We found high population densities of Hydra canadensis (up to 30,000 m ‘) on macrophytes in Lake Opinicon (Ontario, Canada) and tested whether they influenced the survival of larval bluegill, Lepomis macrochirus. Hydra were observed to capture and ingest bluegill larvae in laboratory trials, IndiAduals that ingested fish larvae turned black; those fed a variety of crustacean species remained brown. The ingestion rate of Hydra on larvae in 8-liter laboratory microcosms was higher during the night (0.197 larvae predator-l h-l) than during the day (0.111 larvae predator-l h-l), Many larvae also died after escaping from the stinging tentacles of Hydra (on average 26% of larval mortality). Population densities of Hydra were highest on the macrophyte Myriophyllum spicatum. Bluegill colonies were surrounded by dense beds of M. spicatum in some areas, and larvae in the colonies had to swim through the plants (with the attached Hydra) in order to reach open water. We sampled Hydra in the vicinity of bluegill colonies on the morning after larval swim-up. The number of black Hydra decreased with increasing distance from a colony; 72% black within colonies, 45% at 0.5 m outside colonies, and 31% at 2 m outside of colonies. We estimate that up to 20% of the larvae produced by a colony csn be killed by Hydra within this 2-m zone. Many more larvae likely die as a result of encounters with Hydra that are abundant throughout the lake.
Hydra collected from lake, placed in tanks with flow-through lake water