Meeting Abstract

22.3  Monday, Jan. 4  The biogeography of Symbiodinium from giant clams (Tridacnidae) WEBER, M.X.*; FAY, S.A.; LIPPS, J.H.; Univ. of California, Berkeley; Univ. of California, Berkeley; Univ. of California, Berkeley mxweber@berkeley.edu

Tridacnidae, the giant clams, are one of many groups of host organisms for Symbiodinium, a diverse clade of dinoflagellates that structurally and energetically support coral reef communities. These algae are found in symbiosis with corals, sponges and foraminifera as well as giant clams. Using DNA sequence data I identified Symbiodinium hosted by various populations of tridacnids in the Red Sea, the Indian Ocean and the Pacific Ocean. This study described Symbiodinium ecology and biogeography over the complete distribution of a single host clade. Phylogenetic analysis grouped the symbionts into three major subgeneric clades, A, C and D. Giant clam symbionts were most diverse in Papua New Guinea and along the East African coast although most Tridacna populations hosted several different Symbiodinium lineages. However, at the extreme eastern end of the host distribution, I identified only one species of clam, T. maxima hosting only one type of symbiont. The Red Sea T. maxima populations hosted a unique lineage of symbionts not found in other tridacnids from other regions. I analyzed patterns in the symbiont distribution with respect to several ecological variables including temperature, reef environment and three spatial dimensions: latitude, longitude and depth. Giant clam symbiont diversity was limited compared to other hosts. The same symbiont phylotypes were identified in clams from disparate localities despite evidence for a variety of alternative types available in other hosts such as corals or foraminifera. Biogeographic patterns can be attributed to the geologic history of the Indo-West Pacific and the ecology of the symbiosis. These data suggested that giant clams select specific Symbiodinium lineages to optimize the mutualism.