Multiple components in ink of the sea hare Aplysia californica are aversive to the sea anemone Anthopleura sola

Abstract

Sea hares of the genus Aplysia rely on an array of behavioral and chemical defenses, including the release of ink and opaline, to protect themselves from predation. While many studies have demonstrated that ink and opaline are repellent to predators, very little is known about which components of these secretions are active against predators. Ink was previously shown to facilitate the escape of Aplysia from predatory anemones ( Anthopleura) by eliciting tentacle retraction and/or shriveling, and gastrovascular eversion, but the metabolites mediating this interaction were not identified. We investigated the metabolites in Aplysia californica secretions that were aversive to the anemone Anthopleura sola, as demonstrated by tentacle shriveling and/or retraction. We found that ink elicited tentacle shriveling and/or retraction, while opaline elicited a feeding response. The active components in ink do not appear to be diet-dependent, as ink was aversive regardless of diet (natural seaweed diet vs. Gracilaria ferox). Furthermore, metabolites extracted from G. ferox were not aversive, suggesting that the aversive components are produced by the sea hares. We then examined escapin, a protein in ink with antimicrobial properties. Escapin quickly forms reaction products when mixed with the amino acids l-lysine and l-arginine, which would occur when ink and opaline are released into the sea hare mantle cavity. Neither escapin alone nor escapin mixed with its amino acid substrate l-lysine elicited aversive behaviors either immediately before or 2 min before applying to the tentacles. In addition, escapin mixed with opaline and applied to tentacles after 2 min did not elicit a significant aversive response. Using bioassay-guided fractionation, we attempted to isolate the components in A. californica ink that are aversive to A. sola. We determined that multiple components in ink, including both lipophilic and hydrophilic constituents, elicited aversive responses. We hypothesize that these components may facilitate A. californica's escape from A. sola by eliciting tentacle shriveling and/or retraction, which lead to anemones dropping ensnared sea hares.