A new method for live-staining protists with DAPI and its application as a tracer of ingestion by walleye pollock ( Theragra chalcogramma (Pallas)) larvae

Abstract

The efficacy of DAPI (2,4-diamadino-6-phenylindole), a fluorescent DNA-specific stain, to live-stain protists was evaluated for use as tracers in laboratory studies of feeding by larval pollock, Theragra chalcogramma (Pallas). At 4 μg · ml −1, DAPI effectively live-stained all eight species of heterotrophic protists examined. DAPI also stained four species of autotrophic protists although staining was more variable. No toxic effects of DAPI were detected. DAPI did not appear to alter the swimming behavior of stained protists. Further, the growth rates of a ciliate, Euplotes sp. and an autotrophic flagellate, Rhodomonas salina (Wislouch) Hill and Wetherbee, exposed to DAPI, did not differ significantly from growth in controls. When DAPI-stained Euplotes sp. were presented to larval pollock, ingested cells were easily seen in larval guts. This is the first report of ingestion of an aloricate ciliate by larval pollock. The lack of toxicity of DAPI and the easy visualization of ingested DAPI-stained protists, make this method a useful new tool for examining the ingestion of protists by metazoa and other protists.