PARASITE INFLUENCES ON HOST LIFE-HISTORY - ECHINOSTOMA REVOLUTUM PARASITISM OF LYMNAEA ELODES SNAILS

Using field surveys and experimental infections, we investigated the i nfluence of a trematode parasite on life history traits of adult Lymna ea elodes snails. We found that parasitism significantly affected the growth, fecundity, and survival of host snails. Within five of the six natural L. elodes populations we sampled, shell length of echinostome -infected hosts was significantly greater than for uninfected conspeci fics. Furthermore, we show that gigantism occurs among experimentally infected snails due to an accelerated growth rate and size-selective m ortality following an Echinostoma revolutum infection. The fecundity o f infected snails sharply decreased beginning at 3 weeks post exposure (PE) and all egg production eventually ceased for most hosts by 5-6 w eeks PE. Energy constraints, imposed by parasite development, alter th e host energy budget. Early in the infection, parasite depletion of ho st energy reserves reduces host reproduction, but sufficient resources remain to allow accelerated host growth. Mortality was increased amon g host snails at two distinct stages: shortly after exposure and sever al weeks after cercariae were first released. We did not observe tissu e degradation in snails during the first 4 weeks after exposure to the parasite, but destruction of host tissues was noted among snails dyin g later in the infection.