Figure 2.11 Unique adaptations to saltwater. When aquatic vertebrates digest proteins, they produce ammonia as a by-product. (a) Most aquatic organisms, such as dolphins, excrete this ammonia in their urine. (b) Sharks and rays convert this ammonia into urea and then retain some of the urea in their bloodstream. The result is a higher solute concentration that helps offset the challenge of being hyposmotic in saltwater.