Discrimination, previous experience and substrate selection by the amphipod Corophium.

The experiments reported below were undertaken to give more information about why animals prefer one habitat or substrate to another. They are a continuation of work already reported on substrate selection by Corophium volutator (Meadows, 1964 a, b, c). Consider an experiment in which animals may choose between two substrates, A and B, and in which at equilibrium 75 % are found in A and 25 % in B. A is the animal's typical habitat or substrate (mud or muddy sand in the case of Corophium volutator) and B is an atypical substrate (coarser sand, for instance) in which animals are not normally found. The 25 % in B would then be defined as being less discriminating in their choice than those found in A. Let the animal be a species that is mobile and can move at will between A and B; then if the ratio 75/25 is caused only by differences between substrate A and B, and if the experimental animals are an unbiased sample from the population as a whole, either (i) 75 % of the population always prefer A and 25 % B, or (ii) all animals in the population prefer A for 75 % of their time and B for 25%. (i) and (ii) may be differentiated by offering animals a choice of A and B, and then re-offering the AjB choice to the animals which chose A and to the animals which chose B. The experiment is described below (Table 2). Another approach is to ask whether animals can be conditioned into preferring B substrates by keeping them in B for some while before giving them an A\B choice. Mechanisms of this kind might be influencing substrate preferences in the field, for on occasion populations of Corophium volutator are found in atypical B substrates. The hypothesis was tested by experiment (Table 3). However, before the results of these experiments can be assessed, it is necessary to know how repeatable the ratios in A\B choice experiments are, and what factors influence them. The first series of experiments (Table 1) were designed to answer these questions.