How tasty are brachiopods? Examining predation on a "refugia" taxon

Background/Question/Methods Brachiopods are rare in modern marine communities and their distribution is anti-tropical.  However, prior to the end-Permian extinction, brachiopods were the dominant marine macrobenthos world-wide. These patterns have led workers to postulate that modern brachiopods are “refugia” taxa, restricted to specific regions by increased predation. However, Thayer argued that modern brachiopods are toxic, a suggestion not entirely consistent with the “restricted by predation” hypothesis. The present study reexamines brachiopod predation and differs from previous experiments in that (a) predators and prey were collected together, ensuring familiarity of predators with prey; and (b) different prey species were presented to predators separately; thus, failure to consume brachiopods would not be due to predator preference. Two brachiopods, Terebratalia transversa and Terebratulina unguicula, two bivalves, Chlamys hastata and Acila castrensis, and three predators, Cancer productus, Pycnopodia helianthoides, and Fusitriton oregonensis, were collected by dredge from 50 m depth off Iceberg Point, Lopez Island, Puget Sound. Treatments involved presenting individual predators with 12 individuals of one prey species in sea-tables with running sea-water (12-14° C). Subsequent treatments presented each predator with a different prey species. Treatments lasted for five days and were separated by a starvation period.  Order of prey presentation differed among conspecific predators.

Results/Conclusions At least one individual of each predator species consumed brachiopods. Each individual that consumed brachiopods ingested at least two brachiopods. There was some delay in the predator attacking the first brachiopod, but minimal times between the first and subsequent kills. No adverse effects on the predators were observed. All predators consumed the bivalve taxa in separate treatments; however, predators consumed bivalves at a greater rate than predators consumed brachiopods (permutation, p < 0.01). These results suggest that co-occurring predators were capable of consuming brachiopods without obvious detrimental effects, casting doubt on the suggestion that these brachiopods have toxins capable of repelling invertebrate predators. Brachiopods are more tightly articulated than many bivalves and are able to remain closed for long periods of time (some brachiopods survived digestion). Thus, predators may prefer bivalves as prey (greater consumption rate) because they are generally fleshier and take less time to kill, providing a greater net energetic gain. The initial delay in attacking brachiopods may be a function of the time it took the predator to recognize that bivalves were absent and that brachiopods were the only available prey. Although brachiopods may lack toxins, they may be less vulnerable to predation than is often supposed.