The first record of the land crab, Discoplax rotunda (Quoy & Gaymard, 1824) (Decapoda, Brachyura, Gecarcinidae) from East Africa: the problem of the island species

Abstract

A mature male Discoplax rotunda (Quoy & Gaymard, 1824) (carapace width 50 mm, length 40 mm) was collected by Mrs. Valentina Vigiani in September 2000, at the edge of the swimming pool of the Reef Hotel, Mombasa, Kenya. The specimen was compared with material from the Aldabra Islands preserved in the Zoological Museum “La Specola”, Florence, Italy, where it has been deposited (MZUF 2611) (fig. 1). Discoplax rotunda is a land crab inhabiting islands in the Indo-West Pacific oceans, from Aldabra and Mauritius to the Ryukyu Islands, Hawaii, and French Polynesia (Turkay, 1974; Yaldwyn & Wodzicki, 1979; Ng & Guinot, 2001). It commonly lives in the supralittoral or in inland habitats. The Gecarcinidae, a circumtropical family, are successful in invading terrestrial habitats, with many species colonizing islands (Turkay, 1974; Hartnoll & Clark, 2006). There are several decapods commonly distributed on Indo-Pacific islands but unknown from continental coasts, such as the coconut crab, Birgus latro (L., 1767) (cf. Burggren & McMahon, 1988), the land hermit crabs, Coenobita perlatus H. Milne Edwards, 1837 and C. brevimanus Dana, 1852 (Anomura, Coenobitidae: Forest, 1954; Haig, 1984; Burggren & McMahon, 1988), the grapsid crabs, Geograpsus crinipes (Dana, 1852), G. grayi (H. Milne Edwards, 1853) (cf. Hartnoll, 1975; Holthuis, 1977), and Metasesarma obesum (Dana, 1851) (cf. Ng & Schubart, 2003). There is no satisfactory explanation for such restricted geographical distribution. The larvae, when released, probably do not migrate nor drift far, and thus settle