923

Zeidler, Wolfgang, 2021, A review of the hyperiidean amphipod family Brachyscelidae Stephensen, 1923 (Crustacea: Amphipoda: Hyperiidea), Zootaxa 5026 (3), pp. 405-439 : 407

Family BRACHYSCELIDAE Stephensen, 1923

Diagnosis. Body length up to 20 mm, but usually 6–10 mm. Head of females evenly rounded, of males usually slightly pointed antero-ventrally, as long as first 4–5 pereonites in both sexes. Eyes large, occupying most of head surface. Coxae separate from pereonites. Antennae 1 of females with 2-articulate peduncle, enlarged first flagellar article (callynophore), followed by two small terminal articles. Antennae 1 of males with 2-articulate peduncle, enlarged, curved first flagellar article (callynophore), with two-field brush of aesthetascs medially, and three smaller, slender articles inserted on antero-distal corner. Antennae 2 absent in females. Antennae 2 of males with five slender articles folded back on one another; juncture of articles 2 and 3 supported in pocket of basis of G1. Mandibles with 3-articulate palp in males; palp absent in females. Maxillae 1 reduced to tiny, rounded lobes. Maxillae 2 extremely reduced, most likely absent. Gnathopods 1 and 2 distinctly chelate. Pereopod 5 the longest. Pereopods 5 and 6 with greatly enlarged basis, slightly shorter than remaining articles combined, which are inserted terminally on basis for P5 and sub-terminally for P6. Pereopod 7 reduced in size, but with full complement of articles, with enlarged basis, which is longer than remaining articles combined. Uropods all with articulated endopods and exopods. Telson triangular, usually extending to limit of U3, not fused with double urosomite. Gills with folds on pereonites 2–6. Oostegites on pereonites 2–5.

One genus. Brachyscelus Spence Bate, 1861 .

Remarks. Bovallius (1890), in a footnote (pp. 18–19), divided the old family Tryphaenidae into two; Lycaeidae and Euthamneidae (for Brachyscelus and Thamneus ) and changed Thamneus to Euthamneus to avoid confusion, and fit with the new family name. However, Stephensen (1923) quite rightly points out that Brachyscelus is the older generic name, and “the family must be named after the older genus and so be called Brachyscelidae ”. Not all subsequent authors have accepted this arrangement and have restored Brachyscelus and Thamneus to the family Lycaeidae (e.g., Bowman & Gruner 1973, Shih & Chen 1995), while others support the recognition of Brachyscelidae (e.g., Vinogradov et al. 1982, 1996, Vinogradov 1999).

Following my review of the families and genera of the superfamily Platysceloidea ( Zeidler 2016) , the family Brachyscelidae is here maintained for the genus Brachyscelus because it differs significantly from the family Lycaeidae in a number of characters as follows: i) the head of males is usually pointed (round in Lycaea and only slightly pointed in Simorhynchotus ); ii) the second antennae of males have the juncture of articles 2 and 3 supported in a pocket of the basis of gnathopod 1 (absent in Lycaeidae ); iii) the second antennae of males consist of a relatively long basal article, followed by three slightly longer articles, and one shorter one, all folded back on one another, the terminal one being about one-quarter as long as the preceding one, and orientated anteriorly (in Lycaeidae the basal article is only about one-third as long as the following article, and the terminal article is minute, is not folded, and is orientated posteriorly); iv) the basis of pereopods 5 and 6 is more operculiform, and in pereopod 6 the terminal articles are inserted sub-terminally to the basis; v) the coxa of pereopod 7 is not fused with the pereonite; vi) the endopods and exopods of the uropoda are never fused with the peduncle; and vii) the telson is not fused with the double urosomite. In addition, newly hatched juveniles of B. crusculum have specialised appendages that differ from those of the adult. In particular, pereopod 5 is chelate, while in the adult it is simple. Lycaea lacks these specialised appendages, thus supporting the familial status for Brachyscelus ( Harbison 1976) . Some of the above characters, such as the morphology of the male head and the pereopods, are intermediate between the families Lycaeidae and Oxycephalidae . The morphology of the maxillae are also like those of Lycaeidae and Oxycephalidae .

Prior to my review ( Zeidler 2016), this family included Thamneus but, because it has characters that are unique and inconsistent with Brachyscelus (or Lycaeidae ), a new family, Thamneidae , was established by Zeidler (2016) to accommodate it.