One of the primary channels of sensory input for zooplankton are chemical signals. Much zooplankton behavior is triggered by chemical stimuli, including feeding, predator defense, mating, and migration. Chemically regulated zooplankton behavior affects larger scale ecosystem processes like grazing, recruitment and secondary production. Knowledge of how chemicals transmit information about location, food quality, conspecifics, competitors, and predators is critical for understanding how aquatic ecosystems function. This paper reviews the behavioral evidence that planktonic rotifers respond to a variety of chemical stimuli. Although a rich variety of rotifer behaviors are regulated by chemical signals, little progress has been made to isolate and characterize these stimuli. If aquatic ecology is to become a predictive science, knowledge of the mechanisms causing the observed interactions is necessary. Chemical signals need to be isolated, purified, and characterized, and their causal role in regulating population and community processes needs to be demonstrated. Rotifers have chemosensory neurons in their corona and electron microscopy has revealed chemoreceptive pores in the anterior integument of several species. Some rotifers use these chemoreceptors to discriminate food particles based on the flavors on the cell surface. In Asplanchna, prey are discriminated by contact chemoreception. Asplanchna releases a waterborne signal that induces spine formation in several Brachionus species, Keratella cochlearis, K. slacki, and Filinia longisecta. The colonial Sinatherina socialis is defended against fish predation by warts containing unpalatable chemicals that have yet to be identified. Larval settlement in Collotheca gracillipes is determined by the chemistry of aquatic plant surfaces. Larvae prefer the undersurface of leaves where there is a low Ca{++} microhabitat due to photosynthesis. Oviposition in Euchlanis dilatata is restricted to plant surfaces familiar to the maternal female. Hydrogen peroxide and certain prostaglandins stimulate resting egg hatching even in the dark. Sexual reproduction and polymorphism in Asplanchna sieboldi is regulated by dietary tocopherol. A chemical signal that allows assessment of conspecific population density is detected in conditioned media by several rotifer species. Water soluble extracts of Brachionus plicatilis increase mictic female production 1.7 times more than controls. Unknown compounds produced by certain bacteria also increase mixis 4–10 fold over controls. Mate recognition in B. plicatilis is determined by a 29 kD surface glycoprotein called the mate recognition pheromone (MRP). The MRP has been isolated, purified, and a polyclonal antibody against it has been prepared. The structure of the oligosaccharide and protein components of the MRP are currently being characterized. Elucidation of the chemicals regulating rotifer life cycles will make important contributions to the understanding of ecological processes in aquatic communities.
