SEXUAL COMMUNICATION IN COPEPODS AND ROTIFERS

The problem of locating mates and identifying them as conspecifics is similar for Copepoda, Cladocera, and Rotifera since they are all relatively small animals moving in a large volume of water. Chemoreception is a potentially effective means of identifying mates in aquatic environments. In zooplankton, mate recognition systems based on chemoreception have evolved which can distinguish conspecifics from other species and can discriminate males from females. The evidence for the role of chemical signals in copepod mating is indirect, based on observations of mating behavior. The small size and limited metabolic capability of zooplankton, and high diffusion rates at 1-10 mm spatial scales, restrict the volume through which chemical signals can be effectively transmitted. An alternative to using diffusible molecules for sexual communication is to bind signal molecules to cell surfaces, allowing energetically costly molecules like proteins to serve as signals without their loss through diffusion. Contact chemoreception has been described in copepod and rotifer mating and perhaps represents a general solution to the problem of chemical communication by small zooplankters. The types of signals used, their mechanisms of transmission and reception, and their role in maintaining reproductive isolation among species has yet to be characterized for any aquatic invertebrate. In this review, I compare the methods used by copepods and rotifers for mate seeking and recognition, describe the behavioral evidence supporting the existence of chemical cues, and examine experiments describing the biochemical characteristics of the signal molecules. In copepods, male mate seeking behavior occurs without previous female contact, suggesting the reception of a diffusible signal. The signal molecule is small and lacks species specificity in the species investigated. Male rotifers do not respond to females from a distance. Mates are located by random male swimming and contact chemoreception of a species-specific signal. Mate recognition in both copepods and rotifers is based on contact chemoreception of a species-specific signal. The pheromones responsible are not known for copepods, but surface glycoproteins with pheromonal activity have been identified in rotifers. The structure of the rotifer sex pheromone has been probed by selective enzymatic degradation and lectin binding, electrophoretic characterization, and attachment to agarose beads to assess its biological activity. Glycoproteins play a key role in the sexual communication of some algal and ciliate species and have well characterized roles in cellular recognition phenomena like sperm-egg binding. The significance of these studies lies in their contribution to our understanding of zooplankton reproductive biology, the chemical ecology of male-female communication, the molecular basis of chemoreception in the aquatic environment, and the evolution of pre-mating reproductive isolating mechanisms in zooplankton.