Ink secretion protects sea hares by acting on the olfactory and nonolfactory chemical senses of a predatory fish

Ink secretion of sea hares, Aplysia californica, is a mixture of ink from the ink gland and opaline from the opaline gland. Its defensive mechanisms against predatory spiny lobsters include deterrent compounds that are unpalatable and amino acids that stimulate appetitive responses (phagomimicry) or interfere with chemoreception (sensory disruption) by predators. The current study aimed to identify mechanisms whereby sea hares use ink secretion to defend against a fish predator, in this case the bluehead wrasse, Thalassoma bifasciatum. We show that inking by live sea hares decreased the probability that wrasses strike sea hares. Ink protected sea hares by affecting two phases of feeding. First, an ink cloud between a wrasse and food decreased the probability that the wrasse captured the food. Second, if the wrasse captured food treated with ink, then that food was less likely to be accepted. In neither assay did opaline have a significant effect. Inactivating the olfactory sense of fish through nares occlusion eliminated the deterrent effect of ink on food capture but not the effect of ink on food acceptance, thus showing that olfaction mediates responses to deterrents during the capture phase of feeding and that nonolfactory chemical senses mediate responses to deterrents during the acceptance phase. These nonolfactory chemical senses may be intraoral senses, as fish did not reject pellets until after they were captured. Ink did not protect through phagomimicry, since neither ink nor opaline was accepted, despite the fact that mixtures containing the amino acid components of ink and opaline were accepted.