The effect of substrate on periphyton to avoid grazing by Physa integra.

Abstract

The effects of substrate were investigated as to whether certain species of algae were better able to avoid predation by a grazer, Physa integra. Rocks, reeds, and twigs were collected from a large lake in northern lower Michigan and analyzed for host algae which were quantified. Then similar substrate were placed in separate aquariums with 18 snails each. After a period of two weeks of grazing the substrates were also analyzed for algae and quantified. It was found that substrate did not influence grazing efficiency as much as periphyton size or species. Diatoms as a group were more adept at avoiding predation than other non-diatom species. C. americana obtains its nutrients solely through the tubercular tissue, and must cope with the defensive actions of the host that may isolate this structure from the Q. rubra vascular tissues. If growth of the tubercle allows the parasite to maintain its connections with its nutrient source, this may be reflected in increased availability of resources for reproduction. I investigated the relationships between tubercle volume, changes in host root circumference as a result of the parsite infection, and C. americana reproductive effort, as measured by number of capsules produced per plant and total capsule dry weight. The null hypothesis is that reproductive expenditure is independent of somatic expenditure (tubercle volume) and of the effect on host root morphology.