Hydroids (Cnidaria, Hydrozoa) of Coral Reefs: Preliminary Results on Community Structure, Species Distribution and Reproductive Biology in Juan de Nova Island (Southwest Indian Ocean)

Abstract

The first field investigation of the marine life at Juan de Nova gave an opportunity to study hydroid diversity in April-May 2004. Species richness was high, with 95 species belonging to 26 families and 44 genera. Thecates dominated (72%), with three families particularly diverse: Haleciidae, Sertulariidae and Aglaopheniidae. There were four different sub-communities with a few species in common: two intertidal and two subtidal. One was located on reef flats subject to strong hydrodynamic conditions, and included rheophilic and photophilic species such as Millepora exaesa, Nemalecium sp., Dynamena crisoides, Thyroscyphus fruticosus, Thyroscyphus sp. and Aglaophenia cupressina. The second was found where the reef flat experienced calmer conditions, and comprised mainly Plumularioidea and Haleciidae species with small sized colonies (1-3cm) and large populations, densely covering the hard substrata of coral patches. The third sub-community colonized the reef platform (5- 20m), with hydroids widespread and diverse. The fourth was on the outer slope, deeper (30m), and was characterized by the presence of four Solanderia species, several Aglaophenids and Thyroscyphus aequalis. Most of the species were brooders (84%). Present data are discussed regarding environmental parameters, and compared with data from the Îles Glorieuses, other islands of the Mozambique Channel. INTRODUCTION