Rhadinoplax, Castro & Ng, 2008

Castro, Peter & Ng, Peter K. L., 2008, Rhadinoplax, a new genus of Progeryonidae Števčić, 2005, for Carcinoplax microphthalmus Guinot & Richer de Forges, 1981, and a redescription of Paragalene longicrura (Nardo, 1868) (Crustacea: Decapoda: Brachyura: Goneplacoidea), Zootaxa 1777, pp. 53-68 : 54-57

Rhadinoplax , new genus

Type species. Carcinoplax microphthalmus Guinot & Richer de Forges, 1981 , by monotypy.

Composition. Monotypic.

Etymology. From rhadinos, Greek for slender, in reference to the diagnostic relatively slender abdomens of both males and females, and - plax, Greek for plate or tablet, which is derived from the generic name Carcinoplax (from karkinos, Greek for crab), to denote that the species included in the new genus was formerly included in the latter. Gender feminine.

Diagnosis. Carapace round to subovate ( Figs. 1, 2A, B); front lamellar with slight median notch; anterolateral borders curved without distinct teeth; posterolateral borders prominently convex ( Figs. 1, 2A, B). Dorsal surface of carapace strongly convex, without clear indication of regions ( Figs. 1, 2A, B). Orbits small; suborbital border with small inner tooth, cornea very small ( Fig. 2C–F). No visible stridulating mechanism. Thoracic sternum relatively narrow ( Figs. 2G, H, 3, 4I); S3/4 complete but medially shallow, S4/5, S5/6 medially interrupted; S6/7 medially interrupted by very narrow gap; S7/8 complete; longitudinal groove across median portion of sternite 7 ( Figs. 3, 4I); sternite 8 not visible when abdomen closed ( Figs. 4C, D). Episternite 7 expanded, partially covering coxa of P5, penis ( Figs. 3, 4G). Penis coxal ( Fig. 4G). Sterno-abdominal cavity relatively deep, reaching to before margin of S3/4 ( Fig. 3). Press button of male abdomen locking mechanism on posterior half of sternite 5, nearer margin of sternite 6 ( Figs. 3, 4E, F). Male abdomen relatively wide, all somites freely-movable, somite 1 exposed, not covered by posterior carapace margin, transversely narrow ( Figs. 2G, H, 4A–D). G1 slender, with minute spinules ( Fig. 5B–D); G2 slightly longer than G1 with long flagellum ( Fig. 5E). Female abdomen relatively narrow, not covering all of thoracic sternum ( Fig. 4H). Vulva of mature females ( Fig. 4I) oval, extending from 5/6 suture but not reaching suture 6/7; thick, elevated vulvar cover over ventral border.

Remarks. A suite of characters clearly separates C. microphthalmus from all described species of Carcinoplax as well as from the other goneplacid genera described by Castro (2007) as new, some from species of Carcinoplax sensu lato. Carcinoplax microphthalmus is therefore placed in a new genus, Rhadinoplax .

The male abdomen of Rhadinoplax , new genus, is relatively narrow ( Figs. 2H, 4D), more so than in Carcinoplax (see Ng & Manuel-Santos 2007: fig. 10C; Castro 2007: fig. 4B). The lateral portion of sternite 8 is completely covered by the male abdomen in Rhadinoplax , even after the abdomen is completely closed ( Figs. 4C, D, 5A), while varying portions of sternite 8 are visible among the different species of Carcinoplax (see Guinot 1989: figs. 2, 3; Ng & Manuel-Santos 2007: figs. 10C, 11C). The G1 is slender and with a pointed apex ( Fig. 5B–D), while it is dorsoventrally flattened and with a variously truncated or pointed apex in Carcinoplax (see for example Guinot 1989: figs.6A, 7A, 8A, B; Castro 2007: fig. 2A). The G2 has a long flagellum that is as long as the proximal part (peduncle) and with a simple tip ( Fig. 5E), while it has a shorter flagellum, shorter or subequal than the proximal part, and typically with a slightly expanded part ending in 1–3 lateral spinules (see for example Guinot 1989: figs. 6B, 7B, 8C; Castro 2007: fig. 2B). Females have a relatively narrow abdomen which does not cover the outer portions of the thoracic sternites ( Fig. 4H), while it is much broader, covering most of the thoracic sternites in Carcinoplax . The vulva is relatively small and partially covered by a thick vulvar cover ( Fig. 4I), while it is greatly expanded and without a vulvar cover in Carcinoplax (see Castro 2007: fig. 1). Sternal suture 7/8 is complete ( Figs. 3, 4I), while it is interrupted medially in Carcinoplax (see Castro 2007: fig. 1). The carapace is distinctively suboctagonal, with its anterior border having a slight notch between shallow lobes ( Fig. 1), while it is transversely rectangular or quadrate and typically with a straight anterior border in Carcinoplax (see for example Guinot 1989: figs. 22–28; Castro 2007: figs. 3, 4A).

In contrast, Rhadinoplax shares several important characters with the five known species of Progeryon Bouvier, 1922 , and the monotypic Paragalene Kossmann, 1878 , which constitute the family Progeryonidae Števčić, 2005 (see Ng & Manuel-Santos 2007). In all three progeryonid genera, the press button is on the posterior half of S5 closer to S6 (see Fig. 3 for Rhadinoplax ; Ng & Manuel-Santos 2007: fig. 12G for Progeryon ; Fig. 7E for Paragalene ) (on the anterior half of S5, closer to S 4 in Carcinoplax and other Goneplacidae , see Ng & Manuel-Santos 2007: fig. 12C). The lateral part of sternite 8 is completely covered by the male abdomen in Rhadinoplax like in the other genera (see Fig. 4C, D for Rhadinoplax ; Ng & Manuel-Santos 2007: figs. 10G, 11G for Progeryon ; Fig. 7D, F for Paragalene ); and the sterno-abdominal cavity almost reaches the anterior edge of sternite 4 (see Fig. 3 for Rhadinoplax ; Ng & Manuel-Santos 2007: fig. 12G for Progeryon ; Fig. 6D for Paragalene ). Episternite 7 is not prominently expanded but with the posterior part just touching the anterior edge of the coxa of P 5 in Rhadinoplax , and the penis opens near the condyle of the coxa and episternite 7 is not expanded to protect the penis, which is instead protected by abdominal somite 3 (see Fig. 4G for Rhadinoplax ; Fig. 7H for Progeryon ; Fig.7G for Paragalene ). The G1 of Rhadinoplax also resembles the other family members, being slender, nearly straight, with a pointed apex and armed with spinules (see Fig. 5B–D for Rhadinoplax ; Ng & Guinot 1999: fig. 2A–D for Progeryon ; Fig. 8A, B for Paragalene ); and the G2 is slightly longer than the G1 and with a long flagellum (see Fig. 5E for Rhadinoplax ; Ng & Guinot 1999: fig. 2E, F for Progeryon ; Fig. 8C, D for Paragalene ).

Some differences between Rhadinoplax and the Progeryonidae are nevertheless apparent. The male abdomen of Rhadinoplax ( Figs. 2H, 4D) is relatively narrower than in Progeryon (see Ng & Manuel-Santos 2007: fig. 10G) and Paragalene ( Fig. 7F). The carapace of Progeryon , although similar in general shape to that of Rhadinoplax has a depressed dorsal surface and only one pair of small but conspicuous anterolateral teeth (see Ng & Manuel-Santos 2007: fig. 6G). The orbits are similarly small, the eye peduncle slightly shorter and the spherical cornea slightly larger in Progeryon (see Ng & Manuel-Santos 2007: fig. 8C) and Paragalene ( Fig. 6B) than in Rhadinoplax ( Fig. 2C–F).

Paragalene shows some characters that sets it apart from Rhadinoplax and Progeryon . Somites 3 and 4 of the male abdomen are fused ( Fig. 7F), with only the median and lateral parts of the suture just visible, and the first three somites are transversely wider than in Rhadinoplax and Progeryon . The flagellum of the G2 is laterally expanded as a thin, narrow membrane, which is armed with small denticles along the simple, pointed distal portion and apex ( Fig. 8C, D). The carapace of Paragalene has conspicuous transversal gastric and branchial ridges, there are four anterolateral teeth (the last two prominent and acute) and there are two acute teeth on the front ( Fig. 6A).

The family Progeryonidae , together with four other families ( Conleyidae Števčić, 2005 , Goneplacidae , Mathildellidae Karasawa & Kato, 2003 , and Vultocinidae Ng & Manuel-Santos, 2007 ) was referred to the superfamily Goneplacoidea as defined by Ng & Manuel-Santos (2007).