Predation by Hydra on larval fish: Field and laboratory experiments with bluegill (Lepomis macrochirus)

Abstract

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, a model system for environmental studies

“…Ecologically, Hydra play the role of both predators and prey in aquatic ecosystems (Slobodkin and Bossert, 2001). As predators, Hydra have been shown to ingest cladocerans (Schwartz and Hebert, 1989), copepods (Link and Keen, 1995), rotifers (Walsh, 1995), and larval fish (Elliott et al, 1997), as well as their standard laboratory food, brine shrimp, Artemia sp. (Loomis and Lenhoff, 1956).…”

How to use Hydra as a model system to teach biology in the classroom

“…Laboratory cultures of Hydra float when hungry and attach again after they have been fed (Lomnicki and Slobodkin, 1966), therefore the best place to look for them is often at the downstream end of a lake or the pools in the streams immediately below the lake. Hydra can also become a significant planktonic predator (Griffing, 1965;Elliott et al, 1997) but these populations are not as easy to collect in open water because these small soft-bodied Cnidaria do not tolerate plankton tows very well. …”

Phytophilous Fauna of a Small and Artificial Urban Lake

“…Dakić et al (2008) also found a significant correlation between the number of individuals of brown hydra and watermilfoil stem length, fresh macrophyte biomass and epiphyton biomass. Living and eating attached directly to the plant, it would be expected for them to be dependant and numerous on submerged macrophytes (Elliot et al, 1997;Dakić et al, 2008), whilst other invertebrates, although dependant on the substrate, are more motile, especially Cladocera and Copepoda which can easily detach and be part of the plankton community. Plants with dissected leaves, such as Myriophyllum, have a higher ratio of surface area and biomass and therefore provide a better shelter for representatives of invertebrate fauna than the macrophyte of simple architecture (Jackson, 1997;Cheruvelil et al, 2002;Vidaković and Bogut, 2006;Bogut et al, 2007Bogut et al, , 2010Čerba et al, 2009Špoljar et al 2011.…”