Vertical migration by Cerithidea decollata (Mollusca, Potamididae): Addition but not subtraction

The mangrove snail Cerithidea decollata feeds on the ground at low water, and about two hours before high water climbs Avicennia marina trunks settling 20–40 cm above the level that the incoming tide will reach. The moment that C. decollata must start to measure the vertical distance it has to creep up the trunk to avoid submersion corresponds to the point at which creeping switches from a horizontal to vertical movement (the “0” point). Two vertical movements interrupted by a short horizontal movement were added together; however, if the horizontal interruption was wider than 10 cm a new “0” point was set, and snails climbed higher than control snails, according to the level of the horizontal interruption, without adding the two vertical segments. Through forcing snails to both climb and descend, we observed that they do not appear to be capable of subtraction.