Chilled but potent: validating the ability of frozen alarm cues to induce antipredator defences in a freshwater gastropod

Abstract

Damage-released chemical alarm cues released from injured prey serve as an important component of predator–prey interactions in aquatic systems. Other prey can innately recognize these cues, allowing them to reliably estimate the level of risk present in a habitat. Unsurprisingly, researchers widely apply alarm cues across aquatic taxa to study antipredator responses of prey organisms. As alarm cues degrade quickly, scientists often freeze them to retain efficacy over extended periods. While many past studies have successfully used frozen cues to induce antipredator defences, uncertainty remains regarding whether these responses match the magnitude of those elicited by fresh cues. Only rarely have empirical comparisons been conducted between these cues, and in the few instances that have occurred, researchers have primarily assessed short-term behavioural responses. While incorrect short-term responses incur low costs and may thus be triggered by inaccurate cues, the high costs of long-term morphological plasticity should select for greater cue scrutiny. Thus, we comparatively assess both behavioural and morphological responses to these cues and do so for the first time in an invertebrate species, the gastropod Physella acuta. We exposed individuals one time (for behavioural trials) or lifelong (for morphological trials) to either fresh conspecific alarm cues, previously frozen conspecific alarm cues, a fresh water control as well as a previously frozen water control. We then assessed their crawl-out behaviour or their shell thickness, both established antipredator responses in this model system. In line with previous research, exposure to alarm cues induces faster crawl-out behaviour and thicker shells compared to the water control. Furthermore, previously frozen and fresh alarm cues induce comparable responses, suggesting that at least in P. acuta, continuing this convenient practice that also reduces donor animal use does not seem to have unintended consequences.