Functional and ecological aspects of the mucus trails of the intertidal prosobranch gastropod Littorina littorea

It has been suggested that some gastropods might utilise pedal mucus trails as a source of nutrition. Despite much work, there has been little attention on the ecological consequences of such a strategy. Here we aimed at determining the value of mucus trails to snails by investigating (1) snail reaction to naturally aged conspecific mucus trails and (2) the effect of starvation on trail- following behaviour. The mucus trails of Littorina littorea (L.) adhered microalgae in the laboratory but this ability began to diminish after the trails had been exposed on-shore for >1 tidal cycle. Nat- ural floral components of trails were greatest for up to 4 tidal cycles of exposure and then began to diminish. If trails are to be used for foraging purposes, snails would gain most benefit by utilising them while relatively fresh. Correspondingly, snails readily responded to fresh mucus trails, but after 20 tidal cycles only 3 of 24 did so. As the mucus was still present we conclude that some unknown cue is present enabling snails to discriminate trails by their age. Despite previous work to the contrary, no evidence of preference for following trails in a particular direction was found and we conclude that trail following in our snails was not related to mate searching or the formation of aggregations. The starvation level of snails (up to 4 d) did not affect their foraging behaviour in the laboratory. However, snails followed the trails of starved snails for significantly shorter distances than those of fed snails, suggesting a coding of starvation level in the trail mucus. Choices about whether to trail follow may be based on how well fed a conspecific is and how old a trail is —following a fresh trail would increase the likelihood of encountering a conspecific and may have a nutritional benefit if the trail is to be grazed. Previous work on trail following should be re-evaluated in the light of these findings.

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