RESEARCH ARTICLE Patrick Berrebi Æ Ghislaine Cattaneo-Berrebi Pierre Valade Æ Jean-Franc¸ ois Ricou Æ Thierry Hoareau Genetic homogeneity in eight freshwater populations of Sicyopterus lagocephalus, an amphidromous gobiid of La Re ´ union Island Received: 1 March 2005 / Accepted: 3 June 2005 / Published online: 21 July 2005 Ó Springer-Verlag 2005 Abstract In the subfamily Sicydiinae of the family Go- biidae, the species Sicyopterus lagocephalus is one of the common amphidromous fish inhabiting island freshwa- ters in a large intertropical area, widespread from the Comoros archipelago to the Austral Islands. Eight of the main perennial rivers of La Re´union Island (Masc- arene Islands) were sampled in 2000, in order to describe their population genetic structure. A total of 267 adult specimens of S. lagocephalus were screened for 14 enzymatic systems providing 15 presumptive loci, 11 of them being polymorphic. Four other loci were useful as diagnostic markers between S. lagocephalus and Cotyl- opus acutipinnis, a similar, closely related sympatric species. Both species constitute an economic resource locally called ‘‘bichique’’. The focus of this study, S. lagocephalus, accounts for the majority of the bichi- que. The results clearly showed a genetic homogeneity among the eight samples (Fst tests). Within-samples tests (Fis) indicated, in general, a heterozygote deficiency. The multidimensional diagrams (CA) describe an overall structure composed of two main subgroups. This structure was not confirmed in a 2001 sampling. The interpretation of the results takes into account the bio- logical life cycle of this species, characterized by a planktonic phase, from hatching to recruitment, followed by a freshwater life of the adults. All the samples in the survey belong to a single pool. However, the young fish entering the rivers are probably of com- plex origin. We did not find any isolation between rivers and this result should be considered when conservation measures are taken to prevent overfishing. Introduction The freshwater ichthyofauna of isolated islands are characterized by limited biodiversity and diadromy, a biological cycle well adapted to variable climatic and hydrologic conditions. In the Indo-Pacific region, diadromy is widespread mainly in tropical volcanic is- lands, including even large islands like New Zealand, where half the freshwater species are diadromous, most of them being amphidromous (McDowall 1999). Amp- hidromy characterizes those species migrating between marine and brackish or freshwater environments during a stage in their life cycle, which is not the reproductive period—e.g. recruitment (Keith 2003). In the subfamily Sicydiinae of the family Gobiidae, the species Sicyopterus lagocephalus (Pallas 1770) is one of the common amphidromous species inhabiting island freshwaters in a large intertropical area, from the Comoros archipelago (Indian Ocean) to the Austral Is- lands (Pacific Ocean) (Keith 2002). The larvae hatch in freshwater and are rapidly carried by river currents away to the sea where they begin a pelagic life (Manacop 1953; Keith 2003). Larvae are transported mostly by currents as has been frequently observed for other fish (Borkin 1991; Hare and Cowen 1996), but larvae may also ac- tively swim as described in gobiids (Balon and Bruton 1994) and in several other fish larvae (Cowen et al. 1993; Leis et al. 1997; Stobutzki and Bellwood 1997; Fisher et al. 2000). Communicated by S.A. Poulet, Roscoff P. Berrebi (&) Æ G. Cattaneo-Berrebi Æ T. Hoareau Laboratoire Ecosyste`mes Lagunaires, UMR 5119, cc093, University Montpellier 2 place Bataillon, 34095 Montpellier, Cedex 05, France E-mail: berrebi@univ-montp2.fr Tel.: +33-4-67143732 Fax: +33-4-67143719 P. Valade Æ J.-F. Ricou Æ T. Hoareau Association Re´unionnaise pour le De´veloppement de l’Aquaculture—Centre des Eaux Douces, BP 16, Z.I. Les Sables, 97427 Etang-Sale´, La Re´union, France T. Hoareau Laboratoire d’Ecologie Marine, Universite´ de La Re´union, 97715 Saint-Denis mes., Cedex 9, France Marine Biology (2005) 148: 179–188 DOI 10.1007/s00227-005-0058-6