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In their paper, EUMEDONINAE DU VIETNAM (Crustacea), R. Serene, T.V. Duc and N.V. Luom (1958) give an account on the genera and species of the subfamily Eumedoninae. But unfortunately some species are not sufficiently studied, especially those not collected and examined by the authors and only worked out by the reference of other publications. The present note is intended to suffice, if not all, the insufficiency in the above mentioned paper. The species studied in this note include: Proechinoecus sculptus Ward 1934. Ceratocarcinus longimanus Adams & White 1848. Zebrida adamsi White 1847. Rhabdonotus pictus A. Milne Edwards 1878.

The association of Parthenopid crab Zebrida adamsii and flower urchin Toxopneustes pileolus is reported for the first time from Gulf of Mannar. The Parthenopid crab Zebrida adamsii has been found to be associated exclusively with these sea urchins and this obligatory association has been studied by introducing another host sea urchin Salmacis virgulata to understand host preferences.

The symbiotic assemblage associated with the tropical sea urchin, Salmacis bicolor was studied in the An Thoi archipelago, Vietnam. Five species of macrosymbionts were found on the surface of this echinoid host, including the polychaete, Ophiodromus sp. 3 (Hesionidae), gastropod, Vitreobalcis sp. (Eulimidae), shrimp, Allopontonia iaini (Palaemonidae), crab, Zebrida adamsii (Pilumnidae), and Ophiuroidea sp. This is the first record of A. iaini in association with S. bicolor in the coastal waters of Vietnam. The polychaete, Ophiodromus sp. 3 was located on the oral side of echinoids circled around the oral opening, while Vitreobalcis sp. was attached to the peristomial membrane. The shrimp, A. iaini and the crab, Z. adamsii were found among the spines; the latter was attached to the spines by chaelipeds. Of the 60 S. bicolor examined, 28 (46.6 %) were infested, with an average number of symbionts per host of 0.52. The prevalence increased with increasing sea urchin test size. We did not find symbionts on urchins whose test diameter was less than 35 mm. Almost all S. bicolor were inhabited by one symbiont, and in only three cases did we find two symbionts belonging to two different species on one host. Polychaete, Ophiodromus sp. 3 occurred most often, and only 1–6 specimens of the other species were found. The composition of the assemblage associated with S. bicolor was compared with that known for other assemblages associated with tropical sea urchins. The ecological characteristics of the association and the possible host-symbiont relationships are discussed.

A new genus and species of eumedonid crab from the Gulf of Carpentaria, northern Australia, Zebridonus mirabilis, is described. Zebridonus gen. nov. is allied to Eumedonus H. Milne Edwards, 1834, Gonatonotus White, 1847, and Zebrida White, 1847, but differs from Eumedonus and Gonatonotus in the following aspects: (1) the lateral lobes of the carapace are sublamelliform and upturned; (2) presence of two sublamelliform teeth on the chelipedal carpus; (3) presence of a sharp spine at the distal end of the anterior margin of the ambulatory merus; (4) the thoracic sternum is flatter and smoother; and (5) having a proportionately broader abdomen. Zebridonus differs from Zebrida White, 1847, in having fewer spines on the chelipedal carpus, absence of a subcheliform process formed by the ambulatory dactylus and propodus, the posterior margin of the epistome is lower, the rostrum is not as deeply clefted and the orbit is straight instead of oblique.

The complete larval development of the brachyuran crab Zebrida adamsii White, an obligate symbiont of sea urchins, is described and illustrated. This species has 4 zoeal and 1 megalopal stages. The larval features indicate that the family Eumedonidae should be treated as a separate family within the superfamily Xanthoidea.

A new genus and new species of eumedonine crab, Cebudonus poppeorum, is described from the central Philippines. Superficially similar to Eumedonus H. Milne Edwards, 1834, Gonatonotus White, 1847, Zebrida White, 1847, Zebridonus Chia, Ng & Castro, 1995, and Tiaramedon Chia & Ng, 1998, Cebudonus n. gen. is easily characterised by possessing two long pseudorostral spines, a gently convex and unarmed dorsal carapace surface, elongated chelipeds that have the chela and merus unarmed and not cristate, elongated and slender ambulatory legs in which the merus is not cristate, a relatively narrow anterior male thoracic sternum, and a proportionately broader male abdomen. The general structure and colour pattern (with longitudinal stripes) suggests Cebudonus n. gen. is an obligate symbiont with echinoids.

A small crabZebrida adamsii, an obligate symbiont of the sea urchin, lives solitary on a host from the earliest stage of its benthic life. In the breeding season males frequently move between hosts to search for mates. Analysis of daily positional changes of individually marked crabs indicates that males often land on the substrate to migrate between separate hosts. Females also change hosts before they copulate, but after oviposition they stay on the same hosts for a long time, similarly to crabs in the nonbreeding season.