Ithyomus antarcticus Larsen, 2013, n. sp.

Ithyomus antarcticus n. sp.

(Figs. 1 & 2)

Material examined. Holotype: neuter (Reg. # ZMH K-43955) dissected, ANDEEP-I, DZMB-HH 10589, station 105-2, 61 °24.14’S, 58°51.15’W, 2290 m, 14 February 2002.

Diagnosis. Maxilliped endites with two small acute denticulations; palp article 2 with outer seta. Cheliped carpus and propodus lacking crenulations, carpus wide (l/w 1.4). Pereopods 1 and 2 propodus with dorso-subdistal seta. Pereopods 4 to 6 basis robust, ischium with two robust setae.

Etymology. Named after the continent of Antarctica.

Description. Holotype, neuter 2.9 mm.

Body (Fig. 1 A). Cylindrical, 13 times as long as wide. Cephalothorax shorter than the lengths of pereonites 1 and 2 combined. Eyes and eye-lobes absent. Pereon lateral edge even, without setae. Pleon long (more than 0.25 times total body length). All pleonites subequal and with pleopods. Pleotelson shorter than two pleonites combined.

Antennule (fig. 1B). Shorter than cephalothorax. Article 1 shorter than rest of antennule, with one simple distal seta. Article 2 less than half as long as article 1, with two simple distal setae. Article 3 shorter than article 2, with two simple distal setae. Article 4 longer than article 2, with five distal setae and one aesthetasc.

Antenna (fig. 1C). Almost as long as antennule. Article 1 wider than following articles, naked and fused to cephalothorax. Article 2 shorter than article 1, with one simple dorsodistal seta. Article 3 longer than article 1, with one simple dorsodistal seta. Article 4 longer than other articles, with five simple distal setae. Article 5 longer than article 3, with one simple distal seta. Article 6 decreasing in width at midlength, with two short subdistal and three long distal setae.

Mouthparts. Labrum (fig. 1D,E) elongate, protruding from under the carapace, naked. Mandibles (fig. 1F,G) without molar or lacinia mobilis, incisors pointed. Labium not recovered. Maxillule (fig. 1H) endite with seven spiniform terminal setae; palp with two long, but unequally so, setae. Maxilla (fig. 1I) stout, naked. Maxilliped (fig. 1J) basis naked; endites narrower than basis, with distal seta; palp article 1 naked, article 2 with three simple inner setae and one outer seta, article 3 with four inner setae, article 4 with five long setae. Epignath (fig. 1K) elongate, naked.

Cheliped (fig. 2A). Pseudocoxa prominent (three times as long as basis), naked and only connected to ventroproximal part of basis. Basis as long as merus, with ventral seta. Merus with one ventral seta. Carpus robust (l/w 1.4), as long as propodus including fixed finger, with one simple dorsal seta in each end and two robust ventral setae. Propodus with one seta at dactylus insertion and five inner setae increasing in length ventrally. Fixed finger with two robust ventral setae and three on inner margin. Dactylus as long as fixed finger, with long stout seta midlength.

Pereopod 1 (fig. 2B). Coxa apparently naked. Basis longer than the three succeeding articles combined, naked. Ischium with one ventral seta. Merus shorter than carpus, widening distally, with two distal setae. Carpus longer than propodus, with three distal setae (possible an artefact as we would expect four setae). Propodus less than half as long as basis, with one dorso-subdistal seta, one bayonet seta as long as dactylus/unguis ventrodistally, and dorsal spine. Dactylus with dorsoproximal setae and combined with unguis as long as propodus. Unguis longer than dactylus.

Pereopod 2 (fig. 2C). As pereopod 1 except: carpus with three bayonet seta and one spiniform distal seta.

Pereopod 3 (fig. 2D). As pereopod 2 except: propodus apparently without dorsal seta (probably an artefact). Pereopod 4 (fig. 2E). Coxa not visible. Basis with one setulose ventromedial seta arising from a process. Ischium with two thick ventral setae. Merus with two spiniform setae with bi- or trifurcate apex. Carpus longer than merus, with one bone-shaped seta and four spiniform distal setae with bi- or trifurcate apex. Propodus with two ventral and one dorsal bayonet setae and dorsal spine. Dactylus naked and combined with unguis marginally longer than propodus. Unguis, shorter than dactylus.

Pereopod 5 (fig. 2F). As pereopod 4.

Pereopod 6 (fig. 2G). As pereopod 4 except: propodus with two ventral and three dorsal bayonet setae.

Pleopods (fig. 1L). Well developed; endopod length subequal to exopod, with one outer and 13 inner plumose setae, the first of which have a complex apex. Exopod broader than endopod, with strongly convex inner margin and 31 plumose inner setae, the most proximal of which arises from an incompletely fused proximal vestigial article

Uropod (fig. 1M). Basal article shorter than first endopod article. Endopod with two articles; article 1 more than twice as long as article 2, with three distal setae, article 2 with one medial and four distal setae. Exopod fused to basal article, more than half as long as endopod article 1, with blunt apex, with three setae.

Remarks. This species can be separated without dissection from the only other species in the genus, Ithyomus conopygus Bird, 2004 by the robust cheliped carpus and the carpus and propodus lacking crenulations. Furthermore it differs in the armament of the maxilliped endites, the dorso-subdistal seta on the pereopod 1 and 2 propodus, and pereopods 4 to 6 having a robust basis and two robust setae on the ischium.