Jesogammarus Bousfield, 1979

Tomikawa, Ko & Kimura, Naoya, 2021, On the brink of extinction: a new freshwater amphipod Jesogammarus acalceolus (Anisogammaridae) from Japan, ZooKeys 1065, pp. 81-100 : 81

Genus Jesogammarus Bousfield, 1979

Type species.

Anisogammarus jesoensis Schellenberg, 1937

Diagnosis.

Pleonites not carinate dorsally, with slender and robust setae (robust setae often lacking). Dorsal margins of urosomites with 4 (3), 4 (2), 2 (4) clusters of setae or single robust seta; urosomite 2 without prominent median tooth. Antenna 1 longer than antenna 2; article 1 of peduncle subequal to or slightly longer than article 2. Male antenna 2, flagellum with or without Sternophysinx calceoli . Maxilla 1, palp article 1 without setae. Female gnathopods 1 and 2 strongly dissimilar. Coxal gills on gnathopod 2 and pereopods 3-7, gills 2-5 each with 2 accessory lobes, gills 6 and 7 each with 1 accessory lobe. Uropods 1 and 2, rami extending beyond peduncle of uropod 3. Uropod 3, inner ramus not longer than 0.4 times of that of outer ramus; terminal article distinct. Brood plate 2 of female broadly expanded anteroproximally.

Remarks.

The presence of a calceolus on the flagellum of male antenna 2 is a major diagnostic feature of Jesogammarus , which distinguishes it from Ramellogammarus ( Bousfield 1979; Morino 1985). However, the discovery of the new species, J. acalceolus , which lacks a calceolus, indicated that the calceolus was not critical for diagnosis. The genus Jesogammarus is distinguishable from Ramellogammarus by the dissimilar female gnathopods 1 and 2 and the expanded brood plates of the female. The genus Jesogammarus shares a similar coxal gill type with marine Locustogammarus Bousfield, 1979 and Spasskogammarus Bousfield, 1979 but differs from these two genera in terms of the following features (features of Locustogammarus and Spasskogammarus in parentheses): from Locustogammarus , in terms of longer antenna 1 than antenna 2 (subequal in Locustogammarus ), dissimilar female gnathopods 1 and 2 (similar in Locustogammarus ), uropods 1 and 2 with rami extending beyond the peduncle of uropod 3 (not extending in Locustogammarus ), and a distinct terminal article of uropod 3 (very small in Locustogammarus ); from Spasskogammarus , in terms of dorsal margins of pleonites with slender setae (lacking in Spasskogammarus ), longer antenna 1 than antenna 2 (subequal in Spasskogammarus ), and slender pereopods 5-7 (short in Spasskogammarus ).