Host reproductive fitness: the influence of increasing parasite pressure in aBiomphalaria glabrata/Schistosoma mansonisystem

Summary Host life history traits are often shaped by trade-offs between the current and potential future costs of parasitism. Reproductive tissues are not normally essential for host survival and diversion of resources elsewhere is a common effect of parasitic infection. Variations in reproductive output may therefore indicate overall fitness correlated to the host response to parasite pressure. Here, we investigated reproductive fitness in a Biomphalaria glabrata—Schistosoma mansoni system, a laboratory model for schistosomiasis. Five matched groups of unselected B. glabrata snails were individually exposed to doses of 1, 2, 5, 10 or 20 S. mansoni miracidia, respectively. A sixth group remained unexposed providing a control. Fertility (defined as actual reproductive performance, measured as the number of offspring produced) and fecundity (defined as potential reproductive capacity, measured as number of eggs and embryos formed) were monitored for each group at weekly intervals. Our results revealed that both parasite dose and infection status had a significant effect on the potential reproductive capacity of the host, but this was not always reflected in the actual reproductive success. Egg mass production showed a negative association with increasing parasite dose in patently infected snails. In contrast, snails exposed to the parasite, but within which infection did not establish, demonstrated a positive association between egg mass production and parasite dose. This suggests the existence of a fecundity compensation mechanism occurring within the post-patent period of infection. This is, to our knowledge, the first report of such an effect in a snail-trematode system and, indeed, in any host-parasite association.