Macroarthrus, Hendrycks & Conlan, 2003

Hendrycks, EA & Conlan, KE, 2003, New and unusual abyssal gammaridean Amphipoda from the north-east Pacific, Journal of Natural History 37, pp. 2303-2368 : 2338

Macroarthrus gen. nov.

Diagnosis

Eye absent; rostrum short, acute; antenna 1 elongate, about 1.5×length of body, peduncle 3 very short, accessory flagellum very long, about 0.3×length of primary flagellum; antenna 2 about 0.3×length of antenna 1, flagellum short; upper lip, weakly incised, symmetric; mandible, incisors slightly asymmetric, palp strong, threearticulate; maxilliped, inner plate not reaching base of palp, outer plate nearly reaching base of article 2 of palp, palp four-articulate, powerful; gnathopods dissimilar, gnathopod 1 large, propodus strongly tapering, stout, weakly subchelate; gnathopod 2, propodus small, subchelate; peraeopods 3–4 gracile, ischium extremely elongated, almost as long as basis; peraeopods 5–6 subequal; peraeopod 7 dissimilar to peraeopods 5–6, very long, slender, about as long as body; epimeron 2–3 with a posteroventral tooth; urosomite 1, with a posterodorsal, bidentate tooth; urosomite 2, with a slender, posterodorsal tooth; uropods 1–2, lanceolate, very long, more than 2×length of the urosome and strongly spinose; uropod 3, rami elongate, subequal, outer ramus two-articulate, article 2 minute; telson short, slightly longer than wide, lateral margins parallel, cleft v-shaped about 55%, lobes acute.

Type species Macroarthrus victoriae sp. nov. by monotypy.

Etymology

The genus name is from the Greek makros (long) and arthrus (joint), referring to the immensely elongated ischium of peraeopods 3 and 4.

Remarks

Macroarthrus has a wealth of unique characters. The extreme elongation of the ischium of peraeopods 3 and 4 is remarkable and highly distinctive, a character previously unknown in the Gammaridea. The Lysianassoidea possess an elongated ischium of gnathopod 2, but not of peraeopods 3 and 4. Hyperiopsidae have an elongated merus of peraeopods 3 and 4, but the ischium is very short.

Macroarthrus differs from all pardaliscids in the following combination of characters: maxilla 1, inner plate with four apical setae; dissimilar, stout gnathopods with gnathopod 1 the larger; antenna 1 very long, about 3×antenna 2, accessory flagellum extremely long, about 0.25×the length of antenna 1, 18-articulate; peraeopod 7, strongly dissimilar and elongated relative to peraeopods 5 and 6 and the elongated, strongly spinose uropods.