| Predation is one possible source of mortality for planktonic marine invertebrate larvae. Using a combination of laboratory and in situ experiments, I investigated predation for a variety of predator-prey combinations. Laboratory experiments manipulated prey densities and the presence of background plankton. Experiments examined predation on echinoid, cirriped, or bivalve larvae by 8 predators. Predation was dramatically reduced when prey were presented at natural prey densities and with background plankton.; In situ experiments with natural plankton assemblages captured in corrals lasted 24 h. Echinoid, bivalve, and gastropod larvae were marked with calcein and known numbers were added to corrals. Potential predators included a heterotrophic dinoflagellate, a variety of large crustaceans (e.g., copepods, amphipods, and zoeae), various polychaetes, chaetognaths, fish, ctenophores, and hydromedusae. Recovery of marked larvae, whether unconsumed or in predator guts, was usually 100%. No predation was observed in 5 of the 10 sets of observations. In two experiments where predation was observed, the dinoflagellate Noctiluca scintillans produced instantaneous mortalities of {dollar}-{dollar}0.035 and {dollar}-{dollar}0.07 day{dollar}sp{lcub}-1{rcub}{dollar} in marked bivalve veliger populations. Other predators which consumed veliger larvae include a spionid metatrochophore (1 veliger consumed), a juvenile threespine stickleback Gasterosteus aculeatus (1 veliger consumed), and the hydromedusae of Proboscidactyla flavicirrata (3 veligers consumed). Only one echinoid pluteus larva and no marked gastropod veligers were consumed in 10 observational experiments.; Low predation rates may result from low predator-prey encounters. However, model estimates of encounter rates within the corrals indicate that numerous predator-prey encounters should have occurred. One explanation for low predation in spite of encounter estimates is the potential influence of background plankton on predation. Background plankton may serve as substitute food or reduce encounters. Corral manipulations of prey density and background plankton support laboratory findings that natural prey densities and background plankton can reduce or eliminate predation. While N. scintillans was a significant predator in 2 of 10 experiments, the majority of observations showed that veliger and pluteus larvae suffered little or no predation. Thus, planktonic predation may not always be a major source of larval mortality. |
