Pleojassa Conlan 2021, n. gen.

Genus Pleojassa n. gen.

Description of male. Maximum body length 10.8 mm.

Head lobe: squared, dorsal angle more acute, lower angle lobe more rounded.

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

Antenna 2: stouter and longer than antenna 1, setation variable, filter setae shorter and sparser in larger individuals, flagellum spination variable, first article considerably longer than following articles.

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

Gnathopod 1: coxa rectangular; carpus, anterior margin length <propodus length; propodus, palm defined by 1–2 medial defining spines, this central or slightly proximal of centre; dactyl facially striated.

Gnathopod 2: without a gill; coxa not deeper posteriorly; basis without filter setae; carpus a quarter of propodus length or less, lobe apically setose; propodus without anteroproximal setae, hinge tooth rectangular cuboid or conical, shallowly or deeply bifid, or multiply incised, palm concave to the single medial defining spine, there produced or not into a short “hook” or long thumb; dactyl shorter than the propodus, variably expanded at the hinge tooth, tip apposing the defining spine, or if thumb present, its posterior margin, cusps reduced and interspersed with short setae.

Pereopod 3: coxa deepest centrally or slightly posterior of centre; basis a little slenderer in larger individuals; merus, anterior margin bearing well-spaced single or clustered setae; carpus, overlap by merus variable; propodus not posteriorly spinose.

Pereopods 5–7: basis variably posterodistally produced or not produced, anterior margin spinose or setose; merus not posterodistally spinose; carpus with a cluster of spines posterodistally at least on pereopod 5; propodus not strongly expanded anteriorly; dactyl not facially serrated, posterior (outer) margin not cusped distally, anterior (inner) margin, setation variable.

Pleopods: rami very short, length ± depth of the pleon, each with 2 coupling hooks.

Urosome: segment 1 with dorsal pair of erect setae.

Uropod 3: peduncle mid-ventrally setose, without mid-dorsal spines, but 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 associated seta and serrations, cusps variable, but never as on Jassa; inner ramus with or without a spine or spines mid-dorsally in addition to the single apical spine.

Telson: each dorsolateral corner with a pair of cusps accompanied by setae (2 long, simple, and 2 short, plumose) but not spines.

Description of adult female. Maximum body length 9.8 mm. Character states as in the male except as follows.

Brood plates: broad, setae well separated, abundant, at least some hook-tipped.

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

Gnathopod 2: propodus much larger and different in shape from propodus of gnathopod 1, palm concave, defined by a single medial spine and without a thumb; dactyl tip apposing the defining angle and spine, dactyl cusps strong.

Pereopod 3: basis somewhat broader than in the male.

Variation. Antenna 2 peduncular setal change appears to be similar to that of Jassa, with the male’s setae shorter in larger specimens and the female’s remaining long. Male gnathopod 2 thumbing is not homologous, however, because the thumb develops at the palmar defining spine rather than distal to it. Consequently the thumb’s setation pattern is quite different.

Type species. Jassa wandeli Chevreux, 1906.

Included species. Pleojassa wandeli (Chevreux, 1906), P. multidentata (Schellenberg, 1931), P. moorei n. sp., P. lowryi n. sp. and P. orientalis n. sp.

Remarks. Distinguishing features from Jassa are a single spine defining the palm of gnathopod 2, and hence different thumb setation, closure of the second gnathopod’s dactyl at, rather than distal to, the defining spine, lack of a gnathopod 2 gill, reduced pleopods and lack of a double cusp on the uropod 3 outer ramus.

The males of P. wandeli and P. multidentata produce a long thumb if sufficiently larger than the adult female and thus would superficially appear to be a Jassa. Thumb development appears from specimens to be a progressive transformation, not at a terminal molt, however (although this has not been tested experimentally as it has for Jassa). Males of P. orientalis probably also produce a thumb as this species closely resembles P. multidentata. Pleojassa moorei and P. lowryi are not known to produce thumbs in the males and sufficient specimens were available to find males of similar or larger size than adult females to indicate that the males were adult as well. However, the five species resemble each other in characters that are conservative within Jassa, such as the tendency toward setal reduction in the antenna 2 of the male compared to the female, and in similar morphologies of the mouthparts, gnathopod 1, female gnathopod 2, female brood plates and third uropod hooking.

Key to World species of Pleojassa (both sexes)

1 Uropod 1, posteroventral peduncular spinous process nearly as long as the outer ramus. Gnathopod 1, carpus without a seta at the anterodistal junction of the propodus. Female gnathopod 2, palmar setae not densely plumose (Fig. 22)....................................................................................................... P. moorei n.sp.

- Uropod 1, posteroventral peduncular spinous process ½ to ¾ the length of the outer ramus. Gnathopod 1, carpus with or without a single seta or cluster of setae at the anterodistal junction of the propodus. Female gnathopod 2, palm bearing abundant plumose setae (Figs 27 and 28).......................................................................... 2

2 Pereopods 3 and 4, carpus nearly fully overlapped by the merus (Fig. 25). Female body size at maturity 2.5–5.1 mm. Gnathopod 2, propodus, palm densely plumose in both sexes; larger male ~ 3 mm body length or more, palm defined by a small hook but not by a thumb (Fig. 25)..................................................................... P. lowryi n.sp.

- Pereopods 3 and 4, carpus 1/2 to 3/4 overlapped by the merus (Figs 27–29). Female body size at maturity 5.9–9.8 mm. Gnathopod 2, propodus, only the female palm densely plumose; larger male ~ 6 mm body length or more, palm defined by a thumb (Fig. 30)............................................................................................ 3

3 Female gnathopod 2, propodus, dactylar hinge tooth shallow (Fig. 33). Antenna 2, large male and female, posterior margin of article 5 and flagellum plumose (Fig. 33)............................................ P. wandeli (Chevreux, 1906)

- Female gnathopod 2, propodus, dactylar hinge tooth deep (Figs 27 and 28). Antenna 2, adult female and comparably sized male, posterior margin of article 5 and flagellum not plumose (Figs 27 and 28)......................................... 4

4 Gnathopod 1, carpus with a single or cluster of setae at the anterodistal junction of the propodus (Fig. 27). Known only from South Georgia (Fig. 2)..................................................... P. multidentata (Schellenberg, 1931)

- Gnathopod 1, carpus without a single or cluster of setae at the anterodistal junction of the propodus (Fig. 28). Known only from Macquarie Island (Fig. 2).................................................................. P. orientalis n. sp.