THE FIRST RECORD OF THE LAND , DISCOPLAX ROTUNDA (QUOY & GAYMARD, 1824) (, BRACHYURA, ) FROM EAST AFRICA: THE PROBLEM OF THE ISLAND SPECIES

BY

GIANNA INNOCENTI1,3) and MARCO VANNINI2) 1) Sezione Zoologica “La Specola”, Museo di Storia Naturale, Università di Firenze, via Romana 17, I-50125 Florence, Italy 2) Dipartimento di Biologia Animale e Genetica “Leo Pardi”, Università di Firenze, via Romana 17, I-50125 Florence, Italy

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 (Türkay, 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 (Türkay, 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 , Coenobita perlatus H. Milne Edwards, 1837 and C. brevimanus Dana, 1852 (Anomura, : 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

3) e-mail: gianna.innocenti@unifi.it © Koninklijke Brill NV, Leiden, 2007 Crustaceana 80 (5): 635-637 Also available online: www.brill.nl/cr 636 NOTES AND NEWS

Fig. 1. Discoplax rotunda (Quoy & Gaymard, 1824) (MZUF 2611), male, collected in Mombasa (Kenya) (photo: S. Bambi). nearby (Garth, 1966; Fletcher & Amos, 1994; Wolanski & Sarsenski, 1997). Megalopae of ruricola (L., 1758) were found to return to the same area where they had been released as zoeas within 18 days, with no apparent reasons why the zones of larval release and landing coincide (Hartnoll & Clark, 2006). It is unclear why species such as carnifex (Herbst, 1794), a gecarcinid related to D. rotunda,orGeograpsus stormi De Man, 1845 (a congeneric of the two inland Geograpsus), or Coenobita rugosus H. Milne Edwards, 1837 and C. cavipes Stimpson, 1858 (very close in their ecology and behaviour to the inland Coenobita species, cf. Burggren & McMahon, 1988), are commonly distributed across the Indo-West Pacific. The lack of terrestrial predators can probably explain why a slow and shell-less B. latro is absent from continental coastlines, but this hardly applies to the shell-encased land hermits and swift moving land crabs. Recently, “insular” species such as C. perlatus and C. brevimanus, were found in Kanamai, 30 km north of Mombasa (Vannini & Ferretti, 1997), and G. crinipes was found in Somalia (Vannini & Valmori, 1981) and in Kenya (M. Vannini, pers. obs.). The extreme rarity of these species along the East African coastline confirms their “insular” character, yet poses the question of the reasons for such a seemingly odd distribution.

ACKNOWLEDGEMENT The authors wish to thank Prof. Bella S. Galil, Israel Oceanographic and Limnological Research, Haifa, for her useful suggestions.