Haplostoma depressum Kim & Boxshall 2021, sp. nov.

Haplostoma depressum sp. nov.

(Figs. 100, 101)

Type material. Holotype ♀ (MNHN-IU-2009-5190), 4 ♀♀ paratypes (MNHN-IU-2014-21515), and 2 ♀♀ para- types (dissected, MNHN-IU-2014-17385) from Aplẚdẚçpsẚs pyrẚfçrmẚs (Herdman, 1886); Kerguelen, MD42 - SIBEX cruise, RV “Marion Dufresne”, Stn 4/CP25 (53°11’S, 74°05’E), depth 285-375 m, MNHN coll., 14 January 1985.

Additional material. 4 ♀♀ (MNHN-IU-2014-21516) from Aplẚdẚum fuegẚense Cunningham, 1871; Kerguelen, no other locality data; 7 ♀♀ in A. pyrẚfçrmẚs, SSE Kerguelen Is., MD04 -BENTHOS cruise, R. V. “Marion Dufresne”, Stn DC 275 (49°58’S, 70°29’E), depth 234 m, Boury-Esnault coll., 15 March 1975.

Etymology. The body of the new species is dorsoventrally depressed, hence its name.

Description of female. Body (Fig. 100 A-C) eruciform, slightly depressed dorsoventrally, consisting of cephalosome, 4-segmented metasome, and small genitoabdomen. Body length 1.91 mm in dissected specimen (Fig. 100B); maximum width 740 μm across last metasomite and maximum dorsoventral depth 542 μm. Cephalosome distinctly narrower than metasome, metasome gradually broadening posteriorly, obscurely segmented in dorsal and ventral views (Fig. 100A, B), but metasomites distinctly defined by dorsal and ventral constrictions (Fig. 100C). Compound last metasomite consisting of fused fourth and fifth pedigerous somites. Genitoabdomen (Fig. 100D) as long as wide (262×258 μm), 2-segmented; anterior somite subcircular, with traces of 2 suture lines on dorsal sur- face; small posterior somite (free abdomen) distinctly defined from anterior somite, wider than long in contracted specimen (Fig. 100D), but longer than wide in relaxed specimen (Fig. 100E). Caudal ramus (Fig. 100F) strongly tapering, armed with 2 setae (1 dorsal and 1 on outer margin) and 2 distal spines. Distal spines (ventral and dorsal) on caudal ramus unequal; ventral spine smaller and simple; larger dorsal spine bifurcate, with pointed dorsal and blunt ventral branches; ventral branch invariably ornamented with 1 or 2 minute spinules subdistally.

Rostrum as small, semicircular anterior protuberance on cephalosome. Antennule (Fig. 100G) 3-segmented; articulation incomplete between 2 distal segments; armed with 2, 2, and 13 small setae on first to third segments, respectively; one seta on second segment consisting of extremely swollen proximal half and slender distal half. Antenna (Fig. 100H) 2-segmented; proximal segment unarmed; distal segment 2.6 times longer than wide (43×16 μm), as long as proximal, armed with 4 simple spines, proximal 2 smaller than distal 2; all spines distinctly articulated at base.

Labrum (Fig. 100I) bearing 4 broad lobes on posterior margin, lateral lobes longer, prominent, medial lobes short. Labium well-developed, located posterior to labrum. Mandible (Fig. 100J) distinctly 2-segmented; tapering proximal segment unarmed; distal segment 1.8 times longer than wide, tipped with 2 equal setae. Maxillule and maxilla absent. Maxilliped (Fig. 101A) 4-segmented; broad first segment unarmed; second segment also wider than long, bearing 2 leaf-like setae; short third segment unarmed; fourth segment with 1 small seta distally; terminal claw unequally bifurcate.

Legs 1-4 each consisting of protopod, exopod, and endopod; protopods lacking outer seta. Exopods incompletely articulated from protopods. Endopods prominent, semicircular, indistinctly defined from protopods. Exopod of leg 1 (Fig. 101B) armed with 1 seta and 4 simple spines, proximal 2 spines smaller than distal 2. Exopod of leg 2 (Fig. 101C) armed with 1 seta and 3 simple spines; spines becoming larger from proximal to distal. Exopod of leg 3 (Fig. 101D) armed with 1 seta and 2 simples spines. Leg 4 shaped and armed as leg 3.

Leg 5 (Fig. 101E) short, wider than long, strongly tapering towards blunt apex; armed with 1 subdistal and 2 distal, small setae. Leg 6 (Fig. 101F) 1 small spine, 2 unequal, spiniform processes, and 1 small setule on genital operculum; 5 internal dentiform elements present near leg 6.

Male. Unknown.

Remarks. The combination of armature elements on the exopods of legs 1-4 of e. depressum sp. nov. is 5, 4, 3, and 3, as in e. albẚcatum, e. dentatum Ooishi & Illg, 1977, e. elegans Ooishi & Illg, 1977, e. mẚnutum, and e. eruca. eaplçstçma depressum sp. nov. can be clearly distinguished from those five species as well as all other congeners by the three following features: (1) the caudal ramus is armed with 2 spines and 2 setae; (2) the labrum bears 4 lobes on the posterior margin; and (3) the mandible is 2-segmented. Although the second feature is shared with e.

ambẚguum, the 4 lobes on the labrum of the latter species are equally weak, unlike the combination of 2 large lateral and 2 weak medial lobes in the new species.