Thesis Project Outline

Fig. 1 - click to enlarge	My primary interest for my thesis is to try to understand the scale at which stepping-stones of intermediate habitat are important in connecting patchy marine populations.  The importance of stepping-stones is intuitively obvious.  They have long been invoked, post hoc, to explain patterns of genetic connectivity observed along a coastline (e.g. Barber et al. 2002), or within archipelagos but not among them (e.g. Planes et al. 1996, Doherty et al. 1995).  From a marine biogeographic point of view, the absence of stepping-stones is often recognized as a barrier to movements of individuals as well as species (e.g. Vermeij 1987). If I could design an idealized experiment in which I sought to investigate the relative importance of stepping-stones to marine population connectivity, I would set it up as in figure 1.  There are two  species, co-distributed on each of two landmasses and alike in every aspect, including the possession of a planktonically dispersing larva, but different in that one lives in freshwater streams and one is intertidal. Scattered between the landmasses are small islands or atolls that have no freshwater on them, and so are only accessible by the marine intertidal species.  If I were to measure gene flow between the landmass populations of both species, I would hypothesize that the marine intertidal species would have a higher rate of gene flow, due to the numerous atoll stepping-stones on which its larvae can settle and produce new larvae to send onward. As it happens, there is a study system that approaches this ideal.  The gastropod family neritidae is formerly part of the archaeogastropoda, now they are part of the neritopsina, sister group to the vetigastropoda (e.g. Trochus and abalones) of Ponder and Lindberg (1997).  Holthuis (1995) proposed that lineages of this group moved from freshwater to saltwater 2 or 3 seperate times, based on her most parsimonious reconstruction of reproductive morphology (figure 2).  Members of both the Neritilia and the Neritina + Septaria freshwater clades have a pan-tropical distribution, as do the marine Nerita, and all of these are found throughout the islands of the South Pacific.  Both the freshwater and marine species have planktonically dispersing larvae (figure 3).  The larvae of the freshwater species are flushed out of fast-flowing high island streams, and into the ocean.  As an undergraduate, I studied this initial period of larval freshwater tolerance. The South Pacific provides many opportunities to replicate the above experiment, in which two high islands (or two archipelagos of high islands) are separated by waterless atolls.  One such place is the region betwen Fiji and Samoa (figure 4).  These islands are separated by the islands of the Lau and Tonga groups - mostly low islands without freshwater.  Another place is the Marquesas and the Society Islands, which are separated by the arid Tuamotu atolls.  I will compare measures of gene flow from these places to gene flow measured across the North Fiji Basin between Fiji and Vanuatu, which has no stepping-stones of any kind.  This region acts as a "control" for the stepping-stones "treatment", I would expect both species to have equal levels of gene flow. I have sampled snails from both freshwater and marine lineages from all of the locations mentioned above.  I will infer levels of gene flow from coalescent analysis of haplotype frequencies at the mitochondrial COI locus, and hopefully from multiple nuclear markers as well.  Nuclear markers will also help to delineate species boundaries. Back to Homepage
Fig. 2 - click to enlarge
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