Immunochemical Detection of Predation on Ciliate Protists by Larvae of the Northern Anchovy (Engraulis mordax).

New approaches are needed for investigating planktonic prey-predator interactions in situ, free from enclosures and long term incubations. Lengthy incubations can lead to several artifacts, including selective mortality of planktonic organisms (1) lysis of some taxa upon fixation (2), alteration of prey-predator encounter rates through perturbations of the turbulent flow field (3) and modification of natural search and avoidance behaviors of predators and prey (4). Microscopic analysis of the gut contents of predators is a feasible alternative only if prey leave digestion-resistant hard parts. Here we describe a different approach involving immunochemical methods. We report the development and first application of an immunoassay that permits detection of predation on soft-bodied, nonloricate ciliates (Strombidium sp.). Polyclonal antibodies raised against Strombidium sp. recognize both intact ciliates and partially assimilated ciliate antigens occurring in a predator’s gut. We demonstrate, by both immunochemical and conventional methods, unequivocal predation by first-feeding larvae of the northern anchovy (Engraulis mordax) on nonloricate ciliates. The intensity of the immunochemical reaction, quantified by enzymelinked dot blots and reflection densitometry, is proportional to prey density and to the predator’s ingestion rate. Polyclonal antisera were produced in New Zealand white rabbits (5) against Strombidium sp. cultured on a diet of the bacterium Vibrio natriegens (6). Immunoglobulin G (anti-Strombidium IgG) was isolated from the antiserum by precipitation in 45% (w/v) ammonium sulfate, desalted by gel filtration, then fractionated by diethylaminoethyl ion exchange chromatography. Detailed

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