Brachyscelus macrocephalus Stephensen, 1925

Zeidler, Wolfgang, 2021, A review of the hyperiidean amphipod family Brachyscelidae Stephensen, 1923 (Crustacea: Amphipoda: Hyperiidea), Zootaxa 5026 (3), pp. 405-439 : 419-422

Brachyscelus macrocephalus Stephensen, 1925

( Figs 7–8)

Brachyscelus macrocephalus Stephensen, 1925: 177–179 , 230 (table), fig. 66.— Shoemaker 1945: 243.— Shoemaker 1948: 14.— Hure et al. 1969: 603 & 605 (tables).— Thurston 1976: 388–389 (table), 434.— Shulenberger 1977: 379 (table).— Vinogradov et al. 1982 /1996: 396/489 (key), 400–401/495–496, fig. 216.—Vinogradov 1990: 77, 95 (table).— Vinogradov 1991: 261 (table).— Zeidler 1992: 115 (key).— Vinogradov 1999: 1193 (key).— Gasca & Shih 2001: 498.— Escobar-Briones et al. 2002: 367 (list).— Vinogradov et al. 2004: 16, 24 (table).— Zelickman 2005: xvii (list).— Gasca et al. 2009: 1497 (table).— Burridge et al. 2016: passim.— Zeidler 2016: 47 (key).— Espinosa-Leal et al. 2021b: passim.

Type material. There are several syntypes (approx. 36 specimens) of Brachyscelus macrocephalus in the NHMD, in alcohol. The type locality is the Mediterranean Sea, from several locations, mainly from the eastern basin; Thor stations 39, 43, 161, 277, 297, 298, 339, 340, 341, 385 ( NHMD- 87114–24; formerly CRU-9618–28 respectively). Stephensen (1925) illustrates a male and female (about 5 mm) from station 277 [33°20’N 27°30’E], 132 metres of wire out (mw), 6 April 1911; partly dissected and labelled “Type” ( NHMD- 85006; formerly CRU-7162).

Material examined. Type material of B. macrocephalus as detailed above and the following. In NHMD: N.E. Atlantic, off Morocco, Dana station 4016 [28°07’N 15°24’W], 50 mw, one female (233044). Tropical W. Indian Ocean, near Maldives, Dana station 3918 v [00°35’N 66°09’E], 50 mw and Dana station 3920 x [01°12’S 62°19’E], 50 mw, numerous specimens, mostly females (233042 & 2233043). In SAM: S.W. Indian Ocean, off South Africa, Meiring Naude station SM 27 [28°S 34°45.4’E], 212–0 m, one female and station SM 30 [28°03.7’S 32°42’E], 212–0 m, 2 females, 2 males. In SAMA: Tasman Sea, off eastern Australia [30°17’S 153°16’30”E], 25 m, one female (C5258).

Diagnosis. Body length up to 5.0 mm for females and 6.0 mm for males. Head relatively large and rounded in both sexes, depth about twice length in females, 1.5 x length in males. Antennae 2 of males; length of terminal article about one-quarter preceding one. Gnathopod 1 basis slightly shorter and more robust than for G2, with inflated anterior margin in females, with distinct pocket for A 2 in males; carpus with antero-distal corner produced into small lobe, partly overlapping propodus, especially in males; carpal process with large teeth, with minor serrations; postero-distal corner of propodus armed with one large tooth followed by two smaller teeth with small serrations in-between; dactylus relatively sharp, length 0.4–0.5 x propodus (longer in females). Gnathopod 2 similar to G1 but basis without pocket and antero-distal corner of carpus barely produced. Pereopods 3 & 4 morphologically similar, P4 marginally longer than P3; merus slightly inflated anteriorly, length about 0.4 x basis; carpus equal in length to merus; propodus 1.5 x carpus length for P 3 in females, 1.3 x in males and about 2 x carpus length for P 4 in females, about 1.5 x in males, posterior margin slightly serrated. Pereopod 5 similar in length to P6; basis oval-shaped, length about 1.5 x maximum width, slightly more narrow in males, antero-distal margin without lobe; merus about 0.3 x basis length, slightly shorter than carpus, anterior margin with or without slight serrations; propodus about 1.5 x carpus length in females, 1.2 x in males; dactylus relatively short, slightly more than 0.2 x propodus. Pereopod 6 basis length 1.7 x maximum width for females, 1.5 x for males, anterior margin straight, antero-distal lobe relatively broad with rounded distal margin produced slightly beyond ischium; posterior margin with distinct bulge proximally tapering gradually to minor rounded lobe distally, overlapping ischium; merus about 0.3 x basis length, about 1.5 x carpus; propodus about 1.6 x carpus length; anterior margin of merus, carpus and propodus with small serrations; dactylus relatively long, about 0.4 x propodus. Pereopod 7 basis with posterior margin inflated proximally, more narrow in females, almost 2 x width, only about 1.5 x in males, slightly longer than remaining articles combined; merus slightly longer than carpus; propodus slightly shorter than carpus, with complex projections on antero-distal corner; dactylus hook-shaped, partly retractile. Uropod 1 endopod slightly longer than peduncle and exopod. Uropod 2 endopod slightly longer than peduncle, about 1.4 x exopod or slightly longer. Uropod 3 endopod about 3.0 x peduncle length, marginally longer than exopod, broadened distally, without distinct excavation terminally on inner margin. Rami of all uropods with serrated margins. Double urosomite about 1.6 x wider than long. Telson slightly shorter than width at base, apex rounded.

Remarks. This is one of the smallest species of Brachyscelus with records of ovigerous females at about 3.0 mm long ( Stephensen 1925). It is most similar to B. globiceps as discussed under that species.

Distribution. A relatively rare species, known from a few isolated records, mainly from tropical regions, from all of the world’s oceans. In the Mediterranean Sea it is most common in the eastern basin. In the Atlantic it has been recorded from near the Canary and Cape Verde Islands in the north and the tropical western part from Cuba to the Gulf of Mexico. In the Pacific it has been found in the Northern Central Gyre ( Shulenberger 1977) and off Chile and Peru, with one record from the Tasman Sea (see Material examined). Specimens from near the Maldives and off South Africa (see Material examined) are new records for the Indian Ocean. Most records are from near surface waters.