Feeding behavior and population structure of Ampithoe valida : a bicoastal comparison

Eelgrass (Zostera marina) beds are highly valued in temperate estuaries worldwide, and decades of declines have led to conservations and restoration programs in many regions. Understanding variation in factors that determine eelgrass success is an important part of these programs. Small invertebrate grazers on eelgrass are generally thought to benefit plants by removing competing algae; however, an amphipod, Ampithoe valida, putatively introduced to San Francisco (SF) Bay, also directly consumes eelgrass and threatens restoration efforts. This feeding behavior differs from that observed in the range of A. valida where it primarily grazes on micro- and macroalgae. The mechanisms as to why feeding preference for eelgrass differs between regionally distinct populations of A. valida are not well understood. The results of a series of feeding tests demonstrate that A. valida in SF Bay is adapted to preferentially consume local eelgrass when offered a choice, and also to more readily consume eelgrass from regionally local estuaries (SF Bay, Bodega Bay and Tomales Bay) over Virginia eelgrass, while Chesapeake Bay A. valida did not exhibit preferences among the same foods. Morphological and chemical differences between eelgrass populations did not clearly drive SF Bay A. valida feeding preferences. This study suggests evolution of feeding behavior in the SF Bay amphipods. Alternatively, Chesapeake A. valida may simply have not acclimated to local eelgrass, perhaps due to greater availability of non-eelgrass foods including macroalgae and epiphytes. Increased amphipod density did not induce a feeding shift from Gracilaria to eelgrass in an experiment with SF Bay amphipods and SF Bay foods; in fact, A. valida consumed eelgrass equally to Gracilaria at all densities. Phylogenetic analysis of COI sequence data from Atlantic and Pacific A. valida showed cryptic diversity; however, analysis of two nuclear genes did not corroborate these findings, suggesting that these are cryptic subspecies that have only recently diverged. The genetic data also suggest a Pacific origin, rather than Atlantic, of A. valida.