Diversity of dinoflagellate symbionts (zooxanthellae) in a host individual

Zooxanthellae are phototrophic dinoflagellates that exist in symbiosis with a variety of marine invertebrates. The traditional view of zooxanthella-in vertebrate symbioses suggests that individual hosts harbor taxonomically ho mogeneous symbiont populations. To assess the diversity of the zooxanthella assemblage inhabiting an individual host. zooxanthellae from 6 species of clam (Tridacna gigas, T. squamosa, T. crocea, Hippopus hippopus, H. porcell anus and Corculum cardissa) and 1 species of sea anemone (Aiptasia sp.) wer e studied using temperature-gradient gel electrophoresis (TGGE), coupled wi th polymerase chain reaction (PCR) using zooxanthella-specific primers that were designed to target hypervariable regions of the small subunit ribosom al RNA (ssrRNA) gene. Results revealed that 1 clam may harbor 2 or more gen otypically distinct zooxanthellae, with 1 or more dominant taxa occurring a t a time. The clams studied associated with at least 4 zooxanthellar taxa. Nucleotide sequencing of the TGGE bands and phylogenetic reconstruction rev ealed that the zooxanthellar taxa in clams were Symbiodinium spp.; 1 was id entical to previously cultured clam symbiont isolates, 1 appeared identical to a previously studied unculturable clam symbiont, and the other 2 clams represented novel strains of Symbiodinium. individual Aiptasia sp. harbored only 1 zooxanthellar taxon, which had a ssrRNA sequence identical to that of S. pulchrorum, previously isolated from Aiptasia pulchella. This study h as shown that individual tridacnid and cardiid clams can harbor heterogeneo us zooxanthellae.