Amphipoda of the Atlantic and Arctic Coasts of North America: Anonyx (Lysianassidae)

In place of the widely ranging, abundant, and variable species Anonyx nugax, previously recorded from Canadian Atlantic and Arctic waters, eight less variable species of this genus are here recognized, one of which (sarsi) is new to science. Detailed descriptions, figures, distribution maps and a key applying to all sizes are given. The study of the species is based on examination of available type-specimens and the use of new characters. The systematics of the genus is discussed in a chronological review of all the relevant literature, and the status of several species is revised.Since the young of large and small species are similar among themselves, and have sometimes been wrongly described as distinct species, they are also described and illustrated at two sizes. Mature males are distinguished by elongated second antennae equipped with calceoli, in most species by the armature of the second and third uropods, and sometimes by their enlarged eyes.Three species are characterized by a row of spines accompanying single setae on the hind margin of segment 6 of peraeopods 3–4, and by a rather unprominent upper lip. One may distinguish them as follows. The large (> 33 mm) A. laticoxae has short spines on peraeopods 3–4, an anteriorly well-expanded first coxa, and an unconstricted short-spined inner ramus of uropod 2. Anonyx compactus is smaller (12–17 mm), has one elongated ventral spine on each of the first two flagellar segments of antenna 1, a posterior projection marked off by an angle from the lateral plate of metasome segment 3, a strongly parachelate peraeopod 1 with a perpendicular palm, and a fairly strongly subchelate peraeopod 2. Anonyx sarsi is of medium size (20–31 mm), has a very low and unprominent upper lip and, on the outer ramus of uropod 2, small similar-sized spines on the proximal two-thirds of the inner margin (no such spines in laticoxae and compactus); the female has tufts instead of a row of setae on the dorsal margin of the fourth peduncular segment of antenna 2, but otherwise the species is morphologically similar to A. nugax.The other five species are characterized by a row of pairs of long setae on the hind margin of segment 6 of peraeopods 3–4 and, except nugax, have a rather prominent upper lip. Except in ochoticus and lilljeborgi, uropod 2 bears spines on the inner margin of its outer ramus. Anonyx ochoticus is the smallest (9–11 mm) species, and has on urosome segment 1 a prominent dorsal keel well overlapping the next urosome segments. Anonyx debruyni, rather small (15–21 mm), has a parachelate peraeopod 1 with an oblique palm, a very strongly subchelate peraeopod 2, most prominent and pointed upper lip and interantennal angle, and, on the outer ramus of uropod 2, only two small spines on the distal half of the inner margin. Anonyx lilljeborgi is small (9–18 mm), has a very prominent but high and usually evenly rounded upper lip, a concave cephalic ventral margin (like debruyni), and a lateral plate of metasome segment 2 with a nearly toothless postero-ventral corner. Anonyx nugax is the largest (31–44 mm) species, with a little-projecting upper lip, a prominent tooth at the postero-ventral corner of metasome lateral plate 2, a short posterior projection, with a nearly straight ventral margin, on the lateral plate of metasome segment 3 and, on the inner margin of the outer ramus of uropod 2, a row of similar-size small spines not extending into the distal third of the ramus. Anonyx pacificus is also large (27–34 mm) and similar to nugax, but differs in the more prominent and pointed upper lip, the longer and ventrally concave posterior projection on metasome lateral plate 3, and, on the outer ramus of uropod 2, spines which increase in size distally and reach into the distal third of the inner margin.Three species, nugax, lilljeborgi, and sarsi, have a circumpolar arctic-boreal distribution, the latter chiefly in shallow water (< 50 m). Four (pacificus, laticoxae, ochoticus, and compactus) are colder-water forms apparently of Pacific origin and all new for the North Atlantic. The distribution of debruyni is uncertain.