Cerithidea decollata: a snail that can foresee the future?

Individuals of Cerithidea decollata periodically migrate along mangrove trees, resting on the trunks during high water, and foraging on the surrounding muddy platform during low water. Groups, ranging from tens to hundreds of snails, aggregate on the trunks well before the incoming tide (1–2 h) at a level that will not be reached by the high tide, that is, from a few centimetres (around neap tides) to over 1 m (around spring tides) above the ground. We addressed two questions: (1) how can snails determine when to stop climbing and (2) how do snails determine the length of the path necessary to stop at that level. We used plastic pipes, 2 m long and 20 cm in diameter, along which snails crawled just as on their own trees. Experiments in which the pipes were lowered, raised or tilted indicated that snails were able to perform both tasks. Overloading the snails with lead weights showed that the estimate of the required travelling distance was probably related to the energy used to cover that distance. However, it was not possible to define how snails were able to establish the distance they needed to travel to avoid the incoming tide, although we were able to exclude any role of for visual cues or chemical marks present on substratum.