Deciphering the known unknowns in the behavioural ecology of the intertidal gastropod Littorina littorea

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A considerable amount of attention has been devoted to understand the role of chemoreception in the behavioural ecology of gastropods. There is still, however, a limited amount of information related to their sensory ecology and their subsequent ability to navigate through topographically complex landscapes, such as intertidal rocky shores, despite the documented role of substrate spatial structure in the movement rates and directionality of gastropods. Responses to gravity and visual cues have been documented in a range of intertidal gastropods, but often led to contradictory conclusions. We examined the response of the gastropod Littorina littorea, a keystone grazer on most temperate intertidal ecosystems of the northern hemisphere, to substrate slope, topographic discontinuities and light using purpose-designed experimental set-ups under laboratory-controlled conditions. In contrast to previous results, we showed that L. littorea did not exhibit a strict geotactic behaviour. Instead, they were predominantly thigmotactic following an encounter with a topographic discontinuity, and we suggest that the observed seemingly negative geotactic responses are only a by-product of thigmotaxis, i.e. a thigmotactically-induced negative geotactic response. L. littorea also showed a strong preference for dark areas and shapes. Taken together, our results suggest that thigmotaxis and scototaxis are evolutionary advantageous to navigate through topographically complex and thermally stressful environments, and that the behavioural repertoire of gastropod may be much wider than previously thought.