Pseudocryptocoeloma Ward, 1936

Ng, Peter K. L. & Rahayu, Dwi Listyo, 2023, Review of the pilumnid crab genus Lophoplax Tesch, 1918 from the western Pacific, with descriptions of two new species, and the clarification of the identity of Pseudocryptocoeloma parvus Ward, 1936 (Crustacea: Brachyura), Zootaxa 5244 (5), pp. 428-454 : 446-447

Pseudocryptocoeloma Ward, 1936

Pseudocryptocoeloma Ward, 1936: 3 ; Ng 1987: 97; Davie 2002: 426; Ng et al. 2008: 144.

Type species. Pseudocryptocoeloma parvus Ward, 1936 , by original designation; gender neuter.

Diagnosis. Carapace subquadrate, broader than long; regions poorly defined; surface smooth, without granules or obvious rugosities, frontal and anterolateral margins covered with numerous soft setae that obscures margins, rest of carapace glabrous, appearing polished ( Figs. 13A , 14A , 15A ); frontal margin with 2 very low lobes, barely separated by faint concavity; lateral lobe not discernible ( Figs. 13A , 14A , 15A ); anterolateral margin convex; external orbital tooth very low, wide; first and second anterolateral teeth low, wide ( Figs. 13A , 14A , 15A ); posterior margin of epistome with wide median triangular lobe with median fissure, separated from lateral part by short fissure, lateral margin almost straight ( Figs. 14A , 15B, C ). Merus of third maxilliped quadrate, with auriculiform anteroexternal angle, exopod tip almost reaching distal edge of merus ( Fig. 15D ). Chelipeds asymmetrical, relatively short, chela high, pubescence only on dorsal margin, outer surface smooth ( Figs. 13A, B , 14A–D ).P 2–5 short, stout; surface smooth; margins covered with pubescence, merus without cristae on dorsal margin ( Figs. 13A , 14E ). Sternopleonal cavity reaching imaginary line connecting submedian part of coxae of chelipeds; sternite 8 exposed when pleon closed ( Figs. 13B , 14F , 15E ); pleon triangular, transversely narrow, somite 1 and 3 wide, reaching to P5 coxae ( Figs. 14F , 15F ). G1 strongly sinuous, distal part strongly curved ( Fig. 15G –J ).

Remarks. Ward (1936: 3) established this genus for one small-sized species, Pseudocryptocoeloma parvus Ward, 1936 , on the basis of the following characters: “Carapace three-quarters as long as it is broad. The posterior portions of the carapace are flat, smooth and shining. The epistome is twice as wide as it is long. The anterior margin of the buccal frame is raised and has an obsolete fissure on each side. The merus of the external maxilliped has the anterolateral angle sub-auriculate. The buccal frame is completely filled by the external maxillipeds.” He did not compare it with allied genera except Cryptocoeloma Miers, 1884 , and Heteropilumnus De Man, 1895 . Compared to Cryptocoeloma , he noted that his new genus differed in that the eyes are visible in dorsal view (versus hidden below the frontal margins, the anterolateral margins are lined with setae of unequal lengths (versus lined with a fringe of setae with interspersed longer setae) and the exposed dorsal surface of the carapace is smooth and polished (versus surface duller, the regions more developed and sculptured on the branchial regions). From Heteropilumnus he commented that Pseudocryptocoeloma differed in the anterolateral margin being entire, the wider posterior carapace margin and the structure of the chela.

Despite Ward’s (1936) choice of the genus name, Pseudocryptocoeloma is actually morphologically very different from Cryptocoeloma (type species Cryptocoeloma haswelli Rathbun, 1923 ; cf. Ng & Holthuis 1989; Ng et al. 2022). The differences highlighted by Ward (1936) are valid at the genus level; the fringe of setae mentioned by Ward for Cryptocoeloma is in fact very dense and formed by two types of long stiff setae which form a thick brush that completely obscures the frontal and anterolateral margins (cf. Ng 1989; Ng et al. 2022). In Pseudocryptocoeloma , the frontal and anterolateral setae are composed of soft plumose setae, not forming a brush, and the rest of the dorsal surfaces are very smooth and glabrous ( Fig. 13A ). The smoothness of the carapace is also unlike that of species of Heteropilumnus , which have better defined carapace regions and are not as polished in appearance (cf. Ng & Tan 1988; Ng & Davie 1991; Yeo et al. 2004; Maenosono 2016; Ng et al. 2018). Another allied genus whose identity was only recently clarified is Pseudolitochira Ward, 1942 (type species Carcinoplax integra Miers, 1884 ), which superficially resembles Heteropilumnus . Ng et al. (2021) showed that it can be easily distinguished from Heteropilumnus as well as Cryptocoeloma and Pseudocryptocoeloma by posterior margin of the epistome being entire, without any fissure or cleft separating the median triangular lobe from the rest of the structure (see also Ng & Clark, 2022). The chela of Pseudocryptocoeloma is distinctive, with the chela high and the outer surfaces smooth even though the specimens are small ( Fig. 14C, D ). The male chelae of species of Lophoplax and Pseudolitochira are not enlarged, relatively more slender and with the outer surface covered with small granules (cf. present study; Ng & Clark, 2022). In Heteropilumnus , the male chelae of most species are asymmetrical, with the larger chela longer and more slender (cf. Ng & Tan 1988; Ng & Davie 1991; Yeo et al. 2004; Maenosono 2016; Ng et al. 2018).

Edmondson (1951) described a second species, Pseudocryptocoeloma symmetrinudum , from Samoa, but it clearly does not belong to this genus and is here referred to Lophoplax (see Remarks under that species).

Figures

FIGURE 13. Pseudocryptocoeloma parvum Ward, 1936, male (8.7 × 6.2 mm) (NHM 1937.7.15.20), Australia.A, overall dorsal view; B, ventral view of cephalothorax.

FIGURE 14. Pseudocryptocoeloma parvum Ward, 1936, male (8.7 × 6.2 mm) (NHM 1937.7.15.20), Australia.A, frontal view of cephalothorax; B, dorsal view of right cheliped; C, right chela; D, left chela; E, right P5; F, male pleonal somites 1–4.

FIGURE 15. Pseudocryptocoeloma parvum Ward, 1936, male (8.7 × 6.2 mm) (NHM 1937.7.15.20), Australia. A, left side of carapace; B, frontal view of carapace showing antennae, antennules, orbit and epistome; C, posterior margin of epistome; D, left third maxilliped; E, anterior thoracic sternum; F, pleonal somites 1–4; G, left G1 (ventral view); H, distal part of left G1 (ventral view); I, distal part of left G1 (dorso-mesial view); K, left G2. Scales: A–F = 1.0 mm; G, K = 0.5 mm; H–J = 0.2 mm.