Genetic diversity and population differentiation of small giant clam Tridacna maxima in Comoros islands assessed by microsatellite markers

The giant clam subfamily Tridacninae (Schneider and Foighil 1999) is the most widespread of the bivalves and is distributed throughout the Red sea and Indo-Pacific Ocean, from French Polynesia to East Africa (bin Othman et al. 2010). There are currently eight species from the genus Tridacna in the world: Tridacna. gigas (Linnaeus, 1758), T. maxima (Röding, 1798), T. crocea (Lamarck, 1819), T. squamosa (Lamarck, 1819), T. derasa (Röding, 1798), T. tevoroa (Lucas, Ledua and Braley, 1991), T. rosewateri (Sirenko and Scarlato, 1991), T. costata (Roa-Quiaoit, Kochzius, Jantzen, Zibdah and Richter, 2008) (bin Othman et al. 2010). Recently, T. noae was separated from T. maxima by their genetic and morphological description (Su et al. 2014). Among these bivalves, T. maxima has commonly the largest distribution range (Lucas 1988). All those giant clams are settled on the coral reef in shallow water and live in symbiotic photosynthetic with xanthophyllae algae (genus Symbiodinium) that grow in the mantle tissues (Soo and Todd 2014).

Like other marine bivalves, small giant clam species (T. maxima) are sedentary as adults, reproduce by broadcast spawning with high fecundity (10⁶ eggs per female), and have pelagic larval dispersal about 9 days (Lucas 1988). Based on these aspects, population genetics studies can provide more information about the ecological interactions, larval dispersal, distribution patterns, as well as evolution of the species. To date, most of studies have been conducted on T. maxima about spawning (Lucas 1994; Soo and Todd 2014), larval and post-larval development (Jameson 1976), and growth (Hart et al. 1998; Smith 2011; Toonen et al. 2011). Whereas only a few studies have been done on genetic diversity and genetic structure of T. maxima. Indeed, genetic variations studies using allozyme analysis (Campbell et al. 1975; Laurent et al. 2002) and, recently, mitochondrial markers (Nuryanto and Kochzius 2009), have provided information on highly genetic variability, larval dispersal and also the connectivity of different sites of Indo-Pacific Ocean that can be explained by marine currents or geographic isolation (Benzie and Williams 1992a, b).

Small giant clam is listed in Appendix II of CITES (United Nations Convention on International Trade in Endangered Species of Wild Fauna and Flora) and classified as lower risk conservation dependent on the IUCN (International Union for Conservation of Nature) Red List of Threatened Species. This status indicates that the population densities have declined in a large geographical region by their overexploitation and the degradation of their natural habitat (Lucas 1994; bin Othman et al. 2010; Hui et al. 2011). It seems to be still abundant according to the population densities data in some part of countries (Australia, up to 3.83 × 10¹ individuals per m² and French Polynesia, 5.84 per m², for instance, see bin Othman et al. 2010). Therefore, it is crucial to intensify the conservation efforts of marine biodiversity as well as to preserve the natural marine species for sustainable development.

Comoros islands are separated from each other by a small distance, which indicate that the area is relatively narrow geographically (Fig. 1). Despite that, the area benefit a considerable interest in conservation due to the presence of abundant marine species, such as T. maxima but no relevant information is available to now. Recently, a research was conducted to identify and determine the marine mollusks species in Comoros islands using the photo-identification method and documentation of previous studies (Ramadhoini and Nirina, unpublished). Likewise an ecologic description have been studied on Tridacnidea family from Mayotte island (Deuss et al. 2013). Some microsatellite primers were developed from T. maxima by Grulois et al. (2014) and showed very high genetic diversity. In this study, we selected nine microsatellite markers (Grulois et al. 2014) to estimate the level of genetic diversity of T. maxima distributed in three islands of Comoros including Grande-Comore (Gc population), Anjouan (An population) and Moheli (Mo population). At the same time, we investigated the population differentiation in order to implement the conservation strategies of the T. maxima.

Map showing the sampling collections of T. maxima in Comoros islands