Ever wondered if a gorgonian octocoral species found on coral reefs throughout the Caribbean and the single celled algae that live in its tissue have similar or different population structures? Perhaps not, but this is what two colleagues, Andrew Baker (RSMAS, University of Miami) and Kevin Feldheim (Field Museum of Natural History), and I (Herman Wirshing) recently investigated when I was a graduate student at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, the results of which were published in the September 2013 issue of Molecular Ecology (Vectored dispersal of Symbiodinium by larvae of a Caribbean gorgonian octocoral. Molecular Ecology, 22: 4413–4432).
Using SCUBA and freediving, we collected small samples (~3cm branch clips) from the gorgonian octocoral species Eunicea flexuosa from 9 reefs that spanned the Florida reef tract, along with samples from Panama, Saba, and the Dominican Republic. We used genetic markers called microsatellites to examine if E. flexuosa and the dinoflagellate algae, Symbiodinium spp., that live in their tissue (we call them symbionts) from these different locations form similar or different population patterns, and in what direction is the overall movement of their larvae (the octocoral “babies”).
We found that E. flexuosa from sites within the Florida reef tract are genetically well connected and form a single genetic population that is separate from a second population made up of individuals from Panama. However, the algal symbionts, Symbiodinium B1 (a specific genetic lineage of Symbiodinium), from the same E. flexuosa colonies were divided into at least 5 well-structured populations along the Florida reef tract together with another population found in Panama. How could each member of this symbiotic relationship show different population patterns? It is likely that the larvae of E. flexuosa travel relatively long distances (to different reefs) before it settles on the reef surface and grows into an adult colony. However, our data suggest that over time, E. flexuosa colonies likely end up associating with local Symbiodinium B1 found on the reef on which they have settled - unlike E. flexuosa, their Symbiodinium B1 partners don’t disperse as far so easily. That would explain the different population patterns found from E. flexuosa and its Symbiodinium B1 partners.
However, and quite unexpectedly, we found that 5 colonies of E. flexuosa and their symbionts from Key West contained the same genetic signals as other colonies from Panama. So, did these 5 colonies of E. flexuosa in Key West come from Panama? Using computer programs that infer the direction of gene flow (which way the octocoral “babies” are going), we determined that the 5 unusual E. flexuosa colonies found in Key West are most likely to have come from Panama and not vise versa (not including areas we did not sample). Therefore, we determined that the symbionts found in these particular E. flexuosa colonies likely hitched a ride as “stowaways” in the tissue of these octocoral larvae. Moreover, when we looked at the relationship between the host (E. flexuosa) and their Symbiodinium B1 partners at the genetic level, we did not find it to be strict relationship, but a flexible one. Therefore, we concluded that Symbiodinium B1 can not only hitch rides as “stowaways” to different reefs, but can then move to different E. flexuosa colonies with different genetic signatures, effectively becoming “stowaways on the run”. This process of symbionts hitching rides as “stowaways” and then moving from one E. flexuosa colony to another could contribute to the dispersal of symbionts and hosts with viable genetic combinations to different reefs in response to environmental change.