Feeding, prey selection and prey encounter mechanisms in the heterotrophic dinoflagellate Noctiluca scintillans

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Abstract
The heterotrophic dinoflagellate Noctiluca scintillans has a negligible swimming ability and feeds predominantly on immobile prey. How, then, does it encounter prey? Noctiluca scintillans is positively buoyant and, therefore, we hypothesized that it intercepts prey particles during ascent and/or that microscale shear brings it into contact with prey. Noctiluca scintillans has a specific carbon content 1–2 orders of magnitude less than that typical for protists and, thus, an inflated volume. It also has a density slightly less than that of the ambient water and therefore ascends at high velocities (-1 m h−1). In stagnant water, clearance rates of latex spheres (5–80 μm) increased approximately with prey particle size squared. This scaling is consistent with N.scintillans being an interception feeder. However, absolute clearance rates were substantially lower than those predicted by modeling N.scintillans both as a spherical and as a cylindrical collector. The latter model assumes that prey particles are collected on the string of mucus that may form at the tip of the tentacle. Feeding, growth and prey selection experiments all demonstrated that diatoms are cleared at substantially higher rates than latex beads and other phytoplankters, particularly dinoflagellates. We propose that diatoms stick more efficiently than latex beads to the mucus of N.scintillans and that dinoflagellates reduce fatal contact behaviorally. We conclude that N.scintillans is an interception feeder and that the high ascent velocity accounts for encounters with prey. However, the flow field around the cell-mucus complex is too complicated to be described accurately by simple geometric models. Fluid shear (0.7–1.8 s−1 had a negative impact on feeding rates, which were much less than predicted by models. Noctiluca scintillans can survive starvation for long periods (>3 weeks), it can grow at low concentrations of prey (-15 μg C l−1), but growth saturates only at very high prey concentrations of 500–1000 μg C l−1 or more. We demonstrate how the functional biology of N.scintillans is consistent with its spatial and seasonal distribution, which is characterized by persistence in the plankton, blooms in association with high concentrations of diatoms, and surface accumulation during quiescent periods or exponential decline in abundance with depth during periods of turbulent mixing.

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