Does rafting facilitate gene flow in a brooding marine invertebrate?

Background/Question/Methods

Marine invertebrate brooders present something of a conundrum. Typically, they are perceived to have limited dispersal potential because they lack a free-swimming larval stage. However, they may have high propensity for successful long-distance dispersal via rafting. To assess how rafting might influence population genetic structure, I have selected an isopod (Pentidotea resecata) which broods its offspring to a crawl-away phase (i.e. lacks a pelagic phase) and is thought to have low intrinsic dispersal ability. P. resecata inhabits the canopy of the giant kelp (Macrocystis pyrifera) which when detached (e.g. by storms, wave action, herbivory) may form rafts that are transported for days to weeks via ocean currents. I pose two hypotheses: [1] that P. resecata is dominated by local recruitment and high small-scale population genetic structure, versus [2] that P. resecata passively disperses with M. pyrifera and, therefore, like M. pyrifera, shows isolation by distance (IBD) among populations studied in California, where close populations are genetically more similar than distant ones.

Results/Conclusions

Preliminary analyses of cytochrome c oxidase subunit I (COI) reveal P. resecata has weaker population genetic structure (F_ST = 0.10) than one might expect for a brooder and is perhaps more comparable to M. pyrifera. This result suggests phoretic dispersal via rafting likely contributes to gene flow in this species. Preliminary comparison with Leptasterias sp. and Henricia sp. — both invertebrate brooders that lack a close association with kelp — are consistent with these findings. As a next step, I plan to search for signals of natural selection in P. resecata using genetic assignment tests to identify likely source populations of detached kelp and test whether P. resecata genotypes are represented at sites to which they have the potential to disperse.