Appropriate experimental design to evaluate preferences for microhabitat: an example of preferences by species of microgastropods.

The intertidal microgastropods Eatoniella atropurpurea, Amphithalamus incidata and Eatonina rubrilabiata are principally found at mid- to low-shore levels in coralline turf and in patches of sediment close to algal turf on rocky shores. All three species are more abundant in coralline turf (i.e. algal beds composed primarily of Corallina officinalis Linnaeus, often containing patches of sediment) than in sediment, although the latter two are also quite abundant in patches of sediment. Eatoniella atropurpurea seem to live on branches of coralline algae, whereas Eatonina rubrilabiata and A. incidata seem to inhabit the sediment. In this study, we investigated whether or not behaviour of these animals accounts for the patterns observed in the field. Particularly, this study deals with choices amongst three microhabitats (coralline algae, coralline algae plus sediment and sediment) by these three species of microgastropods. Laboratory experiments were designed to examine preferences. The designs of such experiments are complex because demonstrating preference necessitates demonstrating different behaviour when confronted with choices from that showed when no choice is available. This study describes an appropriate series of hypotheses about preferences and experimental designs to test them. Preferences were indicated by differences in occupancy of the microhabitats when presented with a mixture and when each type is presented alone. In such experiments, snails did show preferences, except for A. incidata which did not show a pattern. Thus, the behaviour could, at least partially, account for the patterns of distribution in the field. The designs of experiments used here may be of value in other studies of preferences among habitats.