Pleojassa moorei Conlan 2021, n. sp.

Pleojassa moorei n. sp.

( Figs 22–24)

Description of male. Holotype: Length 4.0 mm.

Antenna 1: missing. (Male, 3.4 mm, not type, CMN IZ1989-013, antenna 2 overlapped by antenna 1 to midway along article 5).

Antenna 2: article 5 bearing a few simple setae only, these about as long as those of the female; flagellum 3 articles, article 1 75% of full flagellum length.

Mandible: palp articles 2 and 3 without a dorsal fringe of setae; raker spines 3 right, 4 left.

Gnathopod 1: coxal margins, anterior 92% of dorsal length; ventral margin straight; basis, anterior and posterior margins each with a single seta distally; carpus, length 80% of propodus length, posterior lobe 47% of anterior margin length, without an anterodistal setal cluster; propodus, palm convex, one defining spine slightly proximal of centre.

Gnathopod 2: coxal margins, anterior 65% and posterior 61% of ventral length, ventral margin sinuous; carpus 1/4 the length of propodus; propodus with 2 hinge teeth, palm sparsely setose throughout, defined by a single spine, this not associated with a discrete thumb-like extension from the palm.

Pereopod 3: basis slender, anterior margin shallowly convex; merus, anterior margin setose only distally, article width two thirds of length; carpus a third overlapped by the merus; propodus width 44% of length.

Pereopods 5–7: slender, setae and spines sparse, basis not posterodistally produced, anterior margin with a few short setae; merus, posterior margin not spinose.

Uropod 1: peduncle, posteroventral spinous process underlying 88% of the inner ramus, inner and outer rami with 1 and 3 mid-dorsal spines respectively.

Uropod 2: peduncle, posteroventral spinous process underlying 21% of the inner ramus.

Uropod 3: inner ramus not mid-dorsally spinose.

Condition. Without antennae 1, right antenna 2 or left pereopod 3. Remaining right appendages, telson and mouthparts slide mounted. Remaining left appendages with the carcass.

Description of adult female. Allotype: Length 4.25 mm. Character states as in the male except as follows.

Antenna 2: only slightly longer than antenna 1, article 5 without plumose setae, simple filter setae about twice the length of those of the comparably sized male.

Gnathopod 2: propodus without a hinge tooth, palm 40% of the posterior margin, setae not so dense as to obscure the palm’s shape.

Condition. Without antennae 1 and right pereopod 6. Right antenna 2 and gnathopod 2 slide mounted. Other appendages with the carcass.

Variation. Maximum body length: male 4.0 mm, female 4.5 mm. Examination of 8 males and 8 females from the type collection showed no variation in the antenna 1 accessory flagellum with consistently 3 articles. No thumb is produced on the propodus of gnathopod 2 to indicate the sexual state of males as in Jassa (Borowsky, 1985). However, there is some indication of adulthood in the larger antenna 2 and propodus of the gnathopod 2 as well as a longer palm and more pronounced hinge tooth in the male than in the female of similar body length (Figs 22 and 23).

Type material examined. Holotype, ♂, Billie Rocks, Signy Island, South Orkney Islands (60°43ʹS, 45°38ʹW), 4 March 1965, M. H. Thurston, coll., 3.1–3.4 m depth, diver collection of Lithothamnia, Desmarestia anceps, D. chordalis, Ascoseira mirabilis, Rhodophyceae, Porifera, and Polyzoa off solid rock inclined at 65° (NHM, registration no. 1987:511, station no. 13(244)). Allotype, ♀, same location (NHM, registration no. 1987:512). Paratypes, ~ 200 specimens, same location (NHM, registration no. 1969:768 (pt.)).

Other material examined. (excluded from type series): South Georgia: no. 10427, (54°30ʹ58ʺS, 36°0ʹ45ʺW), K. von den Steinen, coll., 16 January 1884, 1 juvenile (ZMH K-28907).

South Orkney Islands: 38 collections from Billie Rocks, Berntsen Point, Outer Islet and Paal Harbour, Signy Island, Dec.– July 1964 and Feb.–Mar. 1965, ~1350 specimens (NHM stations 9–12, 15–26, 30, 32–35, 40, 46–49, 54).

Etymology. In appreciation of Dr. Geoff Moore’s contributions to our knowledge of the systematics, ecology and behaviour of amphipods and of his assistance in locating specimens for this study.

Remarks. Table 4 summarizes obvious differences between P. moorei and P. lowryi. Thurston (1974b) collected 2,912 specimens at 29 stations on Signy Island and named this species Jassa falcata form 3. The additional collections examined here were split collections from the same station.