Cancrocaeca xenomorpha Ng 1991
Creators: name: Naruse, Tohru
name: Ng, Peter K. L.
name: Guinot, Danièle
Publisher: name: Zenodo
Year: 2008
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Hymenosomatidae
Cancrocaeca
Cancrocaeca xenomorpha

description: Cancrocaeca xenomorpha Ng, 1991
(Figs. 1, 2)
Cancrocaeca xenomorpha — Ng, 1991: 59, figs. 1–7; Ng & Chuang, 1996: 17, fig. 5; Guinot & Richer de Forges, 1997: 456, fig. 9; Deharveng et al., 2002: 36.
Material examined. Holotype. Male, 4.1 × 4.7 mm, ZRC 1990.11971, Lubang Batu Neraka, Kappang, Maros, Sulawesi, Indonesia, coll. P. Leclerc, 4 Aug. 1990.
Paratypes. One ovigerous female, 5.6 × 6.2 mm, ZRC 1990.11972, data same as holotype; 1 male, 3.6 × 4.0 mm, ZRC 1990.0484, Gua Tanette, Kappang, Maros, Sulawesi, Indonesia, coll. P. Leclerc, 18 Jul. 1989.
Others. Two males, 3.9 × 5.3, 4.0 × 5.3 mm, 1 ovigerous female, 4.2 × 5.5 mm, MZB Cru 1652, Gua Samanggi, Samanggi, Sulawesi Selatan, Indonesia, coll. L. Deharveng, I. Andayani & A. Bedos, 31 Jul. 2001; 1 male, 3.7 × 5.0 mm, 1 ovigerous female, 4.1 × 5.2 mm, ZRC 2007.0118, data same as MZB Cru 1652.
Remarks. Ng (1991) established a monotypic genus Cancrocaeca for one new species, C. xenomorpha, from Sulawesi. He regarded this species as the most highly adapted troglobitic brachyuran, with complete loss of eyes and coloration. The examination of additional larger specimens (MZB Cru 1652) of C. xenomorpha, as well as the type specimens, reveals that all of these specimens actually have remnants of eyes (Fig. 1 a, b). These vestigial structures are very short, rounded, fused to the carapace and immobile, and there is no trace of a clear peduncle, cornea or pigmentation. In larger individuals, these vestigial eyes are relatively bigger when compared to small individuals, with the tips visible in dorsal view. Even so, C. xenomorpha still possesses one of the most reduced ocular structures known for any brachyuran crab. Only the deep sea hydrothermal vent crabs of the genus Austinograea Hessler & Martin, 1989 (Bythograeidae) have eyes reduced to almost the same extent (see Guinot 1990: 892–896, figs. 1, 4).
Cancrocaeca has other unusual characters. The female abdomen of C. xenomorpha is strongly convex externally, forming a prominent dome-shape (Fig. 2 b, c); the second to fifth segments bear biramous and long setose pleopods, which develop from distal outer angles of the inner surface of second to fifth abdominal segments (Fig. 2 c, e–h). The exopods of the third to fifth pleopods lie along lateral margins of the abdomen and cover the margin of the inner surface of the dome-shaped abdomen. There are only a few (ca. 40) relatively large eggs (0.59–0.64 mm diameter) (ZRC 2007.0118, 4.1 × 5.2 mm). However, these eggs do not appear to be attached to the pleopods or any other structures, and will freely roll out when the abdomen is opened. The median parts of thoracic sternum are fused and form a relatively flat, undivided structure (Fig. 2 a) composed of a relatively thick cuticle, and no eggs are placed inside the cephalothorax cavity. The vulva of C. xenomorpha is also unusual in being raised, with a large, conical basal mount, and it is placed on along an imaginary line joining bases of P3 on the medial fused plate of the thoracic sternum (Fig. 2 a).
Cancrocaeca appears to be most similar to Limnopilos and Hymenicoides in the absence of a rostrum (although L. microrhynchus (Ng, 1995) does possess a very short rostrum (see Ng 1991: 61, fig. 2A, B; Chuang & Ng 1991: fig. 1a). Cancrocaeca is also similar to species of Amarinus, especially A. laevis (Targioni-Tozzetti, 1877), in having freely articulating male abdominal segments (Ng 1991: 61, figs. 2E, F, 4A–C; Guinot & Richer de Forges 1997: fig. 4E) and a stout G1 with a complex distal structure (Lucas 1980: fig. 10A, D; Ng 1991: figs. 3B, 6, 7). The G1 of A. laevis, however, is not strongly bent medially, is not twisted basally, and does not have a lamellar expansion and long setae on the distal outer margin (present study; Lucas 1980: fig. 10), features prominent in species of Cancrocaeca, Limnopilos and Hymenicoides (fig. 1d, e; see Ng 1995: fig. 14A, B; Ng & Chuang 1996: fig. 21H; Naruse & Ng 2007b: figs. 2, 5c, d, 8c). Amarinus laevis also differs from the species of Cancrocaeca, Limnopilos and Hymenicoides by its bell-shaped male abdomen-pleotelson (Lucas, 1980: fig. 7A) (barrel-shaped in Cancrocaeca, Limnopilos and Hymenicoides, Fig. 1 c; Ng 1995: fig. 13G; Ng & Chuang 1996: fig. 21E; Naruse & Ng 2007b: figs. 5a, 8a), and the absence of the basal mount of the female vulva (present in Cancrocaeca, Limnopilos and Hymenicoides, fig. 3a; Naruse & Ng 2007b: 18, 22, 26, 27). These characters indicate that Cancrocaeca may be phylogenetically closer to Limnopilos and Hymenicoides rather than to Amarinus.
The lateral lobes of the sixth abdominal segment in the Hymenosomatidae, usually located at the base of the pleotelson, and sometimes appearing as intercalary platelets (which may be movable), correspond to the location of the deep sockets for the abdominal locking mechanism (see Guinot & Richer de Forges 1997: 466, figs. 2A, 4, 5, 6AE; Guinot & Bouchard 1998: 658, 683, fig. 27). The form of this structure is a good generic criterion. Ng & Chuang (1996) considered Limnopilos as a junior subjective synonym of Hymenicoides, but Naruse & Ng (2007b) resurrected Limnopilos emphasising differences in the locking mechanism of the male abdomen and G1 (Naruse & Ng 2007b: figs. 1, 2a, b, 5a, c, 8a, c). Cancrocaeca xenomorpha is more similar to Limnopilos species than to Hymenicoides species in having a slender shaft of the G1 and less produced lateral lobes of the male pleotelson. Hymenicoides species are distributed along the Indian Ocean coasts (H. carteri Kemp, 1917, near Kolkata (= Calcutta), India and Ganges Basin, Bangladesh; H. robertsi Naruse & Ng, 2007, Myanmar), whereas Limnopilos species are distributed on the South China Sea and Gulf of Thailand coasts [L. naiyanetri Chuang & Ng, 1991, Gulf of Thailand; L. microrhynchus (Ng, 1995), Sabah, Borneo; L. sumatrana Naruse & Ng, 2007, South China Sea, coast of Sumatra], which is geographically closer to the distributional range of C. xenomorpha (Sulawesi). The close geographical distribution as well as the morphological similarities suggest that Limnopilos is closer to Cancrocaeca.
description: Published as part of Naruse, Tohru, Ng, Peter K. L. & Guinot, Danièle, 2008, Two new genera and two new species of troglobitic false spider crabs (Crustacea: Decapoda: Brachyura: Hymenosomatidae) from Indonesia, with notes on Cancrocaeca Ng, 1991, pp. 21-40 in Zootaxa 1739 on pages 22-24, DOI: 10.5281/zenodo.181452