An evaluation of the interactions between freshwater pulmonate snail hosts of human schistosomes and macrophytes

Abstract

An account is given of a laboratory investigation designed to evaluate the extent to which the freshwater pulmonate snail Biomphalaria glabrata (Say) can utilize various species of aquatic plants, mainly macrophytes, when presented in the following forms over different time scales: (i) normal plants; (ii) dried plant material; (iii) homogenized plant material in calcium alginate matrices; (iv) water-soluble filtrates of plant homogenates in the medium. The following propositions, derived from the theory of phased coevolution of components of the module consisting of the epiphytic bacteria, algae, snails and macrophytes, are evaluated on the basis of the present results and others including those obtained in this laboratory, (i) That as the snails had become specialized to exploit surface communities of epiphytic algae, decaying plant material and dissolved organic matter (DOM) early in their evolutionary history they would continue to exploit these resources when they later become associated with aquatic macrophytes, (ii) That pulmonate snails would tend to be feeding generalists capable of adapting to food of varying chemical composition, given sufficient time, provided it was sufficiently small or flaccid, (iii) That although macrophytes and snails show a strong positive relationship, the living macrophyte tissue would be little used by the snails, (iv) That the hard outer envelope, inherited from their terrestrial ancestors, would remain as the major defence mechanism of aquatic macrophytes against attack by snails and other aquatic invertebrates, (v) That aquatic macrophytes would invest little in the nutrient deficiency strategy to reduce attack by invertebrates such as snails, (vi) That truly aquatic submerged macrophytes would not possess secondary plant compounds (SPC) that would be molluscicidal. (vii) Emergent parts of sub- aquatic or aquatic plants might be expected to be better sources of SPC with molluscicidal factors than submerged aquatic plants, (viii) Species of epiphytic or planktonic algae might be better sources of SPC with molluscicidal effects than aquatic macrophytes, (ix) That the strategies developed by pulmonate snails for obtaining their energy supplies would not be conducive to rapid speciation. The analysis of the present and other related results supports these propositions. Predictions based on the theory of mutualism involving the snails, macrophytes and other components of the module also receive some support from an analysis of the present results. The additional empirical work that could be undertaken to test this theory is briefly discussed. Possible reasons are given for the differences between the nature of the interactions involving herbivores and plants in terrestrial and freshwater ecosystems.