9507239 Walsh The broad outlines of the evolution of how animals excrete their waste nitrogen are reasonably well understood. Fully aquatic animals typically excrete ammonia as their main nitrogenous waste product because the water surrounding them will dilute the toxic ammonia. Terrestrial and semi- terrestrial animals, in order to conserve water, excrete nitrogenous waste products like as urea and uric acid that are less toxic and can be stored safely at higher concentrations. The production of these less toxic compounds is costly, however, because of their higher energy content. In spite of this expense, one fish, the gulf toadfish, has recently been shown to synthesize and excrete urea. This research is aimed at understanding the biochemical pathways the fish uses to make urea, their similarity to those of terrestrial organisms, and the genetic basis for this ability. Field studies will determine what advantage this ability provides in an aquatic environment. Preliminary results indicate that environmental stress induces the fish to make urea. A variety of biochemical, physiological, molecular and field techniques will be used both in the laboratory and in the fish's natural setting to identify synthetic pathways, control mechanisms, and the functional significance of urea excretion. This research should provide unique insight into the genetic mechanisms, physiological significance and evolution of an important physiological trait in vertebrate animals. Ultimately, it may lead to genetic advances that would enable culture of fish that make urea instead of ammonia that could simplify high-density aquaculture by reducing the need to control ammonia levels. If environmental stress is the trigger that initiates urea production, the abundance of the fish may make it a useful bioassay for biologically im portant changes in coastal water environments.
