Factors Influencing the Choice of Substratum in Cumella vulgaris Hart (Crustacea, Cumacea) 1

Dried sand which is put back into water is rejected at first by Cumella vulgaris. However, attractiveness of the dried sand increases with the time of immersion in water, the regaining of attractiveness proceeding more slowly in sand that had been dried in an oven at 55°C than in sand dried at room temperature. In nature C. vulgaris prefers substrata with a median grain diameter of less than 160 µ , and in the intertidal zone established populations occur predominantly below the 5 ft. level. In sand predominantly finer than 150 µ and in mud C. vulgaris feeds as a “deposit‐feeder”, while in sand predominantly coarser than 150 µ . it feeds as an “epistrate‐feeder”, that is, by scraping food off the surface of individual grains. If fractions finer than 150 µ are thoroughly sieved, C. vulgaris cannot employ the latter feeding habit because the sand grains are too small for efficient individual handling by the mouth parts. On the other hand, deposit‐feeding cannot be employed either, since the fine organic debris which in this instance serves as the source of food, has been removed by the process of sieving. Consequently, the attractiveness of the material is lowered markedly. However, if a sieving method is used in which the fractions finer than 150 µ retain their content of organic debris, the material also retains its attractiveness. Sand grains coarser than 0.5 mm are too large to be handled in an efficient manner by the mouth parts of C . vulnaris. The degree of attractiveness to C. vulgaris of a certain substratum must depend on the amount and the kind of food available. The food content seems to be one of the most powerful stimuli to which sand‐inhabiting animals react in choosing substratum.