Rhythmic vertical migration of the gastropod Cerithidea decollata in a Kenyan mangrove forest

In Mida Creek, Kenya (3 degrees 20'S, 40 degrees 5'E), at high water, the snail Cerithidea decollata dwells on the trunks of mangrove trees (Avicennia marina), while during low water it migrates to the ground, foraging at various distances from the trunk, where it aggregates again well before the incoming tide. Snails from the upper shore level are 150-200 m distant from those living at the lower shore level and they cluster at lower heights on trunks. In any case, sufficient height is usually attained to avoid being submersed. An experiment was designed (February and October 2005), exchanging individuals from different shore levels subject to different tide regimes, in order to test whether snails rely on internal information or on external, direct cues, to adapt their behaviour to local conditions. Results show that C. decollata mostly rely on internal information, presumably based on an internal clock. When individuals from upper and lower shore levels were exchanged, their internal clocks continued to govern when to ascend the home trunk and how high to climb for five to six successive tides, after which the behaviour was reset to the new local conditions.