The combined effects of population density and infection by Schistosoma mansoni Sambon on the growth, reproduction, and mortality of Australorbis glabratus say

Abstract

Controlled experiments were conducted to attempt to measure the effects of varied population density and degree of parasitic infection on A. glabratus. The original progenitors of the snails and parasites used were obtained from the University of Michigan through the National Institutes of Health, Bethesda, Maryland. Six hundred seventy-five snails ranging in size from 2.5 to 10.4 mm in diameter were used to establish three experimental groups. Each group was composed of three populations containing 150, 50, and 25 snails. Group N (normal) was maintained as a control. Group A snails were each exposed to 10 miracidia and Group B snails to 40 miracidia. Measurements of shell diameters, egg clutch counts, and egg counts were made at 10-day intervals over a period of 90 days. Snails in the larger control populations grew at a slower rate and laid fewer egg clutches than those in smaller populations. The introduction of S. mansoni compounded the adverse effects of crowding, but to a lesser degree in populations with 40 miracidia per snail. The over-all growth rate of each population in Group A was less than in the corresponding control population, but growth in smaller populations was suppressed below that of the larger ones. This reverse effect was not seen in the populations in Group B. It was postulated that there exists a parasite load threshold which was exceeded by the exposure to 40 miracidia. In other words, the same mechanisms which adversely affect snails in crowded environments exist also in the parasite's environment. Snail mortality was not the result of crowding, but of parasitic infection and increased with the increased parasite load. The highest mortality rates were observed beginning w.th day 40 postinfection in Groups A and B which coincided with the beginning of cercarial emergence. There was no complete cessation of egg laying in infected snails, but there were fewer egg clutches and fewer eggs per clutch laid by infected snails. Egg production was reduced in more dense populations in all groups.