Spines and epibionts as antipredator defenses in the thorny oyster Spondylus americanus Hermann

Observations and experiments on the cemented pelecypod Spondylus americanus Hermann at Discovery Bay, Jamaica were made to identify the predators of this animal and learn whether their spines, or their epibionts, are important in deterring predation. Spine growth was studied also and found to be very rapid (≈ 1mm/day), and broken spines broken were repaired at this rate.The predators of Spondylus include the gastropod Fasciolaria tulipa Linnaeus (responsible for half of the observed predation), and various crushing predators, including the spiny lobster Panulirus argus (Latreille)], rays Aetobatis narinari (Euphrasen) and Dasyatus americanus Hildebrand], the porcupine fish (Diodon hystrix L.), and an unidentified stomatopod. Predators show an increased frequency of attacks in shallow water versus deep, and probably account for the scarcity of Spondylus americanus above −15 m.The concealment effect of epifauna was found to be more important than spines as an antipredator defense. Removal of spines and epibionts significantly increased levels of mortality only when both features were manipulated. There was no statistically significant increase in predation following the removal of epibionts because the shells were rapidly grown over by sponges. Spines alone do not appear to deter predators, but probably serve to attract epibionts and extend them beyond the more vulnerable shell margin. These results do not support the view that spondylid spines evolved as structural deterrents to predators. I argue that they most likely evolved to attract other organisms to settle on the shell and conceal it from predators.