Influence of Natural Food Items on Growth and Egg Production in Brackish Water Populations of Lymnea Peregra and Theodoxus fluviatilis (Mollusca)

Abstract

Natural food choice was examined by faecal pellet analysis. In Theodoxus fluviatilis (L.) diatoms dominated throughout the year; in Lymnea peregra (Muller) they dominated in the spring but were mixed with filamentous algae during the rest of the year. In feeding experiments food items collected directly from the habitat were used, as well as an artificial power food (spinach). Diatoms seemed to constitute a suitable food item for juveniles of both species. Bluegreen algae, however, caused differential growth in juveniles of the two species. Adult L. peregra grew best on bluegreen algae, while diatoms gave better juvenile growth. Spinach consumed by adult L. peregra caused a fairly poor growth but gave a very high number of eggs. Although growth occurred on the naturally occurring diet, maximum fecundity was not reached in the field. This may be a clue to the regulation of natural populations of L. peregra.