Molecular identification of alarm cues in the defensive secretions of the sea hare Aplysia californica

Abstract

Prey species possess numerous strategies to reduce predation. One tactic is to respond with antipredator behaviours when conspecific alarm cues are detected. The sea hare Aplysia californica defends itself from predators in many ways, one of which is releasing ink and opaline upon attack. Previous work showed that a mixture of ink and opaline from A. californica causes conspecifics to respond with antipredator behaviours such as moving away and/or ‘galloping’. We examined the specificity of the alarm response, including identifying the molecules mediating it. Either ink or opaline alone evokes the full alarm response, but conspecific mucus, conspecific haemolymph, odour from predatory spiny lobsters, or odour from algal food do not. Thus, the defensive secretions, ink and opaline, specifically act as alarm cues to nearby conspecifics. We isolated and identified the alarm cues in ink as the base uracil and the nucleosides uridine and cytidine. Each of these molecules individually elicits frequencies of alarm behaviours as great as ink. Ink without its alarm cue molecules does not elicit a significant frequency of alarm behaviours. Thus, these three molecules together are necessary and sufficient to produce alarm responses. Aplysia californica antipredator behaviours are also elicited by ink from the congener Aplysia juliana or Aplysia dactylomela. Furthermore, ink from the squid Lolliguncula brevis or the octopus Octopus bimaculoides also elicits antipredator behaviours by A. californica, owing to the presence of uracil and uridine. Thus, these alarm cues may be common among inking molluscs.