Mirograpsus, Komai & Fujita, 2018

Komai, Tomoyuki & Fujita, Yoshihisa, 2018, A new genus and new species assigned to Macrophthalmidae (Decapoda: Brachyura: Thoracotremata) from the Ryukyu Islands, Japan, Zootaxa 4531 (1), pp. 109-116 : 109-110

Mirograpsus n. gen.

[New Japanese name: Kimen-doro-gani-zoku]

Type species. Mirograpsus asper n. sp. Present designation by monotypy. Gender: masculine.

Diagnosis. Carapace longer than wide; regions on dorsal surface well defined with gastric, cardiac, intestinal and branchial regions markedly elevated; cardiac region with paired rounded prominences; branchial regions each with 1 conspicuous tubercle on posterior part; external orbital tooth directed anteriorly, subacutely pointed, 2 blunt lateral teeth posterior to external orbital tooth; no lateral constrictions. Front broad, markedly bilobed, not constricted at base. Postfrontal ridges absent. Orbit large; upper orbital margin nearly transverse, markedly sinuous; lower orbital margin bordered with minute granules, inner orbital tooth small, blunt; 1 small but conspicuous subconical tubercle just below inner orbital tooth. Ocular peduncle constricted; cornea large, just reaching external orbital tooth. Antennules folded slightly obliquely; basal article large, but not particularly inflated; inter-antennular septum triangular, narrow in upper part. Antennae in orbital hiatus; flagellum greatly reduced, with long apical filamentous seta. Central region of epistome convex, supported by short median ridge in buccal cavity. Maxillipeds 3 fully closing buccal cavity; lateral margin of merus somewhat expanded, noticeably convex; carpus inserted medially on distal margin of merus; exopodal flagellum well developed. Male cheliped moderately stout; merus slightly narrowing distally, without subdistal spine on upper margin; carpus without prominent tooth or spine on inner surface; occlusal margins of fingers each with row of small teeth; tips of fingers pointed, crossing distally. Ambulatory legs moderately slender; meri each without subdistal spine on anterior margin; propodi unarmed at flexor distal angles. Male pleon gradually narrowing toward telson; somites 4–6 functionally fused though sutures demarcating somites 4/5 and 5/6 on external surface still discernible (while no suture delimited on internal surface); somite 6 with low convexity on lateral margins proximally. Male gonopod 1 relatively slender, not recurved, gradually tapering distally. Female unknown.

Remarks. As mentioned above, the new taxon superficially resembles species of the camptrandriid genera Moguai and Paramoguai , but the nearly straight, non-recurved gonopod 1 clearly excludes it from Camptandriidae . Mirograpsus asper n. gen., n. sp. most closely resembles Apograpsus paantu , the sole representative of Apograpsus , in the general structure of the carapace, which is longer than wide with markedly elevated gastric, cardiac and intestinal regions, with the cardiac region possessing paired rounded prominences. The following major differences between the two genera, however, warrant the establishment of the new genus (cf. Komai & Wada 2008 for character states of Apograpsus ): (1) the carapace is devoid of postfrontal ridges ( Fig. 2A ) (versus longitudinal postfrontal ridges are present in A. paantu ; cf. Komai & Wada 2008: fig. 17A); (2) the lateral margins of the carapace are devoid of constriction posterior to the external orbital teeth ( Fig. 2A ) (versus distinctly constricted posterior to the external orbital teeth in A. paantu ; cf. Komai & Wada 2008: fig. 17A); (3) the antennal flagellum is greatly reduced, bearing a long filamentous terminal seta ( Fig. 2C ) (versus multi-articulated, without a prominent terminal seta in A. paantu ; cf. Komai & Wada 2008: fig. 17A, B); (4) the male chela is large and stout, with the fingers pointed and crossing when closed ( Fig. 4A, B ) (versus small and slender, tips of fingers are spoonshaped, being similar to the female chela in A. paantu ; cf. Komai & Wada 2008: fig. 17G); (5) the meri of pereopods 2–5 are unarmed on the anterior margins ( Fig. 4 C–F) (versus each armed with a small subterminal spine in A. paantu ; cf. Komai & Wada 2008: fig. 17H, I); and (6) the male pleomeres 4–6 are functionally fused ( Fig. 2G ) (versus all freely articulated in A. paantu ; cf. Komai & Wada 2008: fig. 17D). Other notable differences between M. asper n. gen, n. sp. and A. paantu are: the front lacks long setae on each anterolateral part ( Fig. 2A ) (versus with a few long setae are present at each anterolateral part of the front in A. paantu ; cf. Komai & Wada 2008: fig. 17A, B); the branchial region of the carapace has one conspicuous tubercle in M. asper n. gen., n. sp. ( Fig. 2B ) (versus tubercle absent in A. paantu ; cf. Komai & Wada 2008: fig. 17A); and the male pleomere 6 bears a low convexity on the proximal part of each lateral margin ( Fig. 2G ) (versus no convexity is present on each lateral margin in A. paantu ; cf. Komai & Wada 2008: fig. 17D).

Ilyograpsus , another representative of Ilyograpsinae , is readily distinguished from Mirograpsus by the carapace being wider than long, the carapace regions being less defined, meri of the ambulatory legs each armed with a subdistal spine on the anterior margin, and the feely articulated pleomeres 4–6 (cf. Komai & Wada 2008: fig. 10).

The family Macrophthalmidae is now represented by 10 genera classified into three subfamilies, Macrophthalminae , Ilyograpsinae and Tritodynaminae ( Ng et al. 2008; Barnes 2010; Davie 2009, 2012; Naderloo 2013; Anker & Ng 2014; Teng et al. 2016), but in all of the known genera, the male pleon consists of freely articulated somites 1–6 + telson. In this regard, consequently, Mirograpsus n. gen. is unique in the family.

Etymology. The genus name is an arbitrary combination of the Latin “ mirus ”, meaning amazing or marvelous, and the generic name “ Grapsus ”, which is a common suffix for the genera of the subfamily Ilyograpsinae . This name alludes to the unusual appearance of this crab among the macrophthalmids. Gender masculine.

Figures

FIGURE 2. Mirograpsus asper n. gen, n. sp., holotype, male (5.8×5.1 mm; CBM-ZC 14607). A, carapace and cephalic appendages, dorsal view; B, posterior part of carapace dorsal surface, posterior view; C, frontal part of carapace, left eye, antennules, left antenna and epistome, anterior view; D, lower orbital margin and suborbital region, ventral view (setae omitted); E, left maxilliped 3, outer view; F, thoracic sternum and gonopods 1, ventral view; G, pleon, outer view; H, left gonopod 1 and penis, ventral view; I, left gonopod 1, dorsal view; J, right gonopod 2.

FIGURE 4. Mirograpsus asper n. gen, n. sp., holotype, male (5.8×5.1 mm; CBM-ZC 14607), left pereopods. A, cheliped, dorsal view; B, same, chela, outer view; C, pereopod 2, dorsal view; D, pereopod 3, dorsal view; E, pereopod 4, dorsal view; F, pereopod 5, dorsal view.