Jassa Leach 1814
Creators: name: Conlan, Kathleen E.
name: Desiderato, Andrea
name: Beermann, Jan
Publisher: name: Zenodo
Year: 2021
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Amphipoda
Ischyroceridae
Jassa

description: Genus Jassa Leach, 1814
Lusyta Nardo, 1847b, p. 20.
Cratophium Dana, 1852b, p. 309.
Macleayia Haswell, 1880, p. 32.
Bruzeliella Norman, 1905, p. 83.
Description of adult male. Body length at maturity 2–24 mm.
Head lobe: oval or squared, but with dorsal angle more acute and ventral angle more broadly rounded.
Antenna 1: accessory flagellum 2-articled, second minute, only second setose.
Antenna 2: stouter and longer than antenna 1, bearing finely pectinate “filter setae” on peduncular articles 3–5 and shorter, pectinate “brush setae” on flagellum, in many species these being interspersed with feathery plumose setae on distal part of peduncular article 5 and proximal part of flagellum; flagellum with at least 2 of its distal articles bearing posteriorly curved spines, first article considerably longer than any of the distal articles.
Mandible: at least 1 molar with 1 seta and 1 lateral flake.
Maxilla 1: inner plate bearing a few short fine setae, palp with 1 or more rows of setae.
Gnathopod 1: coxa rectangular, often anterodistally produced; propodus, palm defined by 3 spines that are usually arranged in medial-lateral-medial sequence.
Gnathopod 2: coxa deeper posteriorly, with 1 gill; basis, setae, when present, simple or short and plumose, and located on the anterolateral flange only; carpus less than 0.25 of length of propodus; propodus with triangular, shallowly bifid tooth directly posterior to dactyl hinge and larger thumb incised anteriorly to palmar defining spines (or associated setae if spines absent), inner margin of thumb not setose, outer margin with 3-4 discrete clusters; dactyl shorter than propodus, extending beyond the thumb to rest against the thumb’s posterior margin when closed, inner margin expanded at location of hinge tooth or more centrally and acutely as a tooth into palmar incision, dactyl cusps reduced to small buttons interspersed with a few short setae.
Pereopods 3 and 4: coxa 3 usually deepest posterior of centre; coxa 4 deepest centrally; basis, margins convex, or anterior margin straight or shallowly concave in large specimens; merus anterodistally produced to half to fully overlap the carpus, anterior margin bearing numerous setae in clusters or singly; propodus not posteriorly spinose.
Pereopods 5–7: basis of at least one of peraeopods 5–7 posterodistally produced; merus and carpus, posterior margin not spinose; propodus variably expanded; dactyl, posterior (outer) margin not cusped distally, anterior (inner) margin bearing 1 seta at the unguis or short setae along its length.
Pleopods: well developed, with 2 peduncular coupling hooks each in most species, but more in large species.
Urosome: segment 1 bearing a pair of erect setae dorsally.
Uropod 1: peduncle usually extended ventrally as a spinous process under the rami.
Uropod 2: ventral spinous process short or absent.
Uropod 3: peduncle finely setose ventrally, but without spines middorsally, and with a crown of spines around the dorsodistal margin and a cluster of setae distolaterally; outer ramus not setose mid-dorsally, tipped by a basally immersed, dorsally recurved spine, 2 large cusps, and many additional minute cusps; inner ramus tipped by 1 single short spine which is not recurved and not accompanied by cusps.
Telson: each corner with 1 pair of button-like cusps accompanied by 1 long pectinate seta and 1 or 2 short plumose setae, but without spines.
Description of adult female. Body length at maturity 2–21 mm. Character states as in the male except as follows.
Brood plates: broad, setae abundant, tips hooked.
Antenna 2: peduncle, posterior filter setae long.
Gnathopod 2: propodus much larger and different in shape from the propodus of gnathopod 1, palm concave or sinuous, without a thumb, defining spines not lost with age; dactyl fitting into a depression between the palmar angle and spines.
Peraeopod 3: basis broadly expanded, margins convex, not straight or concave.
Variation. Antenna 2 may develop plumose setae on article 5 and the flagellum when individuals are approaching maturity, though small individuals that appear to be otherwise adult may not develop plumosity. This occurs in both sexes. The mandibular palp article 2 may bear a row of setae on the dorsal margin in a few species. The maxilla 1 may bear a cluster of setae at its base though most species lack these setae. Akey character is the presence or absence and length of a single or cluster of setae on the anterodistal margin of the carpus at the junction of the propodus in gnathopod 1. If present, these setae may be slightly medial or lateral. The bases of gnathopods 1 and 2 may or may not possess a row of setae and the presence and length of these setae is a key character for species definition. The propodus of gnathopod 2 may have a concave or sinuous palm in the juveniles and adult females. Large subadult males may have a small thumb, termed a “pre-thumb” at the location where the much larger thumb will appear at the last molt. Small subadults of the same species do not exhibit a pre-thumb. At the terminal molt, the thumb may be long or short, sinuous or straight, incised, acute or squared at the tip. “Major form males” have larger thumbs than “minor forms”, but being at the terminal molt, a minor form does not transition into a major form. The minor form may have a tooth on the dactyl that fits into the palmar incision. In the major form, the dactyl is expanded on its inner margin close to the hinge. Most terminal molt species lose their palmar defining spines that are present in the juvenile; the female always retains these spines. For all species the number of spines defining the gnathopod palms may be occasionally 2 or 4 rather than the usual 3. The bases of pereopods 3 and 4 are wide with convex margins in juveniles and females but the bases are slenderer in the thumbed male. In a few species the propodus of pereopods 5–7 may be prehensile. The peduncular spinous process of uropod 1 is absent in some species. The number of major cusps adjacent to the recurved spine that tips the third uropod outer ramus may be rarely 3 or 4 rather than 2. The inner ramus bears extra spines mid-dorsally in some species. Unless noted otherwise, these interspecific differences apply to both sexes and all ages of a species.
Species treatments. The key characters of the 24 species are presented following, with the species grouped by commonality of distribution: trans-hemispheric (Table 10 and Figs 15–36), North Atlantic (Table 11 and Figs 37–59), North Pacific (Table 12 and Figs 60–87) and Southern Hemisphere (Table 13 and Figs 88–104). Key characters are not affected by age or gender and so can be used for the identification of most specimens, with the possible exceptionofhatchlings. Someadditionalgenderandage-specificcharactersareaddedtoassistwithidentification. Followingthespeciestreatmentsisakeytoall 24 species. Whilethedistributionalformatisaimedtosimplify regional identifications, the key should be checked in case the specimen is a new species or a new introduction.
description: Published as part of Conlan, Kathleen E., Desiderato, Andrea & Beermann, Jan, 2021, Jassa (Crustacea: Amphipoda): a new morphological and molecular assessment of the genus, pp. 1-191 in Zootaxa 4939 (1) on pages 43-44, DOI: 10.11646/zootaxa.4939.1.1, http://zenodo.org/record/4580622