MEPS 374:101-111 (2009)  -  DOI: https://doi.org/10.3354/meps07767

ABSTRACT: Chemical defenses are thought to be infrequent in most soft-sediment systems because organisms that live beneath the sediment rely more on avoidance or escape to reduce predation. However, selection for chemical deterrence might be strong among soft-sediment organisms that are sessile and expose at least part of their body above the surface. The phoronid Phoronopsis viridis is a tube-dwelling lophophorate that reaches high densities (26500 m^–2) on tidal flats in small bays in California, USA. We found that P. viridis is broadly unpalatable, and that this unpalatability is most apparent in the anterior section, including the lophophore, which is exposed to epibenthic predators as phoronids feed. Experimental removal of lophophores in the field increased the palatability of phoronids to predators; deterrence was regained after 12 d, when the lophophores had regenerated. Extracts of P. viridis deterred both fish and crab predators. Bioassay-guided fractionation suggested that the active compounds are relatively non-polar and volatile. Although we were unable to isolate the deterrent metabolite(s), we were able to rule out brominated phenols, a group of compounds commonly reported from infaunal organisms. One predator, juvenile Scorpaenichthys marmoratus, consumed all P. viridis extracts and whole P. viridis. However, over time, this predator rejected phoronids when fed a diet that included 2 to 3 phoronids d^–1. The broadly effective defenses possessed by phoronids, which may be unusual among soft-sediment invertebrates, may play a key role in allowing them to reach high densities.

KEY WORDS: Phoronopsis viridis · Chemical defense · Predation · Deterrent metabolites · Soft sediment · Regeneration · Palatability