Response of invertebrate larvae to the presence of the ctenophore Bolinopsis infundibulum, a potential predator

Larval loss of marine benthic invertebrates while in the plankton is extremely high, and although several sources of mortality have been proposed, the relative significance of each remains unknown. Despite suggestions that predation is a significant source of mortality, experimental manipulations using natural densities of prey have measured low predation rates. In part, structural, chemical and behavioural larval defenses may lead to these low rates. In the laboratory, we examined changes in the vertical distribution of echinoplutei (4- and 6-arm developmental stages) of Strongylocentrotus droebachiensis (Mueller) and veligers of Ostrea edulis (L.) in the presence of the ctenophore Bolinopsis infundibulum (Mueller), a known larval predator. Larvae were introduced 2-3 cm above the bottom of experimental plexiglas rectangular columns (60 × 10 × 10 cm). The ctenophores were held in baskets (10 × 10 × 10 cm), made of 0.63-μm Nitex mesh, which hung in the water at the top of the experimental columns. Empty baskets were used in “No predator” treatments. Echinoplutei of both stages, and reared under both high and low food rations, showed a strong response and were found lower in the columns in the presence than the absence of ctenophores. The population mean height above the bottom of the experimental columns was 12.2-15.2 and 9.4-16.2 cm's lower in the presence than in the absence of predators (for 4- and 6-arm larvae, respectively). Interestingly, there was little difference in the vertical distribution of veligers between the two treatments, suggesting that a behavioural defense to predation may not have evolved in all phyla. A behavioural avoidance of predators, such as the one shown by echinoplutei in our study, may reduce larval mortality and increase the probability for eventual recruitment to the benthos.