The causes and consequences of intraspecific variation in the feeding preferences of a marine herbivore

Herbivores that feed on chemically-rich plants must combat the metabolites by behavioral or physiological means. Spatial variation in such herbivore tolerance is well documented for many insects but largely unexplored for other groups, including marine herbivores. I tested the hypothesis that populations within a single species of marine herbivore exposed to chemically-rich seaweeds will have higher tolerance for those seaweeds and their secondary metabolites than geographically separated populations outside the plants' distribution. The herbivore Ampithoe longimana (Smith 1873) is a polyphagous, tube-dwelling amphipod found in estuaries along the eastern United States coastline. One of A. longimana's preferred foods, tropical seaweeds of the genus Dictyota, has a northern limit at North Carolina and as a result, amphipod populations from Virginia to Maine do not encounter Dictyota. Populations of the amphipod sympatric with Dictyota have higher affinity for, and fitness on Dictyota and its diterpenoid metabolites than do populations outside the plants' geographic range. This geographic variation has a substantial genetic basis, since similar patterns were found across several generations raised in the laboratory. Feeding preference for Dictyota is heritable, polygenic in origin, and inherited in a largely additive manner. Integrating these behavioral and fitness data with a molecular phylogeography of several populations (mtDNA and nuclear ITS sequences) suggests that drift cannot completely explain such evolution; rather, local selection must have played a role. My research indicates that (1) marine herbivores can respond evolutionarily to the presence and absence of particular seaweed species, (2) evolving high tolerance for one seaweed does not correlate with reduced tolerance for alternative seaweeds, which may help explain the predominance of a generalist lifestyle among marine herbivores, (3) fish omnivory acts to maintain herbivore genotypes with relatively high preferences for chemically-rich seaweeds, and (4) that either sequential evolution or diffuse coevolutionary cycles characterizes the trajectory of Ampithoe-Diclyota interactions over longer timescales.