Rhynchoplax minutula, Komai, Tomoyuki & Poore, Gary C. B., 2016

Komai, Tomoyuki & Poore, Gary C. B., 2016, Resurrection of Rhynchoplax Stimpson, 1858, with the description of two new species from Japan and Australia (Crustacea: Decapoda: Brachyura: Hymenosomatidae), Zootaxa 4093 (4), pp. 515-524 : 521-523

Rhynchoplax minutula n. sp.

( Figs 4 , 5 )

Material examined. Holotype. Japan, Ryukyu Islands, Yaeyama Islands, Iriomote I., Uehara, 24°25.16'N, 123°48.01'E, 0.5–1 m at low tide, sea grass beds, 14 July 2003, dip net, coll. T. Komai, CBM-ZC 13121 (ovigerous female, cl 1.5 mm).

Paratypes. Same locality as holotype, 8 July 2001, dip net, coll. T. Komai, CBM-ZC 7079 (ovigerous female, cl 1.4 mm); 18 July 2000, dip net, coll. T. Komai, CBM-ZC 13122 (ovigerous female, cl 1.8 mm).

Diagnosis. Carapace 1.15 times as wide as long in female, with prominent protuberance on subhepatic region; anterolateral margin faintly angular, but without conspicuous tooth or tubercle; epibranchial tooth present, sharp; dorsum demarcated by gastro-cardiac, cervical, transverse thoracic grooves, cardiac region with short median ridge. Rostrum 0.34–0.4 times as long as carapace, slender, rod-like, tapered, slightly ascending, without lateral pseudorostral angles; with tuft of 4 or 5 long setae at apex. Eyestalks compact, with protuberance on anterior surface. Maxilliped 3 merus distolaterally rounded. Cheliped merus strongly bowed. Pereopods 1–5 meri each without prominent distal tubercle on upper margin.

Description. Ovigerous female holotype. Carapace marginally surrounded by complete hymenosomian groove, with pair of small, forwardly directed sharp marginal epibranchial teeth at midlength; anterolateral margins faintly angular, but without conspicuous armature; posterolateral margins almost rounded marginally, slightly upturned; width 1.15 times carapace length; regions on dorsal surface demarcated by gastro-cardiac, cervical, transverse thoracic grooves; gastric, cardiac regions slightly convex, latter with short median ridge connecting with cervical groove; subhepatic region forming distinct facet, its lower margin substantially produced into roundly triangular lobe visible from dorsal aspect, anteriorly continuous with lower lateral orbital angle; lower lateral orbital angle slightly produced; pterygostomial region with oblique ridge extending from anterolateral angle of buccal cavern to ventrolateral margin of carapace, small tubercle; subbranchial region shallowly concave, slightly visible in dorsal view, without armature.

Rostrum slender, rod-like, 0.37 times carapace length, tapering distally, slightly ascending, nearly straight or slightly upturned, demarcated from carapace by hymenosomian groove, with tuft of 4 or 5 long setae at apex (longest setae distinctly longer than rostrum); ventral surface rounded, not particularly carinate; supraocular eave convex, covering about fifth of eyestalk, defined anteriorly by oblique subrostral ridge, without pseudorostral angle; postocular margin separated from rounded anterior margin of subhepatic region by notch, without postocular spine

Antennules only narrowly separated; first article moderately broad, not laterally expanded; proepistome poorly developed, apparently absent. Antenna not reaching to end of antennular article 1. Epistome oblique narrow plate, anterior margin produced medially, weakly ridge-like under eyes, antennae; posterior margin forming buccal cavern almost perpendicular, produced medially, gently sinuous on either side of slit-like median notch. Lateral margin of buccal cavern (branchiostegite) keel-like, laterally directed.

Thoracic sternum about twice as wide as long, mostly membranous, with pair of simple, low, papilla-like vulvae anteriorly; margins fringed with thick plumose setae; pair of deep well-defined branchiosternal canal apertures on posterior margin of sternite 8. Boundary between sternites 3/4 bordered by sharp transverse rim.

Pleon deeply excavated, closing only weakly, distinctly wider than long, pleomeres 1, 2 free, pleomeres 3–5 functionally fused (transverse sutures clearly demarcated), pleotelson free. Pleopods 2–5 biramous, together carrying c. 30 embryos, each c. 0.3 mm diameter.

Eyestalk short, stout, nearly as long as wide, with small protuberance on mesial face proximal to base of cornea; cornea narrower than eyestalk. Antennule with stout basal article; lower flagellum apparently absent, upper flagellum stout, with 3 or 4 aesthetascs. Antenna consisting of fused basal article, free 5 articles, with slender terminal seta, arising posterolateral to base of eyestalk, third article longest.

Maxilliped 3 with endopod, exposed exopod fully covering lateral width of buccal cavern when closed; ischium-merus length 1.6 times ischium width, ischium distomesial angle rounded; merus roundly subquadrate, distolateral margin evenly rounded, not markedly produced; mesial margins of ischium, merus with row of stiff setae. Exopod not reaching beyond distal margin of merus, bearing well developed flagellum.

Pereopods 1 (chelipeds) symmetrical, scarcely thicker than ambulatory legs, 1.7 times as long as carapace, smooth, without tubercles or spines; merus strongly curved inward in dorsal view, articulation with carpus strongly oblique; carpus subequal in length to merus, slightly widened distally; chela about 4.0 times as long as high; palm very slightly inflated; dactylus slightly longer than palm; fingers gently curved inward in dorsal view, occlusal margins weakly dentate.

Pereopods 2–5 (ambulatory legs) rather robust, short; length (ischium–dactylus) decreasing from 2.7 (pereopod 2) to 2.2 (pereopod 5) times carapace length; all dactyli falcate. Relative lengths of merus: propodus: dactylus of pereopod 2:1:0.8:0.9; merus 4.8 times as long as wide. Meri without distal tubercles on upper margin. Dactyli each with 8–9 sharp, proximally directed teeth on flexor margin (plus unguis), these teeth evenly spaced, penultimate tooth subequal or slightly stronger than unguis.

Paratypes. Very similar to holotype. Rostrum 0.34 or 0.39 times as long as carapace. Antennule with uniarticulate lower flagellum.

Size. Ovigerous females cl 1.4–1.8 mm,

Etymology. Named after the minute body size even at the ovigerous stage.

Distribution. Known only from Iriomote Island, Yaeyama Islands, southern Ryukyu Islands (24°N, 124°E); 0.5–1.0 m at low tide, seagrass beds.

Remarks. The new species appears closest to R. magnetica n. sp. in the form of the carapace and pereopods. Unfortunately, R. minutula n. sp. is represented only by females. Nevertheless, Rhynchoplax minutula n. sp. differs from R. magnetica n. sp. in the absence of a preocular pseudorostral angle on the lateral margin of the rostrum ( Fig. 4 a) (with a distinct preocular angle in R. magnetica n. sp.; Fig. 3 a), the absence of dorsodistal tubercles on the meri of pereopods 2–5 and the non-inflated subbranchial region of the carapace, not being visible in the dorsal aspect ( Fig. 4 a) (fairly inflated, clearly visible in the dorsal aspect in R. magnetica n. sp.; Fig. 3 a).

The new species differs from R. hondai in the more clearly demarcated dorsum of the carapace and the absence of a prominent dorsodistal tubercle on each merus of the cheliped and ambulatory legs. Furthermore, R. minutula n. sp. is easily separable from R. hondai by the well-produced lower margin of the subhepatic region of the carapace, which is clearly visible in the dorsal view ( Fig. 4 a) (not particularly produced, hardly visible in dorsal view; Fig. 1 a). Other potentially diagnostic characters are the size of the rostrum, the shape of the ischium of the cheliped and dentition of the cheliped fixed finger. The rostrum is relatively smaller in R. minutula n. sp. than in R. hondai ( Fig. 4 c versus Fig. 1 c). The ischium of the cheliped is strongly arcuate in R. minutula n. sp. ( Fig. 4 e), rather than nearly straight in R. hondai ( Fig. 1 f); the occlusal margin of the fixed finger of the cheliped is weakly denticulate in R. minutulus n. sp. ( Fig. 4 f) but it is smooth, unarmed in R. hondai ( Fig. 1 g).

Figures

FIGURE 1. Rhynchoplax hondai Takeda & Miyake, 1971. Holotype, immature female, cl 1.5 mm, KMNH-ZLKU 13088. a, carapace and eyes, dorsal view; b, rostrum and cephalic appendages, lateral view; c, frontal part of carapace, ventral view; d, left maxilliped 3, outer view; e, thoracic sternum and pleon, ventral view; f, right cheliped (pereopod 1), upper view; g, same, chela, extensor (outer) view; h, right pereopod 2, upper view; i, left pereopod 3, upper view; j, right pereopod 4, upper view; k, left pereopod, 5, upper view. Scale bar = 0.5 mm.

FIGURE 3. Rhynchoplax magnetica n. sp. Paratype female, cl 1.97 mm, QM W. 4067. a, dorsal carapace; b, rostrum and eyestalk, ventral view; c, rostrum, antennnules, antennae and epistome, ventral view; d, posterior thoracic sternum, pleomere 1; e, pleon (half scale); f, pereopod 2; g, pereopod 5. Scale bar = 1 mm.

FIGURE 4. Rhynchoplax minutula n. sp. Holotype, ovigerous female, cl 1.5 mm CBM-ZC 13121. a, carapace and eyes, dorsal view; b, rostrum and cephalic appendages, lateral view; c, frontal part of carapace, ventral view; d, right maxilliped 3, outer view; e, right cheliped (pereopod 1), upper view; f, same, chela, extensor (outer) view; g, right pereopod 2, upper view; h, right pereopod 3, upper view; i, right pereopod 4, upper view; j, left pereopod, 5, upper view. Scale bars = 0.5 mm.

FIGURE 5. Rhynchoplax minutula n. sp. Holotype, ovigerous female, cl 1.5 mm, CBM-ZC 13121 a, thoracic sternum; b, pleon, ventral view. Scale bar = 0.5 mm.