The Feeding Ecology of Actinophrys sol (Sarcodina: Heliozoa) in Chesapeake Bay

The occurrence of Actinophrys sol, a planktonic heliozoan, in Chesapeake Bay was monitored over a four-year period (1988-1991). Actinophrys sol was widely-distributed throughout Chesapeake Bay and could exceed densities of 5,000 cells liter 1 . It was most abundant during the warmer months. Feeding experiments were conducted with field populations of heliozoa using 1-μm fluorescent microspheres to label ciliate prey. Two ciliates, a small Strobilidium sp. (30 μm in diameter) and a Pleuronema sp. (45 μm length), were the primary ciliate-prey items in the water column when the experiments were conducted, although a wide range of ciliate taxa was ingested. Two other ciliates not present in situ, a Cyclidium sp. (20 μm length) and a Uronema sp. (40 μm length), were also labeled and added at various concentrations to field populations of plankton containing A. sol. Heliozoan ingestion rates on in situ prey at concentrations of 30 Strobilidium and one Pleuronema ml were 0.2 to 0.3 prey heliozoan ' hour 1 . Ingestion rates increased to a maximum of 1.2 prey heliozoan -1 hour -1 with additions of 100 Uronema ml '. A mean clearance rate of 0.15 ml heliozoan -1 day -1 did not change with increasing prey abundance. The abundance and distribution of A. sol suggests that these sarcodines may exert strong grazing pressure on the planktonic ciliate populations of Chesapeake Bay at certain times of the year, and may be important in shaping the ciliate community composition and distribution.