Jassa (Crustacea: Amphipoda): a new morphological and molecular assessment of the genus Author Conlan, Kathleen E. 0000-0002-2263-7075 kconlan@nature.ca Author Desiderato, Andrea 0000-0002-2263-7075 kconlan@nature.ca Author Beermann, Jan 0000-0002-2263-7075 kconlan@nature.ca text Zootaxa 2021 2021-03-04 4939 1 1 191 journal article 7102 10.11646/zootaxa.4939.1.1 ee8e66ff-2f2c-47e2-978b-be52996d5b0f 1175-5326 4580622 F33F42D0-A139-4CE3-97D7-1314C12CF86B Jassaingens Pfeffer, 1888 ( Table 13 , Figs 99–101 ) Synonyms: see Conlan (1990) . Diagnosis. Both sexes: Mandibular palp : article 2, dorsal margin without a fringe of setae. Maxilla 1 : with a small cluster of setae at the base of the palp article 1. Gnathopod 1 : basis, anterolateral margin with a few to a full row of short setae along its length which may be slender or spine-like; carpus with a single or small cluster of setae at the anterodistal junction of the propodus (setae about 1/3 of anterior margin length and slightly lateral and medial). Gnathopod 2 : basis with a row of small setae along the anterolateral margin (setal lengths <10% of the basis width); carpus and propodus, setae on the anterior margin short and simple (setal length <basis width). Pereopods 5–7 : propodus expanded anteriorly. Uropod 1 : ventral peduncular spinous process underlying about 40% of the longest ramus. Uropod 3 : inner ramus without spines mid-dorsally (with only the single apical spine). Telson : tip without apical setae (only the usual short setae at each dorsolateral cusp). Thumbed male: Antenna 2 : large individuals without plumose setae on the flagellum and peduncular article 5. Gnathopod 2 : propodus, palmar defining spines produced on a ledge, present in both small and large thumbed males. Adult female: Antenna 2 : without plumose setae on the flagellum and peduncular article 5. Gnathopod 2 : propodus, palmsinuous. Remarks. All of the thumbed males available for study had relatively short thumbs with the palmar defining spines present and produced on a ledge, suggesting that these spines are not lost even in the largest animals. The antenna 2 setae are long in small juveniles but shorten drastically in the adult males. The expanded propodus of pereopods 5–7 is distinctive, even in the young. This is the largest species known for the genus, with body length at maturity in the specimens available 19.6–24.1 mm for the adult (thumbed) males and 15.3–20.8 mm for the adult females.