Early ontogenetic expression of specificity in a cnidarian-algal symbiosis

Most cnidarian-microalgae symbioses exhibit some degree of specificity in host-symbiont pairing. Among taxa with aposymbiotic larvae, specificity must be established each generation. Newly settled polyps of the gorgonians Plexaura kuna and Pseudoplexaura porosa rapidly acquired zooxanthellae, in both laboratory and field settings. Initial zooxanthella acquisition by polyps was non-selective and did not reflect adult host specificity. When placed in shallow backreef and forereef habitats, and on the deep forereef, newly settled polyps naturally acquired zooxanthellae belonging to Symbiodinium clades A, B and C (based on restriction fragment length analysis of small subunit ribosomal DNA). Over time the taxonomic diversity changed such that by 3 mo, the majority of polyps (77%) harbored only algae belonging to Symbiodinium clade B, the same algal clade found in the adult hosts. A survey of naturally occurring juvenile gorgonians (those 10 cm or less) showed that all contained only algae belonging to Symbiodinium clade B at this life history stage. These data are the first to characterize the early ontogenic change in zooxanthella diversity and confirm models of specificity in cnidarians in which a broad group of dinoflagellates initially enter the host followed by a change in algal genotypes that leads to the specificity observed in the adult host.