Thyraplax, Castro, 2007

Castro, Peter, 2007, A reappraisal of the family Goneplacidae MacLeay, 1838 (Crustacea, Decapoda, Brachyura) and revision of the subfamily Goneplacinae, with the description of 10 new genera and 18 new species, Zoosystema 29 (4), pp. 609-774 : 672-673

Genus Thyraplax n. gen.

Carcinoplax – Tesch 1918: 154 (part). — Guinot 1969b: 520, 524; 1971: 1081; 1989: 273 (part). — Serène 1968: 89 [in list] (part). — Serène & Lohavanijaya 1973: 62, 63 [in key to species] (part).

TYPE SPECIES. — Carcinoplax crosnieri Guinot & Richer de Forges, 1981 (by present designation; gender feminine).

SPECIES INCLUDED . — Thyraplax cooki ( Rathbun, 1906) n. comb.; T. crosnieri (Guinot & Richer de Forges, 1981) ; T. cristata n. sp.; T. digitodentata n. sp.; T. truncata n. sp.

All species are restricted to the Indo-West Pacific region.

ETYMOLOGY. — From thyra, Greek for “door”, in reference of the presence of a vulvar cover that covers part the vulva like a door, and plax, Greek for “plate” or “tablet”, which is derived from the generic name Carcinoplax (from karkinos, Greek for “crab”), to denote that two of the species in the new genus were formerly included in the latter.

DESCRIPTION

Carapace ( Figs 20A ; 21 ; 23A ; 24 ; 25A ; 26 ) transversely rectangular, wider than long, widest at junction of anterolateral, posterolateral borders; front lamellar, straight, not marked by median notch or projection. Notch between front, inner edge of supraorbital border slight or absent; orbits narrow, not expanded distally; supraorbital borders short, sinuous; suborbital borders rounded, with short, blunt inner tooth not visible dorsally; anterolateral borders short, conspicuously sinuous, with slight carina; posterolateral borders long, arched, maximum width of carapace below anterolateral tooth. Dorsal surface of carapace smooth, slightly or moderately convex, without clear indication of regions. Outer orbital angle with triangular, moderately acute prominence; single anterolateral tooth on each side of carapace; low prominence or notch between outer orbital angle and anterolateral tooth. Basal antennal article short, wide, distalmost (third) article reaches front. Eye peduncles ( Figs 20A ; 21 ; 23A ; 24 ; 25A ; 26 ) short, much shorter than front (0.2-0.3 front width); cornea spherical, only slightly expanded distally. Anterior border of endostome well demarcated from buccal cavern, ridges faint but clearly defined. Third maxillipeds completely close the buccal cavern. No obvious stridulating mechanism other than possible rubbing of proximal portion of cheliped (P1) merus against pterygostomial ridge. Thoracic sternum wide. Median sulcus on thoracic sternite 4 absent; sutures 4/5, 5/6, 7/8 interrupted medially, suture 6/7 complete ( Figs 19 ; 22 ; Guinot & Richer de Forges 1981b: fig. 9A, as Carcinoplax crosnieri ). Anterior end of sterno-abdominal cavity anterior to thoracic sternite 4. Cheliped fingers moderately slender or thick, shorter than propodus, darker in colour; carpus with tooth on inner margin. Dorsal margins of meri, carpi, propodi of ambulatory legs (P2-P5) unarmed; dactyli slender, carinated (smooth in T. cooki n. comb., T. digitodentata n. sp.), setose. Male abdomen with 6 freely-movable somites plus telson, narrowly triangular, somites 4-6 gradually decreasing in length from somite 3 (widest somite). Telson approximately as long as wide. Somite 3 covers space between P5 coxae, somite 2 slightly narrower than or as short as somite 3 so that somites 1, 2 leave small, often triangular portion of thoracic sternite 8 visible, or no portion is visible ( Guinot & Richer de Forges 1981b: fig. 9A). G1 ( Figs 20C ; 23D ; 25C ) sinuous or curved, slightly broadened proximally, stout; truncated or pointed tip. G2 ( Figs 20D ; 23E ; 25D ) slender, slightly or moderately longer than G1, flagellum shorter than proximal part (peduncle), tip pointed or with 2 spinules. Penis arising from P5 coxa, moderate size; broad, soft proximal expansion. Female abdomen (female of T. digitodentata n. sp. unknown) with 6 freelymovable somites, wide. Telson triangular. Somite 3 covers space between P5 coxae, somite 2 only slightly narrower than somite 3, thoracic sternite 8 not visible or only small portion visible ( Guinot 1969b: fig. 60, as Carcinoplax cooki ). Vulva of mature females ( Figs 19 ; 22 ) small, extending from suture 5/6 to suture 6/7, vulvar cover present.

REMARKS

Thyraplax crosnieri n. comb. and T. cooki n. comb. were originally included in the genus Carcinoplax . The distinct morphology of these species in relation to other Carcinoplax species was noted by other authors. Rathbun (1906: 835) remarked that the nature of the anterolateral teeth and the convexity of the carapace of T. cooki n. comb. (as Pilumnoplax cooki ) distinguished the species “from all others”. Guinot (1989: 314) included this two species in a list of eight “non-revised” species of Carcinoplax that proved, with only one exception, to belong to other genera. In describing Carcinoplax crosnieri, Guinot & Richer de Forges (1981b: 252) “provisionally” included the species in Carcinoplax awaiting revision of the genus since it did not resemble any known species in the genus (“dans l’attente de sa révision, ne nous paraît ressembler à aucune autre espèce connue de ce genre”). Serène & Vadon (1981: 126) placed T. cooki n. comb. (as C. cooki ) as “probably” in a group of species of Carcinoplax sensu lato where the outer orbital teeth appear as an extension of the orbit itself. Indeed, the shape of the carapace, with rounded and short anterolateral margins, a single anterolateral tooth, and long posterolateral borders that are markedly arched so that the widest portion of the carapace is actually below the anterolateral teeth are in sharp contrast to the carapace of Carcinoplax , with two teeth (one in C. ischurodous ) on each of the slightly longer anterolateral borders and posterolateral borders that decrease in width behind the last anterolateral teeth. There is also a slight carina or prominence along the anterolateral borders of the carapace, which is absent in Carcinoplax .

Some characters more significant than the general shape of the carapace provide evidence to support a separation of these two species, plus three new ones, from Carcinoplax sensu lato. The G1 varies in shape but it is never as slender and thin as in Carcinoplax ; the G2 is slightly but clearly longer than the G1, a rare occurrence in Carcinoplax ; the vulva is reduced in size and has a vulvar cover (absent in Carcinoplax ); and the 7/8 thoracic suture is clearly complete ( Figs 19 ; 22 ) in contrast to Carcinoplax , although it is shown as incomplete by Guinot & Richer de Forges (1981b: fig. 9A, as Carcinoplax crosnieri ). The P2-P5 dactyli of some of the species is carinated but they have smooth surfaces in Carcinoplax .

Four of the five species of Thyraplax n. gen. have been found only on the eastern and southern limits of the Indo-West Pacific region, areas where species of Carcinoplax are absent or are fewer in number than in the western Pacific Ocean.

Figures

FIG. 20. — Thyraplax cristata n. sp., ♂ holotype,cl 15.1 mm, cw 19.2 mm,Fangataufa Atoll, French Polynesia,SMSRB collections, stn 485, 500 m (MNHN-B 29801): A, dorsal surface of the carapace; B, abdomen; C, left G1, dorsal view; D, left G2, ventral view. Scale bars: A, 5 mm; B, 3 mm; C, D, 1 mm.

FIG. 21. — Thyraplax cristata n. sp., ♂ holotype, cl 15.1 mm, cw 19.2 mm, Fangataufa Atoll, French Polynesia, SMSRB collections, stn 485, 500 m (MNHN-B 29801).

FIG. 23. — Thyraplax digitodentata n. sp., ♂ holotype, cl 8.8 mm, cw 10.8 mm, SMSRB collections, Moruroa Atoll, French Polynesia, 560 m (MNHN-B 29433):A, dorsal surface of the carapace;B, right P5 dactylus; C, abdomen;D, left G1, dorsal view;E, left G2,ventral view. Scale bars: A, 5 mm; B, 1 mm; C, 3 mm; D, E, 1 mm.

FIG. 24. — Thyraplax digitodentata n. sp., ♂ holotype, cl 8.8 mm, cw 10.8 mm, SMSRB collections, Moruroa Atoll, French Polynesia, 560 m (MNHN-B 29433).

FIG. 25. — Thyraplax truncata n. sp., ♂ holotype, cl 8.7 mm, cw 11.6 mm, Fiji, BORDAU 1, stn CP 1481, 441-506 m (MNHN-B 29547): A, dorsal surface of the carapace; B, abdomen; C, left G1 (with rhizocephalan parasite, Thompsonia sp.), dorsal view; D, left G2 (with rhizocephalan parasite, Thompsonia sp.), ventral view. Scale bars: A, 5 mm; B, 3 mm; C, D, 1 mm.

FIG. 26. — Thyraplax truncata n. sp., ♂ holotype, cl 8.7 mm, cw 11.6 mm, Fiji, BORDAU 1, stn CP 1481, 441-506 m (MNHN-B 29547), specimen with a rhizocephalan parasite (Thompsonia sp.) on P1-P4.

FIG. 19. — Thyraplax crosnieri (Guinot & Richer de Forges, 1981) n. comb., ♀, cl 16.7 mm, cw 21.9 mm, Balicasag I., off Panglao I., Philippine Is, 50-500 m (ZRC 2004.0720), thoracic sternum, vulva, and vulvar cover.

FIG. 22. — Thyraplax cristata n. sp., ♀ paratype, cl 14.9 mm, cw 19.9 mm, Moruroa Atoll, French Polynesia, SMSRB collections, stn 476, 470 m (MNHN-B 29806), thoracic sternum, vulva, and vulvar cover.