When animals eat, digest and assimilate a protein-containing meal, nitrogen atoms in the form of ammonia are produced as waste. Ammonia is a very potent toxin to the central nervous system, so it must be rapidly removed from their bodies. In fish and other water-dwelling animals, the surrounding water is a virtually limitless sink for this ammonia, so nitrogen waste is simply and rapidly removed to the water as ammonia. Land-dwelling animals, however, not being immersed in water, have the need to conserve water, so they cannot readily remove ammonia as such. They transform ammonia to urea, which since it is less toxic to the central nervous system, can be stored in higher concentrations before being eliminated in a concentrated urine which conserves water. Transforming ammonia to urea diverts energy from other bodily functions (e.g., growth, reproduction, etc.), so in an evolutionary sense making urea is not a trivial investment. We have discovered a higher teleost (bony) marine fish, the gulf toadfish, that unexpectedly makes and excretes urea. This grant will allow us to study why and how this phenomenon occurs and its ecological and evolutionary significance. In addition to enhancing our basic knowledge in this area of physiology processes, these studies may at some point provide information useful to practical developments in aquaculture and environmental management.
