Ecological investigations of the invertebrate plankton predator Leptodora kindtii.

Abstract

The three chapters of this dissertation examine (1) the role that Leptodora kindtii (Focke) plays in structuring the food web of Lake Michigan, (2) the predatory interaction between Leptodora and the cladoceran predator Bythotrephes, and (3) the general diet requirements of Leptodora. In Chapter 1 I employ both microscopical techniques and field data to demonstrate that Leptodora is a key predator on the cladoceran, Bosmina, in Lake Michigan. The conclusions are based in part on substantial evidence from stomach analyses of Leptodora which reveal that Bosmina is a major diet item. Stomach analyses demonstrate that Leptodora ingests hard parts from its prey and suggest that this organism is not a strict fluid feeder as previously believed. Data also show that Leptodora exhibit a pronounced diet shift during maturation with juvenile Leptodora feeding on rotifers and Bosmina and adults feeding on Bosmina, Daphnia, and copepods. In Chapter 2 I test several hypotheses to explain the non-overlapping distributions of Leptodora and Bythotrephes in Lake Michigan. Based on evidence from experimental enclosures and field data from Lake Michigan, I argue that direct predation by Bythotrephes is a key factor controlling the distribution and abundance of Leptodora. Competition between the predators may also be important but thermal tolerance appears to have no influence on Leptodora's distributional pattern. In Chapter 3, components of the feeding process are quantified with laboratory experiments using radiolabelled cladoceran prey in order to develop a mechanistic basis for the pattern of diet shift described in Chapter 1. Results indicate that the size and profitability of prey are key factors that account for the diet shift. I argue that the growth strategy of Leptodora has evolved largely in response to selective pressures that maximize its ability to utilize prey.