Ceratal autotomy and regeneration in the aeolid nudibranch Phidiana crassicornis and the role of predators

Abstract. Ceratal autotomy by the aeolid nudibranch Phidinna crassicornis is common in the field and was induced in the laboratory by mechanical and predatory stimuli. The ceras detaches from the body wall along an autotomy plane located at its basal constriction. Cerata released copious amounts of mucus during autotomy and exhibited a prolonged writhing response that continued for several hours after detachment. Regeneration of cerata autotomized in the field and in the laboratory was documented. Four days after autotomy, regenerating cerata appeared as small protuberances. By day 24 the regenerates acquired their mature structural organisation and vivid colour. The cerata subsequently increased in length and diameter and were indistin‐guishable from surrounding cerata by 41 to 43 days after autotomy. Regeneration rates of cerata induced to autotomize in the laboratory and regeneration of cerata autotomized in the field were similar, averaging 0.08 and 0.067 mdday, respectively. The sequence of morphological events involved with regeneration following experimental and natural induction of autotomy was identical. The kelp crab Pugettia productn induced autotomy by holding cerata with its chelae. This crab also fed on autotomized cerata and consumed locomotory and ceratal mucus. Ceratal autotomy may be an important mechanism of escape from this predatory crustacean. Other potential predators including hermit crabs and tidepool sculpins did not elicit defensive behaviour in P. crussicornis. Nematocysts were present in the enidosacs and their role in defense was investigated. Fired nematocysts were observed in podia of the asteroid Crossaster papposus following ceratal contact but were not seen in the podia of Pycnopodia helianthoides in a similar trial. For P. crassicornis, the cnidosacs may function primarily as a storage device for safe sequestering of nematoeysts that could pose a threat to the digestive system. They did not play a major defensive role against the predators tested, but may be important in the field against other predators.