Culexisoma niugini, Poore, Gary C. B., Guinot, Danièle, Komai, Tomoyuki & Naruse, Tohru, 2016

Poore, Gary C. B., Guinot, Danièle, Komai, Tomoyuki & Naruse, Tohru, 2016, Reappraisal of species attributed to Halicarcinus White, 1846 (Crustacea: Decapoda: Brachyura: Hymenosomatidae) with diagnosis of four new genera and one new species from New Ireland, Papua New Guinea, Zootaxa 4093 (4), pp. 480-514 : 498-501

Culexisoma niugini n. sp.

( Figs 6–8)

Material examined. Holotype. Papua New Guinea. New Ireland, N coast of Manne I., 02°40.771'S, 150°42.743'E, 4–7 m, coral slope with mud, 8 June 2014, stn KS23, MNHN IU- 2014-2540 (male, cl 1.4 mm).

Paratypes. Collected with holotype: MNHN IU-2013-7975 (6 males, 1.3–1.6 mm; 4 ovigerous females, 1.5– 1.7 mm). Same locality, stn KB18, MNHN IU-2013-7942 (1 ovigerous female, 1.6 mm); MNHN IU- 2014-2541 (3 males, 1.2–1.5 mm; 1 juvenile male, 1.1 mm; 3 ovigerous females, 1.2–1.5 mm). Between Manne and Baudison Is, 02°42.694'S, 150°41.651'E, 12 m, fine mud (stn KS27), MNHN IU- 2014-2538 (1 male, 1.6 mm; 2 ovigerous females, 1.6. 1,7 mm); NMV J62822 (1 male, 1.6 mm, 1 ovigerous female, 1.7 mm).

Additional material (identified in the field, not counted). Papua New Guinea. New Hanover, SE tip of Anelaua, 02°35.242'S, 150°29.120'E, 4–12 m, sand, rubble, silt (stn KS43), MNHN IU- 2014-2587. W of Nago I., 02°36.111'S, 150°45.933'E, 15–16 m, edge of coral drop off slope (stn KS09). NW point of Manne I., 02°41.240'S, 150°41.234'E, 8 m, silty slope with coral debris (stn KS07), MNHN IU- 2014-2178. Same locality (stn KB06). NW corner of Manne I., 02°41.240'S, 150°41.243'E, 3–5 m, dead coral slope (stn KS15).

Diagnosis. Carapace with pair of posterodorsal subhepatic, 3 pairs of branchiostegal tubercles. Rostrum of male greater than 0.9 times carapace length. Cheliped propodus swollen in males, with proximal blade on cutting edge of dactylus occupying about one third of length. Male maximum carapace length 1.6 mm.

Description. Holotype male. Carapace margin surrounded by complete hymenosomian groove lacking marginal teeth or lobes, ovoid with rounded angles antero-, posterolaterally; length 1.1 times carapace width; regions slightly convex, weakly demarcated by gastro-cardiac, cervical, thoracic grooves; subhepatic region inflated, visible in dorsal view, anteriorly continuous with supraocular eave, bearing blunt lateral tubercle, more tapered postocular tubercle (width at this point 0.7 carapace width), flared ventrolaterally to fit maxilliped 3 endopod, exopod; branchiostegal region visible in dorsal view, with row of 3 obsolete blunt tubercles.

Rostrum slender, elongate, 0.9 times carapace length, tapering distally, demarcated from carapace by hymenosomian groove, rostral tip subacute, with tuft of 5 long setae, longest seta about as long as rostrum. Supraocular eave strongly arching, narrow, defined anteriorly by obsolete broadly-based triangular lateral lobe on side of rostrum (basal rostral width at this point 0.25 carapace width), with regularly-spaced lateral setae running on to rostrum; orbit limited posteriorly by postorbital spine dorsal to oblique anterior margin of subhepatic region, ventrally open, without contribution from antennal article 1.

Epistome 4 times as wide as long; posterior margin with rounded, anteriorly-directed median lobe. Lateral margin of buccal cavern (pterygostomian region) keel-like, laterally directed.

Thoracic sternum c. 1.6 times as wide as long, with pleonal cavity reaching anteriorly to obscure margin at posterior margin of buccal cavern, defined laterally by sharp rim defined anteriorly by ventrally-directed rounded tooth on sternite 4, without locking button; sutures between sternites 4, 8 reaching about two-thirds of exposed width.

Pleon 5-segmented with pleomeres 3–4 fused; pleomeres 1, 2 narrow, short; fused pleomeres 3–4 widest (pleomere 2 to telson 1.4 times greatest pleon width). Pleotelson little longer than basal width, apex evenly rounded.

Eyestalk length 0.3 of carapace width, constricted medially, with small tubercle proximal to base of cornea; cornea narrower than eyestalk. Antennule with stout basal articles; lower flagellum thin, 2-articled, upper flagellum stout, with numerous aesthetascs. Antennal peduncle slender, 6-segmented, arising posterolateral to base of eyestalk, article 1 fused to carapace, 2–6 free, third longest. Maxilliped 3 endopod, exposed exopod covering about 0.8 of lateral width of buccal cavern when closed; ischium-merus length 2.6 times ischium width, ischium distomesial angle rounded; merus ovoid; mesial margins of ischium, merus with row of long setae; exopod reaching beyond distal margin of merus, bearing long flagellum.

b c a g e d f Pereopods 1 (chelipeds) symmetrical, stout, 1.7 times as long as carapace length, smooth, without tubercles or spines. Propodus swollen, 2.7 times as long as greatest depth, fingers 0.4 length of total propodus length, fixed finger with irregularly toothed blade over proximal 0.4, broad triangular tooth at about midpoint, concave over distal third with small tooth, apex acute; dactylus with dentate blade (opposing blade on fixed finger), triangular tooth near midpoint, sinuous beyond.

Pereopods 2–5 (ambulatory legs) slender, long; length (ischium–dactylus) 3.5, 3.3, 3.0, 3.0 times carapace length respectively; relative lengths of ischium to dactylus of pereopod 2: 1: 3.0: 1.3: 2.7: 1.7; merus 18 times as long as wide; carpus curved proximally; propodus evenly curved; dactylus sharply curved distally, each with 10–13 sharp teeth on flexor margin.

Gonopod 1 tapering, curving slightly dorsally in situ, with longitudinal groove, with acute apex bearing several short parallel setae, reaching forward almost as far as tooth on sternal rim. Gonopod 2 with swollen base, tapering distal part ending in bifid apex.

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l Ovigerous female. Carapace length 0.93 times carapace width. Rostrum triangular, depressed at midpoint, upturned distally, 0.37 carapace length. Supraorbital eave defined anteriorly by small triangular lateral lobe (rostral width at this point 0.17 carapace width), lateral margins with regularly-spaced long lateral setae. Branchiostegal region with 3 tubercles, more evident than in male.

Thoracic sternum 2.2 times as wide as greatest width, mostly membranous, with pair of simple vulvae anteriorly; pair of deep well-defined branchiosternal canal apertures on ventral face of sternites 8.

Pleon deeply excavated, closing only weakly, almost hemispherical in mature females, about as wide as long, pleomeres 3–5 fused; pleopods 2–4 biramous, together carrying c. 30 embryos, each c. 0.4 mm diameter, pleopod 5 uniramous.

Pereopods 1 (chelipeds) symmetrical, scarcely thicker than ambulatory legs, 1.3 times as long as carapace length, smooth, without tubercles or spines. Pereopods 2–5 (ambulatory legs) slender, long; length (ischium– dactylus) decreasing from 3.1 (pereopod 2) to 2.7 (pereopod 5) times carapace length.

Colour in life. With prominent persistent pair of irregular black spots near posterior margin, pairs on margin near anterolateral boundary of dorsal carapace and on pleomeres, others on pereopodal articles.

Size. Males, 1.2–1.6 mm; ovigerous females, 1.2–1.7 mm.

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g Variation. Male rostrum 0.6 carapace length in juveniles without swollen chelipeds, 0.9–1.0 times carapace length in adults; triangular lateral lobe sometimes poorly developed. Female rostrum 0.3–0.4 carapace length. Postocular tooth varies from blunt to acute, subhepatic tubercles variously well-developed. Cheliped of small males without dentition on fingers. Numbers of teeth on pereopodal dactyli 6–12, with similar numbers on all ambulatory legs of a single individual.

Etymology. From Papua Niugini , Papua New Guinea in the Tok Pisin language; noun in apposition.

Distribution. Papua New Guinea, islands off Kavieng, northern New Ireland; coral rubble habitats, 3– 16 m.

Remarks. Culexisoma niugini n. sp. is most similar to C. ginowan from the Ryukyu Islands, Japan. Males of the two species are virtually identical in terms of carapace length: width ratio, length of pereopods, and shapes of the pleon and maxilliped 3. The two adult males of C. ginowan , carapace length 2.1 and 3.5 mm, are nevertheless larger than the largest of 12 males of the new species (1.6 mm); the largest ovigerous female is 1.7 mm. The new species differs in possessing a pair of subhepatic and three pairs of branchiostegal tubercles whereas in C. ginowan only a slight subhepatic swelling is evident. The rostrum is longer relative to the carapace than in C. ginowan (always greater than 0.9 times, 0.73 times in C. ginowan ). While similar teeth are present on the fingers of the male chelipeds the proportions differ, notably the proximal dentate blade of the dactylus is longer (occupying half the length in C. ginowan , one third in C. niugini ); the exact pattern is somewhat variable. Naruse & Komai’s (2009) figure and description of gonopod 1 of C. ginowan show it to be more elongate than in the new species and without distal setae near the apex. The gonopod 2 appears to be not bifid as in the new species. These differences would seem to justify specific status. The two species are separated by a wide expanse of the tropical west Pacific; neither has been reported yet from the well-studied shallow waters of Taiwan or the Philippines.

Culexisoma niugini n. sp. is remarkable for its strong sexual dimorphism, in overall shape (broader in females than in males), in the shape of the chelipeds (linear in females but swollen in males) and in the rostrum (conspicuously elongate in males but short and triangular in females). Juvenile males possess narrow unarmed chelipeds, mature individuals have a swollen propodus with teeth on the opposing margins of the fingers. Females of C. niugini have the broad and expanded pleon typical of hymenosomatids and carry up to 30 embryos. Less extreme dimorphism is found in Lucascinus coralicola (Rathbun, 1909) and in Nasutoplax rostrata (Haswell, 1881) (observations of Museum Victoria collections) but in these species the female rostrum is at least half as long as that of the male. The rostrum of Rhynchoplax messor Stimpson, 1858 , is a spatuliform blade that varies with size rather than sex (Ng et al. 1999).