Redial populations of Echinostoma revolutum developing in snails of different sizes.

Populations of Echinostoma revolutum rediae were examined in snails (Stagnicola pailstris) that were 2 to 20 mm long when exposed to either one or five miracidia. In both types of infection, the number of mother rediae was proportional to the size of the snail, and the populations were larger when snails were exposed to five miracidia. The number of daughter rediae produced in snails of equal size when infected, and having different rates of growth, indicated that reproduction in mother rediae (as in sporocysts) is adjusted to the growth-rate of the snail. Studies on larval trematode populations in naturally infected snails have shown that the number of cercaria-producing rediae or sporocysts is proportionate to the size of the snail, i.e., large snails of a given species contain more rediae than do smaller ones of the same species (Wisniewski, 1937; Cort, Ameel and Van der Woude, 1948). A large species of snail produces a greater number of rediae or sporocysts of a given species than does a smaller one, even though the frequency of infection is approximately the same (Mathies and Cort, 1956; Cort, Hussey and Ameel, 1957). These observations strongly suggest that the size of the larval population is determined primarily by the space and nutrients available within the snail. However, since observations were not made on the early larval stages (mother sporocysts, mother rediae), it is uncertain whether the larger populations were derived from larger numbers of first generation larvae, an extra generation of larvae, an increase in reproductive rate, or a prolongation of reproduction in one or more generations. In the present study an attempt was made to determine the stage or stages of larval development at which the production of rediae may be influenced by the size of the snail host. MATERIALS AND METHODS Snails (Stagnicola palustris) infected with Echinostoma revolutum were obtained from ponds near Received for publication 23 June 1967. * Present address: Department of Biology, St. Olaf College, Northfield, Minnesota. From a dissertation submitted to the Graduate School of Tulane University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. The study was supported in part by Grant 5-TI-AI-02 from the National Institutes of Health, U. S. Public Health Service. Northfield in southeastern Minnesota. Laboratoryreared progeny were exposed to cercariae to obtain metacercariae for infecting pigeons which in turn provided eggs that were washed from the feces or teased from adult worms. Miracidia were obtained by incubating the eggs in aerated battery jars or in shallow dishes. Hatching was induced by exposing the eggs to bright light. Miracidia were transferred to 30-ml vials in which snails (S. palustris) of various sizes were exposed individually to the desired number of miracidia. All snails were maintained at 20 to 25 C. Snail size, measured with a 0.1 mm interval caliper, was recorded as length in mm from tip of spire to base of body whorl. The width (diameter) of the shell was also recorded but was found to be of little use in the analysis of data. Rediae were fixed in hot 10% formalin and measured with an ocular micrometer.