Diversity of dinoflagellate symbionts (zooxanthellae) in a host individual

Abstract

Zooxanthellae are phototrophic dinoflagellates that exist in symbiosis with a variety of marine invertebrates. The traditional view of zooxanthella-invertebrate symbioses suggests that individual hosts harbor taxonomically homogeneous symbiont populations. To assess the diversity of the zooxanthella assemblage inhabiting an individual host, zooxanthellae from 6 species of clam (<i>Tridacna gigas, T. squamosa, T. crocea, Hippopus hippopus, H. porcellanus</i> and <i>Corculum cardissa</i>) and 1 species of sea anemone (<i>Aiptasia</i> sp.) were studied using temperature-gradient gel electrophoresis (TGGE), coupled with polymerase chain reaction (PCR) using zooxanthella-specific primers that were designed to target hypervariable regions of the small subunit ribosomal RNA (ssrRNA) gene. Results revealed that 1 clam may harbor 2 or more genotypically distinct zooxanthellae, with 1 or more dominant taxa occurring at a time. The clams studied associated with at least 4 zooxanthellar taxa. Nucleotide sequencing of the TGGE bands and phylogenetic reconstruction revealed that the zooxanthellar taxa in clams were <i>Symbiodinium</i> spp.; 1 was identical 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 <i>Symbiodinium</i>. Individual <i>Aiptasia</i> sp. harbored only 1 zooxanthellar taxon, which had a ssrRNA sequence identical to that of <i>S. pulchrorum</i>, previously isolated from <i>Aiptasia pulchella</i>. This study has shown that individual tridacnid and cardiid clams can harbor heterogeneous zooxanthellae.