The effects of food and density on the population dynamics of Australorbis glabratus
Schroder, Gene David
Eisenberg, Robert W.
Master of Arts
The growth and fecundity of populations of Australorbis glabratus, snail vector of Schistosoma mansoni, were measured both in the laboratory and in the field to determine, in part, the effects of quality food and large container size. All experiments were carried out in large (89 gallon) plastic pools. Natural algal growth was supplemented with chopped, dried spinach. The relative effects of food and crowding on snail population dynamics were studied in the laboratory in a group of four replicated pools of the same size. One-half of the pools received 100 snails, and one-half 1000. A ten fold difference in food levels within each density was maintained throughout the 92 day experiment. A total census at two times during the experiment verified that neither food nor density affected survivorship. Snail populations receiving the high food treatment grew faster and produced more eggs than the low food treatments regardless of density. A size cross-section of 75 snails was selected from these pools, including representatives of each treatment, and placed in another pool supplied with a "superabundance" of food. Growth records collected weekly during this 55 day experiment indicate that past history has no effect on snail growth under optimal conditions. In addition, smaller, more stunted snails grew proportionately faster than their larger contemporaries. Data was also collected on snails raised in pools under field conditions which indicate the impact of insect predation on young snail populations. Densities of 100, 500, and 1000 young snails were established in nine large pools, and each received a "superabundance" of food. Two tanks of each density were supplied with screen covers to exclude possible predators. Within 23 days the snail populations of the three open pools were decimated while survivorship remained uniformly high in the covered replicates. The snail growth in the covered tanks was rapid, and a large number of small egg masses were found as early as day 15. A group of 66 large, laboratory raised snails were found capable of producing an average of 141 eggs per snail each day when cultured in a large pool and provided with ample food. The ability of Australorbis glabratus to quickly reach maturity and reproduce has been grossly underestimated in the literature. When raised in a large volume of water and supplied with abundant, high quality food, it has a generation time as short as 21 days, and a capacity to produce approximately 4200 eggs per month. In addition, insect predators have been shown to be a possible factor regulating the size of natural snail populations.