INTRASPECIFIC DIVERSITY AND ECOLOGICAL ZONATION IN CORAL ALGAL SYMBIOSIS

All reef-building corals are obligately associated with photosynthetic microalgal endosymbionts called zooxanthellae. Zooxanthella taxonomy has emphasized differences between species of hosts, but the possibili ty of ecologically significant zooxanthella diversity within hosts has been the subject of speculation for decades. Analysis of two dominant Caribbean corals showed that each associates with three taxa of zooxa nthellae that exhibit zonation with depth-the primary environmental gr adient for light-dependent marine organisms. Some colonies apparently host two taxa of symbionts in proportions that can vary across the col ony. This common occurrence of polymorphic, habitat-specific symbioses challenges conventional understanding of the units of biodiversity bu t also illuminates many distinctive aspects of marine animal-algal ass ociations. Habitat specificity provides ecological explanations for th e previously documented poor concordance between host and symbiont phy logenies and the otherwise surprising lack of direct, maternal transmi ssion of symbionts in many species of hosts. Polymorphic symbioses may underlie the conspicuous and enigmatic variability characteristic of responses to environmental stress (e.g., coral ''bleaching'') and cont ribute importantly to the phenomenon of photoadaptation.