EFFECT OF PARASITISM ON THE GROWTH-RATE OF THE PULMONATE SNAIL HELISOMA-ANCEPS

The effect of trematode parasitism on the growth rate of the pulmonate snail Helisoma anceps was investigated using a multiple mark-recapture protocol. Snails larger than 4 mm in shell diameter grew according to a logistic growth model. In uninfected snails the specific growth rate differed significantly with the cohort age. In the summer, growth rates of prereproductive snails were faster than those of reproductive snails. During fall and spring, when snails were sexually mature, all snails grew at a similar but very slow rate. In parasitized snails, the growth rate was similar to or lower than that of uninfected snails. There was no indication of gigantism in H. anceps following parasitic castration. It is argued that the phenomenon of gigantism as seen in laboratory studies might be a consequence of the ad libitum feeding protocols normally used in these experiments. Under field conditions, however, food is limited most of the time and thus becomes an important factor in the life history of the snails.