Chemicals released by injured prey are presumed to function as a risk signal for conspecifics, indicating potential predation threat. Many prey demonstrate plastic behavioral or morphological responses to such chemicals. However, recent research has shown that the marine snail, Littorina sitkana, reduces predator avoidance behavior when exposed to chemical cues from injured conspecific snails. When L. sitkana detects chemicals from a crab consuming and digesting conspecifics, they increase crawlout (emersion) behavior and time spent in refuge habitat compared to when no cues are present. By contrast, snails spend less time emersed when they are exposed to cues of injured conspecifics than when no cues are present.
We explored two possible explanations for this phenomenon: 1) snails cannot differentiate between unharmed and injured conspecifics, and thus the decrease in emersion behavior is a competitive response to increased conspecific density; and 2) in the absence of other cues indicating predation, snails are attracted to injured conspecifics as a food source (cannibalism). We assessed the effect of snail condition (injured vs. unharmed) and density on crawlout and feeding behavior in a mesocosm experiment. In addition, we used short video observations to determine whether snails were attracted to chemical cues from injured conspecifics.
L. sitkana reduced emersion only in response to injured conspecifics, healthy conspecifics at either low or high density did not change focal snail avoidance behavior relative to the control. Moreover, the response to injured conspecifics was graded: emersion further decreased with increasing number of injured conspecifics – a proxy for cue concentration. In response to live snails, L. sitkana demonstrated a competitive response, increasing their individual grazing rates at higher density of healthy conspecifics, while snails exposed to cues from injured conspecifics did not graze differently than snails in the control. In short term observational studies, we found no evidence that snails were actively attracted to crushed snails, compared to live snails, or a sham control. We were also unable to detect a difference in movement rates of snails in any of these three treatments.
In contrast to other species of snail assayed elsewhere, L. sitkana responds qualitatively differently to chemicals from injured conspecifics compared to other cues indicating predation risk, and in a concentration-dependent manner. This suggests possible influence of other selective pressures than predation in favoring reduced emersion in the presence of injured conspecifics, but as yet, the mechanism for such selection is unresolved.