Population Studies of Hydrothermal Vent Invertebrates
Hydrothermal vents have been discovered at seafloor spreading centers in the Atlantic, Pacific, and Indian Oceans. Many other sites have yet to be explored.
The hydrogen sulfide emitted from hot springs in these areas give rise to specialized communities of organisms. These communities are dependent on the sulfur-oxidizing bacteria that release the energy that fuels the entire ecosystem. We have studied the diverse collection of invertebrates at numerous sites along the East Pacific Rise and Mid-Atlantic Ridges. Vent sites along these ridges are often separated by tens to hundreds of kilometers of cold water with no exploitable source of energy. Many of these areas are ephemeral, lasting only a few decades before venting stops and the organisms die. We have been examining how these particular constraints affect migration and dispersal by using genetic information to compare populations at different vent sites.
We have also been studying the bacterial symbionts that reside within the gills of mussels and clams and within the trophosomes of tubeworms. We are interested in how these bacteria enter in to these symbiotic relationships. Are bacteria transmitted vertically, from mother to offspring through eggs? Or are bacteria transmitted horizontally, where each new generation acquires its symbionts from the environment? (see figure) We use molecular tools and ideas in population genetics to explore some of these issues.
In one study (Peek et al. 1998), we compared phylogenetic trees of vesicomyid clams to phylogenetic trees of their bacterial symbionts. Both trees closely matched each other, providing evidence of long term associations between host and symbiont and the occurrence of vertical transmission. (see figure) In another study (Peek, Vrijenhoek, and Gaut 1998), we looked at nucleotide substitution rates in free-living bacteria, horizontally transmitted symbionts, and maternally transmitted symbionts to examine consequences of populations size and different life-history strategies. Maternally-transmitted symbionts probably have a small effective population size since they do not have a free-living stage. Horizontally-transmitted symbionts presumably do have a free-living stage and hence a larger effective population size. Of the three types, we found that maternally transmitted symbionts had the highest rates of nucleotide substitution. These results support the nearly neutral theory of molecular evolution that predicts higher rates of substitution in small populations compared to larger populations.
Some of the organisms we are studying:
- Southern East Pacific Rise
- Indian Ocean
- NASA article on Indian Ocean expedition - with additional links