Intertidal habitats and decapod faunal assemblages (Crustacea : Decapoda) of Socotra Island, Republic of Yemen

The Socotra Archipelago, situated in the north-western part of the Indian O cean at the entrance of the Gulf of Aden, has a unique zoogeographical posi tion, as the transition between the Arabian and Red Seas and East African s hores. The Socotran marine environment, however, is as yet poorly studied, with only sparse and incomplete reference to the field of crustaceans. The current work presents results from a survey of the intertidal decapod assem blages of Socotra Island conducted in spring 1999. The information from 185 sites sampled around the island is summarized in a map with short descriptions of representative intertidal habitats, their r elative area and distribution. Both rocky shores and cobble beaches have th e largest diversity of decapods. Sandy beaches are dominated mainly by Ocyp ode saratan and Coenobita scaevola, whilst rocky shores are dominated by Gr apsus albolineatus, G. tenuicrustatus, Plagusia tuberculata, Pachygrapsus m inutus, Metopograpsus messor and Eriphia smithii. In cobble beaches, Pseudo zius caystrus, Leptodius exaratus, Xanthias sinensis, Clibanarius signatus and Clibanarius virescens are the most common species. Cardisoma carnifex a nd Uca inversa are common bordering mud flats and coastal lagoons. As fishi ng pressure is low, mud flats surrounding wadis and coastal lagoons host sm all undisturbed populations of Scylla serrata and Fenneropenaeus indicus. T here is only a reduced number of mangrove trees and area of mangrove, most of which is already destroyed or under severe human and environmental press ure. The largest and most representative stand has an unusual structure: sp ecies diversity is strikingly low, it is disconnected from the sea by a san d bar or dune, and is completely dry. Interesting zoogeographical findings are detailed, and a list of intertidal decapod fauna, relating each species to its common habitat is presented. T his list is compared with previous studies, and other intertidal decapod as semblages from the Arabian Gulf, Red Sea and East Africa.