Ecology of ciguateric dinoflagellates in the coral reef complex of Mayotte Island (S.W. Indian Ocean)

Abstract The spatial distribution of ciguateric dinoflagellates was investigated in the coral reef lagoon of the island of Mayotte at the end of the dry season of (1992). Both oceanic and lagoonal waters were oligotrophic. Different substrates were investigated (algae, algal turfs and dead corals) in two areas. One area consisted of a transect between an alluvial coastal bay and the outer barrier reef, through lagoonal patch-reefs, and a second area comprised a lagoonal complex. On nearly all the substrates, the dinoflagellates Gambierdiscus toxicus Adachi and Fukuyo, Ostreopsis spp. and Promcentrmn spp., generally exhibited the following relative abundances: Ostreopsis spp. > Prorocemnim spp. > G. toxicus . Highest abundances of all species of dinoflagellales, particularly G. toxicus , occurred on dead corals, more especially on the inner slope of the barrier reef and on lagoonal patch-reefs, but not at the back of the bay nor on the barrier reef outer slope. On macroalgal substrates, dinoflagellates were moderately abundant, but some substrates, such as several red algae, supported higher abundances suggesting a stimulation. Experimental bio-assays to compare growth in seawatcrs previously incubated with macroalgae, showed pre-conditioned seawaters could be either stimulatory or inhibitory. The red alga, Halymeniu floresia (Cryptonemiaceae), inhibited growth of G. toxicus , while Portieria hornemanii stimulated it. Growth was also slightly stimulated by the brown algae Turbinaria ornatu and Surgassum cf. turbinatifolium (Sargassaceae). Both red algae stimulated the growth of Ostreopsis ovatu to a similar degree.