Pachypygus exilis sp. nov.

(Figs. 82, 83)

Typematerial. Holotype (intact ♀, MNHN-IU-2014- 21241) and paratype (♀, dissected) from the compound ascidian Aplidium nadaense (Nishikawa, 1980) (MNHN-IT-2008-576 = MNHNA1 /APL.B/407), CRRFOCDN 5743-A, Brooker Channel, Calvados Island Chain, Louisiade Archipelago, Papua New Guinea (11°03.09’S, 152°28.62’E), depth 7 m, 01 June 1998.

Additionalmaterial. 1 ♀ (dissected) from Aplidium lineatum Monniot F. & Monniot C., 1996, North Sulawesi, Indonesia, OCDN 1447-I.

Etymology. The specific name is from the Latin exil (= small), referring to the relatively small body size of the new species.

Descriptionoffemale. Body (Fig. 82A) relatively small and stout, bilaterally compressed, and flexed ventrally. Body length 1.55 mm. Posterolateral corners of cephalosome produced into small nipple-like processes. Brood pouch tapering in posterior quarter towards blunt tip.Urosome (Fig.82B) 6-segmented, distinctlynarrowing posteriorly: fifth pedigeroussomite 233 μm wide, distinct butnot articulatedfrom brood pouch. Genital somitemuch widerthan long, 71×180 μm; 4 freeabdominalsomites 91×144, 98×122, 85×91, and 40×69 μm, respectively. Anal somite small, lacking posteroventral protuberance. Caudalramus (Fig. 82C) about 2.8 timeslongerthan wide (62×22 μm), armed with 4 distal claws and 2 setae; claws subequalin length and thickness, 13, 12, 11, and 11 μm long; 2 setae as long as greatest width of ramus, positioned at 49% and 67% of ramus length.

Rostrum (Fig. 82D) 94×82 μm, slightly longerthan wide, tapering strongly in distal half, but with nearly parallel lateral margins proximally. Antennule (Fig. 82E) 9-segmented; first 2 segments broad; secondsegment elongate, about twice as long as wide; armature formula 3, 16 (2 setae omitted in Fig 82E), 6, 4+aesthetasc, 5, 2+aesthetasc, 2, 2+aesthetasc, and 7+aesthetasc; all setae naked. Antenna (Fig. 82F) slender, 4-segmented; coxa, basis, and first endopodal segment unarmed; basis and first endopodal segment subequal in length; compound distal endopodal segment about 3.6 times longer than wide (87×24 μm) and about 1.3 timeslongerthan first endopodal segment, armedwith 8 setae (arranged as 3, 2, and 3) plus terminal claw, more than half as long as segment.

Labrum (Fig. 82G) with posterolateral prominences ornamented with setules laterally and spinules distally; smallposteromedianlobebearing spinules. Mandible (Fig. 82H) with 5 teeth and 2 small setae on coxal gnathobase; basis with 1 seta on medial margin and tuft of setules proximally on outer margin; exopod unsegmented, armed with 5 setae, outermost seta distinctly longer than other 4; endopod with 4 and 10 setae on first and second segments, respectively. Paragnath (Fig. 83A) as usual for genus. Maxillule (Fig. 82I) with 10 setaeon arthrite, otherwise setation as in P. papillosus sp. nov. Maxilla (Fig. 83B) as usual for genus. Maxilliped (Fig. 82J) with 9, 1, and 3 setae on first to third segments, respectively; articulation incomplete between first and second segments.

Legs 1–4 (Fig. 83 C–G) with 3-segmented rami. Inner coxal seta well-developed in legs 1 and 2, lacking in legs 3 and 4. Outer seta on basis large and pinnate in leg 1, small and naked in legs 2–4. Inner distal spine on basis of leg 1 longer than first endopodal segment, with spinulose margins. First to third endopodal segments of leg 1 (Fig. 83D) with 6, 3 or 4, and 2 or 3 sensillae, respectively, on anterior surface. First exopodal segment of legs 2–4 about 3 times longer than wide. Third exopodal segment of legs 2–4 about 2.5 timeslongerthan wide. Armatureformula for legs 1–4 as in P. papillosus sp. nov.

Leg 5 (Fig. 83H) protopod longer than wide, armed with 1 seta at outer distal corner and ornamented with row of spinules distally near base of exopod; free exopodal segment about 3.9 timeslongerthan wide (101×26 μm), armed with 1 small spine and 1 naked seta distally, ornamented with 4 or 5 rows of small spinules on medial surface.

Male. Unknown.

Remarks. Pachypygus exilis sp. nov. most closely resembles P. papillosus sp. nov. as both have paired posterolateral processes on the corners of the dorsal shield of the cephalosome. Both also have 3 setae on the third segment of the maxilliped and share the identical armature formula for legs 1–4. They differ in: (1) the size of the posterolateral processes on the cephalosome, which are small in the new species but large in P. papillosus sp. nov.; (2) the unusually elongate second segment of the antennule is rectangular in the new species, compared to tapering in P. papillosus sp. nov.; and (3) the second endopodal segment of the mandible is armed with 10 setae in the new species, compared to only 9 setae in P. papillosus sp. nov. The most conspicuous difference, however, is that the new species lacks any papillate ornamentation on the body surface, in contrast to the highly ornate P. papillosus sp. nov.