ABSTRACT
A brief review is given here of some aspects of life cycles (reproduction, growth, seasonality) and populations (dynamics and productivity). Life cycles are classified into two basic types (Calow, 1978): iteroparous (when the individual has repeated cycles of reproduction during its lifetime) and semelparous (when there is only one breeding period and parents die after reproduction). The type of life cycle is not necessarily constant for a species throughout its geographical range but can vary among populations according to local conditions. Knowledge of fluctuations in snail populations in a focus of snail-borne disease is fundamental to achieving successful control of the intermediate host. Snails are important too in the flow of energy through freshwater ecosystems; these organisms contribute a major proportion of the invertebrate biomass and produce large quantities of organic and inorganic matter (in the form of the shell). Population fluctuations in African freshwater gastropods are best known for pulmonates, but productivity has been most studied for a few of the prosobranchs.
