Tide-model analysis of the zonation of intertidal prosobranches. II. Four species of trochids (Gastropoda: Prosobranchia)

Abstract

Experiments were carried out in a tide model to determine the patterns of zonation of four species of trochids under laboratory conditions. Calliostoma zizyphinum (L.), Gibbula cinera a (L.) and G. umbilicalis (da Costa) did not show a constant pattern of zonation. Monodonta lineata (da Costa) adopted the same zonation in the model as that previously described for Littorina litorea (L.), i.e. at and just below high tide level, but this was higher than normally found on the shore. Time-lapse monitoring of the movements of the four species showed that, except for Monodonta lineata, they were influenced by the rise and fall of the water level in the tank and tended to move up and down with it. Under conditions of reduced tidal period Calliostoma zizyphinum, Gibbula cineraria, and G. umbilicalis showed, however, a pattern of zonation more like that on the shore. Movement mainly occured during periods of submersion and the number of animals moving at any one time was very low. Monodonta lineata, and to a lesser extent, Gibbula umbilicalis could compensate for the increased rate of movement of water level during the 3-h tidal period by increased rates of movement and greater levels of activity. Thus, not only does the moving water level bring about the initiation of movement of the animals, but it also governs their rate of movement. None of the four species showed any differences in their patterns of movement or distribution whether in complete darkness, or continuous light. When under continuously damp conditions Monodonta lineata adopted a lower position in the model, possibly because of the movement of a layer of water over the substratum. Negative geotaxis was less strong in Calliostoma zizyphinum and Gibbula cineraria in the absence of a moving water level. The results indicate that, with the possible exception Calliostoma zizyphinum, the physical stresses due to emersion were unlikely to be the major control of zonation. It is considered possible that differential zonation shown on the sea shore by British littorinids and trochids might be governed by the different requirements of the species for food.