Chemical Defenses of Sea Hares: Novel and Adaptive Strategies for Escape and Defense Via Inking
Cynthia Kicklighter, Dept. of Biology, Goucher College
26 Jan 2007
Numerous studies have demonstrated that chemical defenses protect a diversity of marine invertebrate prey from predation. Most studies have assumed that these defenses are repellent to predators, but very few investigations have examined the mechanism by which predator behavior is manipulated. When attacked by predators, such as spiny lobsters and sea anemones, the sea hare Aplysia californica releases the secretions ink and opaline. These secretions chemically defend A. californica, facilitating its escape upon attack. Ink and opaline appear to defend sea hares against lobsters via a combination of novel and conventional defensive mechanisms. Lobster behavioral assays and electrophysiology of the lobster chemosensory system suggest that free amino acids in ink and opaline mimic food odors, a phenomenon we refer to as 'phagomimicry.' Lobsters are deceived into attending to a false food stimulus (the secretions), allowing sea hares to escape. In addition, lobsters find opaline unpalatable, due to an as yet unidentified small, polar molecule. Ink and opaline's effects on lobsters differ from those on sea anemones. Against anemones, multiple components in ink, different from those attractive to lobsters, are aversive and elicit tentacle retraction and/or shriveling, while opaline does not. Ink and opaline also elicit anti-predator behaviors in nearby conspecifics, acting as alarm cues. The base uracil and the nucleosides uridine and cytidine in ink and a group of mycosporine-like amino acids in opaline are the molecules that mediate these alarm responses. Investigations of Aplysia californica defenses demonstrate that chemical defenses can operate in ways other than simply deterring predators, may function in multiple ways against a single predator, can affect diverse predators differently, and can function as alarm signals for conspecifics.