HOST CHOICE BY LARVAL PARASITES - A STUDY OF BIOMPHALARIA-GLABRATA SNAILS AND SCHISTOSOMA-MANSONI MIRACIDIA RELATED TO HOST SIZE

Within snail/trematode associations the age/size of the host at infect ion has consequences with regard to miracidial infection success, furt her intramolluscan parasite development and reproduction, and the host response, mainly in terms of growth and reproductive effort. Taking i nto account these differences, we were interested in determining wheth er miracidia could discriminate and make a choice between snails of di fferent sizes. Using the Schistosoma mansoni/Biomphalaria glabrata sys tem, we compared data on the snail infection rate and the mother sporo cyst abundance among three size classes of snails (juvenile, subadult, and adult) exposed separately or together to the parasite larvae. Whe n exposed individually, juvenile snails (3-5 mm) had significantly hig her prevalence and abundance values than did subadult snails, followed by adult snails. In contrast, when snails of the three size classes w ere exposed together in heterogeneous size groups the prevalence and a bundance values were always significantly higher for subadult snails o f the 7- to 9-mm class than for juvenile and adult snails. A host choi ce experiment confirmed that significantly more miracidia were attract ed by subadult snails, suggesting that the parasite has been selected for specific locating and recognition mechanisms increasing the infect ion rate of subadult snails when the latter have been exposed in a het erogeneous size group. Selective forces that may be responsible for su ch a preferential infectivity of the parasite vis-g-vis particular hos t age/size class are discussed in relation to host resources and host responses.