Hemijassa Walker, 1907

Conlan, Kathleen E., 2021, New genera for species of Jassa Leach (Crustacea: Amphipoda) and their relationship to a revised Ischyrocerini, Zootaxa 4921 (1), pp. 1-72 : 30-31

Genus Hemijassa Walker, 1907 .

Hemijassa Walker, 1907 , 38.

Description of male. Maximum body length 22 mm.

Head lobe: squared, angles acute.

Antenna 1: accessory flagellum 2 articles, the second minute, only the second article setose.

Antenna 2: stouter and longer than antenna 1, the filter setae progressively shorter; flagellum with all but the first article bearing spines on the posterior margin, first article considerably longer than any of the following articles.

Maxilla 1: inner plate bearing a few short, fine setae; palp without setae at the base of article 1; article 2 with many scattered rows of facial setae distally.

Gnathopod 1: coxa rectangular; propodus, palm defined by 4 spines located proximally of centre; dactyl not facially striated.

Gnathopod 2: with a gill; coxa not deeper posteriorly; basis, filter setae simple or finely barbed and located on the anterolateral flange only; carpus less than 1/4 propodus length; propodus with a pronunced conical, multiply incised tooth below the dactyl hinge, and a larger acute thumb with 3 minute palmar defining spines at its tip, thumb setose on both inner and outer margins; dactyl shorter than the propodus, inner margin not expanded, tip resting on the tip of the thumb or between the thumb’s anterior margin and the palm; dactyl cusps reduced to small buttons interspersed with a few short setae.

Pereopods 3 and 4: coxae rectangular; basis, margins more parallel than convex; merus slightly overlapping the carpus, anterior margin bearing discrete clusters of setae; propodus not posteriorly spinose.

Pereopods 5–7: at least one basis posterodistally produced, anterior margin with a few short setae; merus and carpus, posterior margin not spinose; dactyl without facial serrations, posterior (outer) margin not cusped distally, anterior (inner) margin bearing a row of setae along its length.

Pleopods: rami long, length> depth of the pleon, each with>2 coupling hooks.

Urosome: segment 1 bearing a pair of erect setae dorsally.

Uropod 3: peduncle mid-ventrally setose and spinose, without spines mid-dorsally, with a ventral row of short setae, with a crown of spines dorsomedially at the insertion of the rami and a cluster of setae distolaterally; outer ramus not setose mid-dorsally, tipped by a basally immersed, dorsally recurved spine and minute serrations dorsally, but without cusps; inner ramus with a single apical spine.

Telson: each corner with a pair of dorsolateral cusps and accompanying setae (1 long, simple and 2 short, plumose) but without spines.

Description of adult female. Maximum body length 20 mm. Character states as in the male except as follows.

Brood plates: broad, setae abundant, hook-tipped.

Antenna 2: peduncle, posterior filter setae long, not shorter in larger individuals.

Gnathopod 2: propodus and dactyl much larger and different in shape from that of the male.

Pereopods 3 and 4: basis slender as in the male.

Type species. Jassa goniamera Walker, 1903 (monotypy).

Remarks. Walker (1903) first suggested the genus name but decided to place his new species ( goniamera ) under Jassa . Walker (1907) transferred J. goniamera to Hemijassa after it was pointed out by other taxonomists that it did not fit in Jassa , Ischyrocerus or Bruzeliella Norman, 1905 (the latter synonymized by Walker 1911 under Jassa ) based on characteristics of the antennal accessory flagellum and uropod 3. However, taxonomists appear to have disregarded the transfer of J. goniamera to Hemijassa and continued to assign H. goniamera to Jassa , as J. goniamera , J. falcata (Montagu, 1808) , J. ingens Pfeffer, 1888 or J. wandeli ( Bellan-Santini 1972; Thurston 1974b; Lowry & Bullock 1976). Sexton & Reid (1951) erroneously submerged the genus and species under Jassa falcata . Hemijassa is formally re-erected here as H. goniamera is clearly not a species of Jassa , as noted by Conlan (1989, 1990). In Hemijassa , gnathopod 2 morphology is unlike that of Jassa . The “thumb” is at, rather than distal to, the palmar defining spines, and occurs in both sexes. The “thumb” in Hemijassa could be considered homologous to the ledge that develops at the defining spines in Jassa morinoi Conlan, 1990 and J. ingens , and thus bears no homology to the Jassa thumb at all. There is no evidence of a transformational increase in thumb length relative to body length as occurs in Jassa . The lack of sexual difference in the shape of the bases of pereopods 3 and 4 is also suggestive of a different sexual life style. In Jassa , the pereopod 3 and 4 bases are broadly convex in non-thumbed males and females but slender in thumbed males. This difference corresponds with abandonment of a tubicolous lifestyle in Jassa once the males develop a thumb and a roving behaviour in search of receptive females to mate with ( Borowsky, 1985).