In aquatic crabs, maintenance of salt balance and excretion of waste in the form of ammonia, are performed by the gills. For those crabs which have taken to living on land, it was assumed that the gills could not serve these functions. Previously it was shown that, although no longer swimming in fluids, land crabs are able to provide sufficient salty water to the gill surface, in the form of their own urine, to allow excellent regulation of salt concentration, a prerequisite for life in a variable habitat. Besides salt balance, nitrogen excretion presents a hurdle to successful colonization of land. Proposed research examines whether urine recycling also promotes excretion of nitrogen wastes by the gill. The amount and form of nitrogen excreted by the possible routes--urine, feces, fluid in the gill chamber, stored internally as uric acid, and air will be measured to locate the routes used by crabs to get rid of nitrogenous wastes. The amount and salinity of the water available to the animals will be varied, since this appears to affect the amount and form of the wastes released. A backpack has been developed which allows the collection of urine directly from its point of release and substitution for urine solutions of known composition. The composition of urine under different conditions of salt and water availability will be studied, and carbon dioxide and ammonia levels measured in the natural habitat. The ability of the gills to reclaim salts and excrete wastes under the changing environmental conditions will also be examined. This research will contribute to a basic understanding of the physiology of land crabs, and their ability to adapt an essentially aquatic body plan and physiology to a terrestrial environment. In addition, land crabs are economically important in the Caribbean. They grow slowly and are subject to local extinction from overharvesting. Low nitrogen levels in the plant diet limit growth rates. The current research will result in a better understanding of the nitrogen budget of the crabs, information which is vital to the success of proposed crab farming.
