ALLEN, Jonathan D*; MCALISTER, Justin S; Bowdoin College; University of North Carolina at Chapel Hill: Testing rates of planktonic versus benthic predation in the field

Predation is thought to be the most important source of embryonic and larval mortality for benthic marine invertebrates. However, few estimates of larval mortality exist and more precise estimates of larval mortality in the field are needed to improve our understanding of the evolution of complex life-cycles. Despite the importance of this information, field studies of mortality have been limited, with few comparisons across species or habitats. This has prevented comparison of the relative risks of development in different habitats. The planktonic environment is assumed to be a safe harbor for the developmental stages of marine invertebrates with rates of predation that are lower than on the benthos. However, no data currently exist comparing predation rates on equivalent developmental stages in these habitats. To determine relative risks of predation for eggs, embryos and larvae, we measured loss rates of agarose baits flavored with the eggs of the sand dollar Dendraster excentricus in planktonic and benthic habitats in Parks Bay, Washington. In addition, we measured loss rates on live, tethered megalopae of the crab, Cancer magister in the same habitats. For both agarose baits and tethered megalopae, predation rates were lower in planktonic than benthic habitats. Predation rates on tethered megalopae were three times greater on the benthos than in the plankton during the day, and loss rates were 18 times greater for benthic megalopae than for planktonic megalopae at night. These results suggest that diel migrations of larvae may significantly reduce mortality rates due to predation. These experiments also confirm a basic, yet untested, assumption of life-history theory, that rates of predation on developmental stages of marine invertebrates are greater on the benthos than in the plankton.