Littoral zooplankton-water hyacinth interactions: habitat or refuge?

Water hyacinth ( Eichhornia crassipes) is one of the most problematic invasive macrophyte in the world. Although it has become a key element of the systems where it was introduced several decades ago, there is insufficient information to determine its role on biological interactions. To elucidate water hyacinth-littoral zooplankton-planktivorous fish interactions, we conducted habitat choice experiments in the absence and presence of fish chemical cues (kairomones) for two tropical littoral cladocerans, Chydorus brevilabris and Simocephalus vetulus. We classified habitat selection as a habitat (preference for the plant without predation risk) and/or as a refuge (preference for the plant with predation risk). Our results showed that E. crassipes is used as a habitat by C. brevilabris and as a refuge by S. vetulus against fish predation. Although C. brevilabris did not use the water hyacinth as a refuge in the presence of kairomones, the results suggested that both C. brevilabris and S. vetulus exhibited behavioral responses that further reduced their predation risk. To our knowledge, this study is the first experimental evidence for littoral cladocerans using free-floating macrophytes as a refuge against fish predation in tropical systems. Our results support the assertion that habitat association is not necessarily an evidence of habitat preference. [ABSTRACT FROM AUTHOR]