Low rates of predation on planktonic marine invertebrate larvae

We conducted in situ observations and experiments to evaluate predation on invertebrate larvae in near-natural plankton assemblages captured in large-volume in situ corrals. In these captured assemblages, we placed known numbers of marked larvae and determined their fate after 24 h. Recovery of marked larvae averaged 99 % (+/-0.25 %, SE), enabling a thorough and direct determination of predators and predation rates. The highest predation rate observed was on bivalve veligers by the heterotrophic dinoflagellate Noctiluca scintillans (7 % lost in 24 h). Gastropod veligers experienced no predation and echinoplutei experienced only rare predation. While N. scintillans preyed on bivalve veligers in some runs, most runs yielded little or no predation. These observations suggest that larvae can encounter relatively safe assemblages. To investigate low predation, corral experiments were conducted which presented prey at near-natural and unnaturally high densities and in the presence or absence of natural background plankton. Predation observed at high prey densities decreased or disappeared at near-natural prey densities. This suggests that low encounter rates may explain some of the low predation. Predation rates also decreased in the presence of natural background plankton. Background plankton may occupy the predator's time and decrease opportunities for encounters with larvae, obscure larvae from detection or capture, or serve as substitute food. Since predation was usually low or absent in diverse corral assemblages, we recommend confirming natural relationships and predation rates for suspected predator-prey combinations before making assumptions about predator effects. It may frequently be the case that planktonic larvae suffer little or no predation by planktonic predators.