Quantifying predation on planktonic larval stages of marine benthic invertebrates
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This thesis enhances our ability to develop hypotheses on effects of predation on the ecology of larval marine benthic invertebrates. I evaluated the mechanisms that influence encounter rates among predators and prey, and the potential impact of predation on larval abundance. I developed an individual-based model that facilitates the prediction of encounter rates between a motionless predator and prey that exhibit directional persistence (i.e., the tendency to maintain direction of travel over time) in an isotropic random walk. Using data from simulations, I (1) showed that common assumptions of diffusive or ballistic prey movement results in overestimates of the rate of search for prey over relevant scales of prey perception and directional persistence; and (2) evaluated the utility of published analytical models for prediction of the rate of search at long time scales. In laboratory experiments, I showed that temperature influences the motility of different larval stages of the acorn barnacle, Semibalanus balanoides, by its effect on directional persistence and swimming speed, and that larval motility was anisotropic. In the field, I found that the potential impact of predation on larval barnacles by a ctenophore, Pleurobrachia pileus, was negligible, primarily due to large differences in the relative abundance of predator and prey. A review of studies that have quantified rates of predation on larval marine benthic invertebrates and fish indicate that predation is potentially ecologically significant in certain instances, but is not always detrimental to larval populations. Observations of ingestion, digestion, and egestion of larval barnacles in the pharynx of P. pileus in the laboratory indicated that the cypris stage may avoid mortality after ingestion, by resisting digestion and inducing egestion. Larvae probably face a gauntlet of predators over their pelagic duration. We need to identify these predators and quantify their potential impacts by developing mechanistic models of the “predation process”, and making observations in the field. My thesis contributed observations to both of these objectives, and demonstrated methods (and their associated challenges) that can be used to quantify rates of search, ingestion, and prey mortality.