Growth and grazing rates of the heterotrophic dinoflagellate Polykrikos kofoidii on red-tide and toxic dinoflagellates

We investigated growth rates, grazing rates, and prey selection of Polykrik os kofoidii when feeding on several species of red-tide and/or toxic dinofl agellates. Polykrikos kofoidii ingested all prey species used in this study , exhibiting positive growth on Lingulodinium polyedrum, Scrippsiella troch oidea, Ceratium furca, Gymnodinium catenatum, Gyrodinium impudicum, Proroce ntrum micans, and the toxic dinoflagellate Amphidinium carterae, but not a P. minimum. Specific growth rates of P. kofoidii increased rapidly with inc reasing density of L. polyedrum. S. trochoidea, C. furca, and G. catenatum before saturating between 500-2,000 ng C ml(-1). Specific growth rates incr eased continuously when P. Kafoidii was fed the other prey species. Maximum specific growth rates of P. kofoidii on G catenatum (1.12 d(-1)) , S. troc hoidea (0.97 d(-1)) and L. polyedrum (0.83 d(-1)) were higher than those on C. furca (0.35 d(-1)) A. carterae (0.10 d(-1)) P. micans (0.06 d(-1)) G. i mpudicum (0.06 d(-1)) and P. minimum (-0.03 d(-1)). Threshold prey concentr ations (where net growth = 0) were 54-288 ng C ml(-1). Maximum ingestion an d clearance rates of P. kofoidii on these dinoflagellates were 5-24 ng C ps eudocolon(-1)d(-1) and 1.0-5.9 mul pseudocolony(-1)h(-1). respectively. Pol ykrikos kofoidii strongly selected L. polyedrum over S trochoidea in prey m ixtures. Polykrikos kofoidii exhibited higher maximum growth, indigestion, and clearance rates than previously reported for the mixotrophic dinoflagel late Fragilidium cf. mexicanum or the heterotrophic dinoflagellates Protope ridinium cf. divergens and P. crassipes, when grown on th same prey species . Grazing coefficients calculated by combining field data on abundances of Polykrikos spp. and co-occurring red-tide dinoflagellate prey with laborato ry data on ingestion rates obtained in the present study suggest that Polyk rikos spp. sometimes have a considerable grazing impact on prey populations .