1. Many species of Polyzoa show marked specificity with regard to the substrate on which they occur. Epiphytic forms are often found mainly on one species of alga.
2. Experiments were performed in which a number of algal species were offered to polyzoan larvae as substrates for settlement. The disposition of algae, and the dishes containing them, was such that the layout conformed to a Youden Square design. This not only achieved economy of materials, but ensured a balanced experiment, made possible a statistical analysis of the results, and eliminated any possible effects of extraneous environmental factors.
3. The larvae showed marked substrate preferences when settling. In the littoral forms Alcyonidium hirsutum, A. polyoum and Flustrellidra hispida, the selection of algae accorded closely with their observed natural distributions: in each case highest settlement took place on Fucus serratus. It seems probable that positive selection plays an important role in determining the distribution of these species on the shore. Celleporella hyalina larvae were also selective, but the preferences were less clearly related to the ecological distribution of the adult.
4. Surface texture appears more important than contour as a factor influencing the choice made by larvae between algal substrates, although the physical and/or chemical factors responsible for the observed differences in attractiveness of algae are largely unknown. However, it is evident that the nature of the surface alters with age, and that this influences favourability. The presence of mucus has an adverse effect on settlement. Once the actual substrate has been chosen, the larvae respond to surface contour and, if possible, select a groove or concavity as a site for fixation.
Selection of a Site for the Ground Disposal of Radioactive Wastes, (I)