Jassa pusilla

Jassapusilla (Sars, 1894)

(Table 11, Figs 37–41)

Synonyms: see Conlan (1990).

Diagnosis.

Both sexes:

Mandibular palp: article 2, dorsal margin without a fringe of setae.

Maxilla 1: without a seta or setal cluster at the base of the palp article 1.

Gnathopod 1: basis, anterolateral margin with a few very short setae; carpus without a single or small cluster of setae at the anterodistal junction of the propodus.

Gnathopod 2: basis with a few minute setae along the anterolateral margin but without long filter setae (most 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 not expanded anteriorly.

Uropod 1: ventral peduncular spinous process underlying about 1/4 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: without plumose setae on the flagellum and peduncular article 5.

Gnathopod 2: propodus, palmar defining spines absent except in small males. Thumb distally acute or squared in minor males and indented in major males. Dactyl expansion variable, centrally toothed or shallowly expanded proximally.

Adult female:

Antenna 2: without plumose setae on the flagellum and peduncular article 5.

Gnathopod 2: propodus, palm concave, palmar defining angle acute.

Remarks. The shape of the male second gnathopod is highly variable but Fig. 39 shows how shapes can grade within a series, this being a single collection made by G. O. Sars from Riŝr, Norway. Juvenile males that approach the size of the thumbed males have pre-thumbs that are small relative to minor and major forms (Figs 39A, Band 40). The pre-thumb is located in the distal half of the palm. Those juveniles that are about to molt into a thumbed stage (showing a thumbed cuticle under the juvenile cuticle) are termed subadult in Fig. 40. These have small prethumbs as well. Minor forms are interpreted as being those that have thumbs that are also in the distal half of the palm (as in minor forms of other species) but that have a larger and longer thumb and never show a thumbed cuticle inside (i.e., will not molt again) (Figs 38 and 39 C–E). Minor forms also have centrally toothed dactyls (Figs 39 C–E). Major form males have longer thumbs that are apically indented at the tip. Their thumbs originate in the proximal half of the propodus (Figs 38 and 39 F–J). The dactyl expansion is shallow and proximal in most major forms (Fig. 39J) but is centrally toothed in some (Fig. 39G). Sars originally named the major form Podocerus pusillus and the minor form Podocerus odontonyx (Fig. 38) (see Conlan 1990). Fig. 39B is interpreted as being juvenile because no thumbed cuticle was visible internally, its pre-thumb is short relative to body length (Fig. 40) and its dactyl is not centrally toothed as in the minor forms (Figs 39 C–E). None of the specimens termed juvenile and subadult in Fig. 40 had centrally toothed dactyls.

Four populations sampled in Väderârna, Sweden, Trondheimsfjord, Norway, Riŝr, Norway and Tórshavn, Farøe Islands showed a similar relationship between thumb length and body length, with the juveniles having prethumbs and the minor forms being rare (Fig. 40). Jassa pusilla is a small species, with the minor forms 2.8–3.8 mm in body length and the major forms 3.0– 4.5 mm in these four populations.

The record of J. pusilla from samples along the coast of Portugal by Lobo et al. (2017) is in error, and is the new species J. laurieae. The three collections of J. pusilla from deep water offshore of the Atlantic U.S. coast (NMNH 6335, 33530, and 106781) (Fig. 9) morphologically resembled this species, not either of the other two species from this coast (J. valida and J. marmorata), which occur shallower. Jassa pusilla is a smaller species with delicate, never plumose antennae, lacking the typical long setae along the basis of gnathopod 2 that the other two possess, and also lacking the diagnostic antero-medial seta on the gnathopod 1 carpus at the junction of the propodus. The minor form thumbed male of Jassa pusilla has a strong tooth on the inner margin of the dactyl, which the minor forms of the other two species lack. One of the specimens from NMNH 6335 was a 3 mm long minor form thumbed male with a strong tooth on the dactyl, its propodus closely resembling that in Figs 38 and 39E. Another thumbed male (NMNH 33530) closely resembled Fig. 39B. The other specimens were female or juvenile. These three collections are the only ones known from the western North Atlantic.