A role for predation in mediating the population dynamics of deep-sea corals?

Deep-sea corals are known for their slow growth and longevity, surviving in dark, cold waters for centuries or more. In contrast, some coral predators (e.g., sea stars) appear to operate on shorter time scales, preying on corals at rates that seemingly outpace the capacity of corals to recover. Does predation on juvenile and adult coral colonies affect the demographic rates of coral populations? We have attempted to evaluate the incidence and importance of predation on several deep-sea coral taxa at depths of 800-1300 m at Sur Ridge, off the central California (USA) coast. Surveys were performed along benthic video transects to measure the incidence of potential predators and other epibionts on coral taxa. Coral colonies were marked and revisited over 2-3 years to assess changes in polyp cover in relation to predator occurrence. In addition, we performed manipulative experiments to assess the behavioral response of predatory sea stars (Hippasteria spp.) placed on coral colonies from several taxa (Paragorgia arborea, Keratoisis sp., Isidella tentaculum). While the incidence of predators and other epibionts was low on all three coral taxa, I. tentaculum had the lowest epibiont density, presumably related to the shield of nematocyst-laden tentacles at the base of colonies. Short field experiments indicated that these basal branches of I. tentaculum were repellent to the sea stars, which quickly moved off basal tentacles. Stars placed on branches of I. tentaculum, Keratoisis sp., and P. arborea were not repelled and in some cases began feeding on the corals. Repeated observations of isidid colonies under active predation by Hippasteria spp. or large nudibranchs (Tritonia tetraquetra) or both indicated that these predators can denude entire coral colonies within months. These results indicate more research is needed to determine how corals survive this apparent paradox of rapid predation damage amidst the slow tempo of coral colony life.