Haplomesus hanseni Kavanagh, Wilson & Power, 2006, sp. nov.

Haplomesus hanseni sp. nov. (Figs 2–7)

Etymology. Named for H.J. Hansen who described the first species of Ischnomesidae without pereopod VII, Haplomesus angustus Hansen, 1916.

Material examined. Holotype female, 4.7 mm, AM P68182, Atlantis II cruise 60, Howard Sanders & party, WHOI stn 245 (subsample B), 14 March 1971, Argentine Basin, 36° 55.7S, 53°01.4'W, depth 2707 m, epibenthic sled. Paratypes, same subsample: adult male 3.7 mm and slide of pereopod I AM P71668; juvenile male 3.6 mm, AM P71669 manca male 2.7 mm, AM P71670; paratype adult male pleopods on slide AM P.71711; paratype adult male mouthparts slide P.71712. Additional paratypes from same sample (218 ind., WHOI 245B): 2 males & 2 females P.71671, adults P.71672, juveniles P.71673, paratype mancas P.71674; (227 ind., WHOI 245A): 1 male fragment AM P.71771, 207 ind. AM P.64981.

Diagnosis. Male and female with subequal spines on pereonites 1 and 4. Male spines elongate, length near width of pereonite 1, female spines short, length near length of pereonite 1. Pleotelson posterolateral margin anterior to pereopods with pedestal spines, in female shorter than uropoda, not reaching pleotelson posterior margin; in male longer than uropoda and extending past pleotelson posterior margin. Antennula with 4 distal flagellar articles. Pereopod I carpus ventral margin with 2 proximal robust setae and 1 distal robust seta on palm. Pleopod I of male with simple setae on distal and lateral margins. Female pleopod II operculum with plumose setae.

Description of female. Body length 4.5–4.7 mm; pereonite 7 reduced. Head length 0.7 width; lobe on ventrolateral margin absent in lateral view; dorsal cuticle densely covered with fine granules. Pereonite 1 width 0.16 total body length. Pereonites 1 and 4 anterolateral simple spines conical shape. Pereonite 5 length 2.9 width, 0.4 total body length. Pleotelson length 1.3 width.

Antenna and antennula. Antennula with 6 articles altogether; article 1 with 1 penicillate seta, article 2 length 1.3 head width, with 2 elongate ventromedial setae, distal articles altogether small, shorter than article 2; article 3 length similar to article 4; article 3 elongate and tubular, much longer than wide; terminal article shorter than penultimate article; aesthetascs absent. Antenna article 3 length 0.2 anterior body length, length 3.9 width, cuticle smooth.

Mouthparts. Maxillula with 11 robust setae on lateral lobe; medial lobe with 1 robust medially­projecting dentate seta. Maxilla with 2 long (approximately as long as lateral lobes) medially­projecting pectinate seta on medial lobe. Maxilliped palp article 2 wider than 3.

Pereopods. Pereopod I propodus ventral margin with 1 robust seta. Pereopod VII absent in adults.

Pleopods and uropods. Pleopod II operculum with narrow proximal neck, laterally convex, broadening posteriorly to rounded angles, posterior margin weakly convex. Uropods extending near posterior margin of pleotelson; length 0.1 length of pleotelson.

Description of male.

Body length 3.7–4.9 mm. Head length 0.5 width. Head cuticular structure granulated. Pereonite 1 width 0.16 total body length. Pereonite 5 length 3.8 width, 0.48 total body length. Pleotelson length 1.1 width; posterolateral margin simple spines approximately twice as long as spines in female.

Antennula and antenna. Antennula article 2 length 1.3 head width. Antenna length 0.2 anterior body length. Antenna article 3 length 2.5 width; flagellum with some segments expanded, wider than long, with more setae on distal margins than female or juvenile.

Pleopods and uropods. Pleopod I distal tip with lateral horns. Pleopod II protopod apex rounded, stylet not extending to distal margin of protopod. Pleopod III exopod with plumose seta on distal margin, with fringe of fine setae on lateral margin. Uropods extending near posterior margin of pleotelson, length 0.1 length of pleotelson.

Distribution. South Atlantic Ocean: Argentine Basin, 2440–2707 m.

Remarks. The females of Haplomesus hanseni sp. nov. (Fig. 2) and H. celticensis sp. nov (Fig. 8) are easily distinguished from one another by a different spination pattern, Haplomesus hanseni has anterolateral spines on pereonites 1 and 4, Haplomesus celticensis has spines on pereonite 1 only. The males share the same spination pattern. Both sexes are easily distinguished by the number of articles in the antennular flagellum: three articles in H. celticensis, and Haplomesus hanseni with four articles. The males also differ in the setation of the first pleopod: H. hanseni males have distal setae on the first pleopod (Fig. 5 A) and H. celticensis males do not (Fig. 11 A).