SPATIAL INTERACTION BETWEEN PARASITE AND HOST WITHIN THE BIOMPHALARIA-GLABRATA SCHISTOSOMA-MANSONI SYSTEM - INFLUENCE OF HOST SIZE AT INFECTION TIME

Host-parasite systems such as mollusc-trematode associations are chara cterized by a high value of the parasite/host mass ratio. The developm ent of larval trematode requires, not only energy, but also space for the integration of the large volume occupied by the infrapopulation of sporocysts or rediae. The spatial constraint imposed by the developme nt of the trematode Schistosoma mansoni on the growth of the digestive gland of the fresh water pulmonate snail, Biomphalaria glabrata, for different sizes of the host at time of infection was studied. Whatever the host-parasite combination examined,the volume of the digestive gl and of the snail is reduced compared with that of controls of similar age. This reduction is more pronounced when snails are infected as juv enile vs adults. Comparison of the volume of the parasite-digestive gl and complex of infected snails with that of the digestive gland of non infected snails of some size showed that the reduction of the volume of the digestive gland results mainly from the limited growth of the h ost when snails are infected as juveniles, and of a competition for sp ace between the parasite and the digestive gland when snails are infec ted as adults. Two patterns of spatial integration of the parasite vol ume within the snail host are proposed (by ''addition'' vs ''substitut ion'') in relation to the size of the host at infection time. The cons equences of this spatial constraint which limits the growth rate of th e digestive gland are discussed relatively to the nutrition and the fe eding behaviour of infected snails.