J. South Asian Nat. Hist., ISSN 1022-0828. May, 1999. Vol.4, No. 1, pp. 65-70; 3 figs. © Wildlife Heritage Trust of Sri Lanka, 95 Cotta Road, Colombo 8, Sri Lanka. A note on the sea cucumber crab, Hapalonotus reticulatus (Crustacea: Brachyura: Eumedonidae) Diana G. B. Chia* and Peter K. L. Ng* * Department of Biological Sciences, National University of Singapore, Kent Ridge, Singapore 119260, Republic of Singapore. Abstract The rare sea cucumber crab, Hapalonotus reticulatus (De Man, 1879) is redescribed and figured in detail. Its familial affinities with the Eumedonidae and Pilumnidae are discussed. Introduction The rare and unusual sea cucumber crab, Hapalonotus Etymology. The name Hapalonotus is derived from reticulatus (De Man, 1879) is redescribed. Its the Latin 'hapalus' and 'notus' for soft back, in relationship with the Eumedonidae and Pilumnidae reference to the original description of the type are discussed, and problems with its classification species which stated its carapace was soft. Gender in the Eumedonidae highlighted. The genus is masculine. The original genus name, Malacomsoma, nevertheless retained in the Eumedonidae until the was derived from the Greek 'malacus' (soft) and family Pilumnidae can be revised and its affinities 'soma' (back), and its gender was neuter. with the closely related Eumedonidae established. Specimens examined are deposited in the IRSNB Diagnosis. Carapace oval, distinctly broader than (Institut Royale des Sciences Naturelles de Belgique, long, dorsal surface very convex, carapace appears Brussels), MZB (Balitbang Zoologi, Museum very high; rostrum very short, deflexed downwards; Zoologicum Bogoriense, Bogor), RMNH (Nationaal inner supraorbital teeth absent; regions poorly Natuurhistorisches Museum (former Rijksmuseum defined; surfaces of carapace, chelipeds and van Natuurlijke Histoire), Leiden), and ZRC ambulatory legs smooth, without granules, covered (Zoological Reference Collection, Department of with dense layer of very short, stiff, transparent setae. Biological Sciences, National University of Antero- and posterolateral margins demarcated; Singapore). The abbreviations G1 and G2 are used anterolateral margin entire, without teeth, lobes or for the male first and second pleopods respectively crest. Antennules folding slightly obliQuely, ca. 15o Measurements provided are of the carapace length from horizontal. Second antennal segment short, and width respectively. length to width ratio of second antennal segment ca. 1.0. Epistome very narrow. Median part of sternites Family Eumedonidae 3 and 4 depressed. Chelipeds smooth, covered with dense layer of very short, stiff, transparent setae; Genus Hapalonotus Rathbun, 1897 carpus with low, rounded tubercle on inner distal angle; merus unarmed except for small rounded Malacosoma De Man, 1879: 67; Alcock, 1900: 294. tubercle on subdistal dorsal margin; chela short, Hapalonotus Rathbun, 1897:164; Tesch, 1918: 275 [in stout, length 2 times length of fingers, height ca. 2-3 key], 277; Balss, 1933:90; Balss, 1957:1650; SerSne, times height of fingers; fingers not carinate, pollex 1968: 74; Schmitt et al., 1973:131; Vandenspiegel not bent downwards. Ambulatory leg segments et al., 1992a: 172. smooth, covered with dense layer of very short, stiff, transparent setae, cylindrical, not cristate; dactylus Type species. Malacosoma reticulatum De Man, 1879, of first pair not distinctly longer or more slender than by monotypy. Gender of genus masculine. those on other legs. G1 long, slender, sinuous. C hi a & N g Remarks. De Man (1897) described the taxon as (Castro, 1971; Fishelson, 1973; Suzuki & Takeda, 1974; Malacosoma reticulatum. Rathbun (1897) commented Ng & Lim, 1990). that Malacosoma was a preoccupied name (Insecta: Many of the above characters make Hapalonotus Lepidoptera) and substituted it with Hapalonotus rather peculiar among eumedonids and it may not instead. The genus Hapalonotus is monotypic, belong to the family after all. However, in the form containing only the type species, M alacosoma of the legs, shape of the antennules, antennae, reticulatum De Man, 1879 (present H. reticulatus). chelipeds, third maxilliped, abdomen and general The familial placement of this genus is difficult. habits, it does not differ greatly from the other Tesch (1918) and Schmitt et al. (1973) referred members of the family. These characters, however, Hapalonotus to the Pinnotheridae, probably because are also in common with most species of Pilumnidae. of its appearance and habits. Balss (1933) thought it For purely ecological reasons, the recognition of the was a Xanthidae. Vandenspiegel et al. (1992a) Eumedonidae has merit, all genera are after all reported males of H. reticulatus for the first time, and obligate symbionts on echinoderms. For this reason they showed that it had features more in common alone, Hapalonotus is retained in the Eumedonidae with the Eumedonidae, i.e. a sinuous G1 and having for the time being. When the limits of the Pilumnidae all seven male abdominal segments free, following are fully revised, the Eumedonidae will probably be the revision of Stevcic et al. (1988). Its possible part of the Pilumnidae, during which time, it might affinities with the Pilumnidae were also discussed. be proper to consider placing Hapalonotus in its own Its placement there, however, is rather difficult as subfamily as well. the taxonomy of the Eumedonidae is still not completely resolved (see Ng & Rodriguez, 1986; Lim Hapalonotus reticulatus (De Man, 1879) & Ng, 1988; Mori et al., 1991). (Figures 1-3) With regards to its habits and general features, Hapalonotus does seem to be a eumedonid. In terms Malacosoma reticulatum De Man, 1879: 67 [type of its carapace shape, eyes, epistome, antennules, locality Ambon, Moluccas = Maluku Islands, ambulatory dactyli, the tip of Gl, large size and host, Indonesia] however, H. reticulatus is very different from the other Hapalonotus reticulatus - Tesch, 1918: 278, pi. 18: Fig. eumedonids. The carapace of H. reticulatus is high, 3 ,3a [Ambon, holotype re-examined]; Balss 1933: bulbous, broad and oval in shape. The eye stalk is 90, Fig. 3, pi. 2: Fig. 1 [Banda Islands, Moluccas = very broad and large, slightly lamelliform and Maluku, Indonesia]; SerSne, 1968: 74 [list only]; expanded at the anterior and posterior margins. The Schmitt et al., 1973:131 [list only]; Vandenspiegel epistome and antennules are highly compressed and et al., 1992a: 172, Figs. 5,6 [Hansa Bay, Papua New narrow. The ambulatory leg also only has a weakly Guinea]; Vandenspiegel et al., 1992b: 242 [Hansa developed dactylo-propodal articulation which is Bay, Papua New Guinea]. very distinct in other eumedonids. The distal tip of Gl is short and stunted in Hapalonotus but sharp and Material examined. Holotype - female (20.0 by 24.0 tapering in other eumedonids. The very short, dense mm) (RMNH 319), Amboina (= Ambon), Indonesia, and almost transparent setae covering the dorsal coll. D. S. Hoedt, 1864. Others: Indonesia: 1 female surfaces is very interesting, and quite unlike anything (19.0 by 24.5 mm) (MZB Cru 070), Banda Islands, known in the Eumedonidae thus far. While many Lesser Sunda Islands, Indonesia, coll. Baidilla. Papua eumedonids, e.g. Harrovia and Ceratocarcinus species, New Guinea: 1 male, 1 female (IRSNB IG27522), can be covered with short, dense setae, these setae Laing Island, in Holothuria sp. - 2 females (IRSNB are coarse, gather dirt and pigmented (Chia et al., IG27754/179-180), Hansa Bay, 6 m, coll. D. 1993; Castro et al., 1995). In Hapalonotus, the hair is Vandenspiegel. -1 male (16.1 by 19.3 mm), 1 female velvet-like and does not gather dirt easily. (20.5 by 24.7 mm) (ZRC 1997.170-171, ex IRSNB Hapalonotus is by far the largest eumedonid IG27598/135, 133), Hansa Bay, seagrass beds off known. It is the only species known to be symbiotic Awar Plantation, 4°08'27"S, 144°51,22"E, 10 m, in the with sea cucumbers. It is also the only species which respiratory of a holothurian, coll. C. Massin, 13 X is an endosymbiont. In most aspects, the ecology of 1989. Hapalonotus is similar to what has been reported for many species of holothurian-associated pea crabs Description. Carapace oval, distinctly broader than (Pinnotheridae) (see Chopra, 1931; Schmitt et al., long, dorsal surface very convex, appears very high; 1973; Vandenspiegel & Jangoux, 1989). All other rostrum very short, deflexed downwards; inner eumedonids are obligate external symbionts on sea supraorbital teeth absent; regions poorly defined; urchins (Echinoidea) or featherstars (Crinoidea) surfaces of carapace, chelipeds and ambulatory legs 66 J. South Asian Nat. Hist. The sea cucumber crab, H apalonotus reticulatus Figure 1. Hapalonotus reticulatus (De Man, 1879). Male, 16.1 by 19.3 mm (ZRC 1997.170-171): A, dorsal view; B, ventral view. Vol. 4, No. 1. 67 Chia & Ng Figure 2. Hapalonotus reticulatus (De Man, 1879). Male, 16.1 leg; F, left third maxilliped; G, ventral view of left Gl; H, by 19.3 mm (ZRC 1997.170-171): A, dorsal view of distal tip of left Gl; I, left G2 (dotted line indicates carapace; B, postero-dorsal view of left cheliped; C, breakage); J, abdomen. Scales for A- G, I-J = 1.0 mm; for H sternum; D, face of carapace; E, fourth right ambulatory = 0.1 mm. 68 J. South Asian Nat. Hist. The sea cucumber crab, H apalon otus reticulatus Figure 3. Hapalonotus reticulatus (De Man, 1879), specimen from Papua New Guinea (photograph courtesy of D. in situ in dissected sea cucumber, showing live colours, Vandenspiegel). smooth, without granules, covered with dense