RESUMO
We investigated predation on bivalve veligers by the scyphozoan Chrysaora quinquecirrha and the ctenophore Mnemiopsis leidyi. We found that the medusa stage of C. quinquecirrha captures, but does not digest, veliger larvae: 99% of oyster veligers (Crassostrea virginica) caught by medusae were egested alive within 7 h of capture, and 98% survived for 24 h after egestion; 98% of oyster, mussel (Mytilus edulis), and clam (Mulinia lateralis) veligers placed on the oral arms of medusae were rejected; all bivalve veligers in field-collected medusae were closed and full of tissue. Our laboratory evidence suggests that the shell of larval bivalves probably offers protection from medusae: 23% of dead, open veligers were ingested by medusae compared with 0.7% of live, closed veligers; open veligers were retained longer than closed veligers; and tissue excised from recently settled oyster larvae was ingested and digested. Freeswimming C. quinquecirrha ephyrae ingested but did not digest veligers. By contrast, the benthic scyphistoma stage ingested 69% of veligers that contacted their tentacles and digested 48% of those ingested. Each scyphistoma consumed an average of 1 veliger/day at densities of 0.3 veligers ml-1. However, larval settlement was not reduced on oyster shells bearing scyphistomae. By contrast to the results on C. quinquecirrha, ctenophores egested only 4% of veligers alive, and 25% of the veligers in their gut contents were digested. Predation on veligers by ctenophores was estimated to be 0.2 to 1.7%/day in Chesapeake Bay. We conclude that C. quinquecirrha medusae are not important predators of bivalve veligers, but rather may reduce their mortality by consuming ctenophores, which do eat veligers.
