Migratory behaviour of the mangrove gastropod Cerithidea decollata under unfamiliar conditions

The mangrove gastropod Cerithidea decollata feeds on the ground at low tide and climbs trunks 2–3 h before the arrival of water, settling about 40 cm above the level that the incoming tide will reach at High Water (between 0, at Neap Tide, and 80 cm, at Spring Tide). Biological clocks can explain how snails can foresee the time of the incoming tide, but local environmental signals that are able to inform the snails how high the incoming tide will be are likely to exist. To identify the nature of these possible signals, snails were translocated to three sites within the Mida Creek (Kenya), 0.3–3 km away from the site of snail collection. The study sites had a much wider tidal range than the original site (up to 160 cm), were dominated by Rhizophora mucronata trees and uninhabited by C. decollata. If cueing signals were linked to the upper mangrove belt (site-specific signals), the Avicennia marina area, translocation should affect snail behaviour; conversely, we could conclude that information on the height of the incoming tide should be widely available within the whole creek (widespread signals), independently from cues linked to the home area and home site. Tests were performed by releasing the snails close to vertical plastic pipes (2 m high) following a standardized procedure. Results revealed that snails, even in such unfamiliar surroundings, climbed higher and earlier on pipes where the tide level would be higher, thus allowing rejection of the hypothesis of site specific signals. Where the tide exceeded 80 cm, however, snail effort was not sufficient to avoid submersion and snails had to climb higher. Hypotheses on widespread signals possibly involved in modulating the migratory behaviour of C. decollata are discussed.