Rhynchoplax magnetica, 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 : 516-520

Rhynchoplax magnetica n. sp.

( Figs 2 , 3 )

Halicarcinus hondai —Lucas, 1980: 183–185, figs 3H, 6O, 7H, 9K. Not Rhychoplax hondai Takeda & Miyake, 1971.

Material examined. Holotype. Australia. Qld, Picnic Bay, Magnetic I., coral in intertidal zone, QM W.4068 (male, cl 1.67 mm).

Paratype. Australia, Qld, Townsville, coral rubble, QM W.4067 (ovigerous female, cl 1.97 mm).

Diagnosis. Carapace as wide as long in male, 1.1 times as wide as long in female, with prominent protuberance on subhepatic region; anterolateral margin without conspicuous tooth or tubercle; epibranchial tooth present, blunt; dorsum demarcated by gastro-cardiac, cervical, transverse thoracic grooves, cardiac region with short median ridge. Rostrum 0.45 (male), 0.5 (female) times as long as carapace, slender, with lateral pseudorostral angle absent in male, obtuse in female; with tuft of 4 long setae at apex. Eyestalks compact, with protuberance on anterior surface. Maxilliped 3 merus distolaterally expanded. Cheliped merus gently curved. Pereopods 2–5 meri each with low distal tubercle on upper margin. Gonopod 1 with swollen base, sharply recurved dorsoproximally, almost straight over distal half, tapering to acute apex anteroventrally directed.

Description. Holotype male. Carapace marginally surrounded by complete hymenosomian groove, with pair of small, forwardly directed blunt marginal epibranchial teeth at midlength; anterolateral margin slightly concave, without armature; posterolateral margins tapering, rounded, slightly upturned; as wide as long; regions 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, lower margin substantially produced into low protuberance visible from dorsal aspect, anteriorly continuous with lower lateral orbital angle; lower lateral orbital angle produced into semicircular vertical plate; pterygostomial region with oblique ridge extending from anterolateral angle of buccal cavern to ventrolateral margin of carapace; subbranchial region shallowly concave, clearly visible in dorsal view, unarmed.

Rostrum slender, 0.4 times carapace length, angled upwards, tapering distally to rounded apex, demarcated from carapace by hymenosomian groove; with tuft of 4 long setae at apex (longest setae distinctly longer than rostrum), shorter setae laterally; supraocular eave convex, covering about one-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.

Antennule with broad first article, not laterally expanded; antennules separated by a short, low rounded proepistome (interantennular ridge). Antenna not reaching to end of antennule article 1. Epistome an oblique broad plate, anterior margin curved, ridge-like under eyes, antennae, posterior margin produced mesially, with slight median notch. Lateral margin of buccal cavern (pterygostome) keel-like, laterally directed.

Thoracic sternum 1.6 times as wide as long, pleonal cavity reaching nearly to boundary between sternites 3/4, about 0.9 of length, delimited by sharp marginal edge; without locking button. Penial papilla on inner face of sternite 8 as long as wide.

Pleon with pleomeres 1, 2 of male free, 3–4 fused, pleomere 5, pleotelson free; pleomere 1 wider than pleomere 2; pleonal margin tapering strongly between pleomeres 4, 5.

Eyestalk short, stout, nearly as long as wide, with conical protuberance on mesial face proximal to base of cornea; cornea narrower than eyestalk. Antennule with stout basal article; lower flagellum short, upper flagellum stout, with 4 aesthetascs. Antenna consisting of fused basal article, 4 free articles, slender terminal seta, arising posterior 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 twice ischium width, ischium mesial margin straight, with oblique ridge on outer face; merus subovate, distolateral margin laterally produced beyond articulation of palp; mesial margins of ischium, merus with row of short stiff setae; exopod reaching distal margin of merus, bearing well developed flagellum.

Pereopods 1 (chelipeds) symmetrical, scarcely thicker than ambulatory legs, twice as long as carapace, smooth, without tubercles or spines; merus slightly curved inward in dorsal view, articulation with carpus strongly oblique; carpus subequal in length to merus, slightly widened distally; chela about 3 times as long as high; palm slightly inflated; dactylus shorter than palm; fingers curved inward in dorsal view, occlusal margins almost smooth.

Pereopods 2–5 (ambulatory legs) rather robust, short; length (ischium–dactylus) decreasing from 2.5 (pereopod 2) to 1.9 (pereopod 5) times carapace length; all dactyli falcate. Relative lengths of merus: propodus: dactylus of pereopod 2: 1:0.88:1; merus 3.7 times as long as wide. Meri each with low distal tubercle on upper margin. Dactyli each with 8–11 sharp, proximally directed teeth on flexor margin (including unguis), increasing in length distally, distance between first two spines about 1.3 times distance between others.

Gonopod 1 with swollen base, then sharply recurved dorsoproximally, almost straight over distal half, tapering to acute anteroventrally directed apex. Gonopod 2 not seen.

Paratype female. Carapace more ovoid than in male, width 1.1 times carapace length. Rostrum almost parallelsided, 0.5 times carapace length, angled upwards, with rounded apex; supraocular eave convex, covering about one-fifth of eyestalk, defined anteriorly by oblique subrostral ridge, well defined pseudorostral angles.

Thoracic sternum mostly membranous, with pair of simple, low, vulvae anteriorly; with pair of well-defined branchiosternal canal apertures on posterior margin of sternite 8.

Pleon deeply excavated, closing only weakly, distinctly wider than long, pleomeres 1, 2 free, pleomeres 3–5 functionally fused (transverse sutures clearly demarcated except medially), pleotelson free. Pleopods 2–5 biramous.

Chelipeds missing. Pereopods 2–5 (ambulatory legs) rather robust, short; length (ischium–dactylus) decreasing from 2.25 (pereopod 2) to 2.0 (pereopod 5) times carapace length; all dactyli falcate. Relative lengths of merus: propodus: dactylus of pereopod 2: 1: 0.88: 1; merus 4.7 times as long as wide. Meri with distal tubercles on pereopods 2–5 more prominent than in male. Dactyli each with 12 sharp, proximally directed teeth on flexor margin (plus unguis).

Size. Ovigerous female cl 1.96 mm; male cl 1.67 mm.

Etymology. Latin, magnetica , from the type locality.

Distribution. Known only from Townsville and Magnetic I., Queensland, Australia (19°S, 147°E), intertidal on coral.

Remarks. Lucas (1980) attributed two adult specimens from Queensland, Australia, to Halicarcinus hondai (Takeda & Miyake, 1971) but biogeographic considerations and the morphological differences revealed here suggest the likelihood of them belonging to this Japanese species is slight. The holotype of R. hondai differs from the Australian female specimens in possessing more pronounced dorsodistal tubercles on the meri of pereopods 2– 5, absence of lateral pseudorostral angles and lacking carapace grooves except the gastro-cardiac groove. The species is compared with R. minutula n. sp. below.

Figures

FIGURE 2. Rhynchoplax magnetica n. sp. Holotype male, cl 1.67 mm, QM W. 4068. a, dorsal carapace; b, rostrum and eyestalk, ventral view; c, lateral anterior carapace and rostrum; d, rostrum, antennules, antennae, epistome, and buccal area, ventral; e, left thoracic sternum (showing penial process); f, pleon; g, left maxilliped 3; h, right cheliped, upper view; i, right cheliped, propodus and dactylus; j – m, pereopods 2 – 4; n, gonopod 1, lateral view (twice scale). Scale bar = 1 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.