Chemical defenses and community impacts of the phoronid Phoronopsis viridis in Bodega Bay, California

Associational defenses can reduce the intensity of predation, particularly in environments where environmental stress may be buffered by the presence of biogenic structures. In tidal flat communities, tube-building organisms create structure and provide refuge for other organisms. I have looked at the effects a large, chemically-defended tube-building lophophorate, Phoronopsis viridis (syn. P. harmeri) on the associated epifaunal and infaunal populations in Bodega Harbor, an estuary in Central California. A chemical deterrent to predation may be allowing P. viridis to maintain high population densities, thereby altering the distribution of infauna on the tidal flats of Bodega Harbor by providing a structurally-mediated refuge. Bioassay guided feeding assays of P. viridis parts and fractionation of organic extracts indicate that the deterrent is (a) strongest in the lophophore; (b) chemically mediated; and (c) non-polar and volatile (Chapter 1). Patch selection experiments were conducted in the laboratory using pair wise combinations of live phoronids, phoronid structural mimics and bare sediment to determine whether P. viridis offers an associational refuge to other infauna and the relative importance of the structure and chemical defenses. Nutricola tantilla had a structurally-mediated refuge from predation by Hemigrapsus oregonensis, though was not as effective against the nonindigenous Carcinus maenas. (Chapter 2). A survey of adjacent high and low P. viridis density patches showed that soft-sediment areas with high densities of P. viridis have higher infaunal abundance and richness than similar areas with low densities of P. viridis (Chapter 3). To determine whether this difference was driven by P. viridis and whether this difference may be attributed to the whole organism or the structure alone, we conducted a field experiment with live phoronids, phoronid mimics, phoronid-free sediments (bare) and unmanipulated sediments. Although the field experiment did not detect differences in the overall abundance or richness of infauna among the manipulated treatments, some of the individual species did show a response to the presence of phoronids and phoronid structure. In particular, the polychaete Boccardia proboscidia, the amphipod Corophium uenoi, and harpactacoid copepods were facilitated by the presence of phoronids and phoronid structure when there was sediment disturbance (Chapter 3).