The symbiosis ecology of reef-building corals

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Marine Biology and Fisheries

First Committee Member

Peter W. Glynn - Committee Chair


The diversity of symbiotic ("zooxanthellae") contained within reef-building corals was surveyed using Restriction Fragment Length Polymorphisms (RFLPs) in large subunit ribosomal rRNA genes. In total >800 samples from >110 species of reef coral from the western Atlantic (Bahamas, Panama), eastern Pacific (Panama, Galapagos, Mexico) and Indo-west Pacific (Australia) were surveyed. These methods, combined with molecular sequencing of large subunit rDNA, distinguished 17--19 symbiont genotypes in four clades of Symbiodinium. The distribution of these genotypes showed strong ecological, biogeographic and host-systematic patterns. Many (>35%) of the >100 scleractinian coral species surveyed contained multiple symbiont genotypes (sometimes in single coral colonies) which often showed light-related patterns of zonation, both among colonies at different depths and within colonies across sunlit and shaded surfaces. Given the extremely conservative nature of: (1) the molecular methods, (2) the per-species number of samples, and (3) the number of sites visited, it is clear that inter- and intraspecific symbiont diversity is a common feature of reef-building coral biology. Despite being sampled from a much wider diversity of coral hosts, symbiont diversity in tropical Pacific scleractinian corals showed significantly lower phylogenetic diversity than their Caribbean counterparts. This biogeographic split is hypothesized to be a result of historical vicariance and paleoclimatic change, which caused temperature-related changes in the symbiont communities of tropical Caribbean corals. Symbiont change was investigated empirically by reciprocal depth transplantation; results supported the notion that the coral-algal association is dynamic and indicated that qualitative change in symbionts is facilitated by an acute stressor (bleaching) to first remove existing symbiont populations. Unusual opportunistic or weedy symbiont taxa can also repopulate these newly vacant hosts. These weedy symbionts were found to be highly resistant to an episode of El Nino-related coral bleaching in the eastern Pacific, further indicating that symbiont identity can have an important influence on the biology of the coral host. Taken together, these results argue for an explicit recognition of symbiont diversity in future studies of reef-building corals. Because these organisms are first and foremost symbioses, their biology, ecology and/or biogeography may only be fully understood when the identities of both partners are known.


Biology, Ecology; Biology, Genetics; Biology, Oceanography; Biology, Zoology

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