Daphnid morphology deters fish predators.

Spine and helmet production in zooplankton are thought to provide protection from invertebrate rather than vertebrate predators. We examined selectivity for Daphnia lumholtzi, a species that exhibits extreme cyclomorphosis with a large helmet and long tail spine (total length can exceed 5 mm), by juvenile bluegill (15-80 mm) in the laboratory and field. Bluegill consumed more D. pulex than D. lumholtzi when the species were presented alone. When the daphnids were offered together in equal numbers, bluegill selected against D. lumholtzi. Bluegill foraging behavior helped explain the observed nonrandom feeding. Bluegill capture efficiency foraging on D. pulex was high (85-100%) and handling times were low (usually too short to detect), whereas efficiencies were lower (40-96%) and handling times were longer (1-3 s) when foraging on D. lumholtzi, particularly for fish <50 mm. As they gained experience, bluegill <50 mm that oriented towards D. lumholtzi rejected them more often than striking. In addition, more D. lumholtzi were rejected and expelled than were D. pulex. From these experiments, we conclude that larger bluegill (>50 mm) are able to forage more successfully on D. lumholtzi than smaller fish. Selectivity by bluegill collected from a reservoir infested with D. lumholtzi verified our laboratory conclusions. Smaller bluegill selected against D. lumholtzi, whereas it was a preferred diet item for bluegill >50 mm. These results show that the morphology of D. lumholtzi interferes with predation by small planktivorous fish, posing foraging constraints for these fish more similar to those of piscivores, where handling time is important, than to those of planktivores, where prey density is of primary importance.