Most creatures need to excrete urea, or suffer their waste product's toxic effects. But a little urea isn't a bad thing for many, and for elasmobranchs,it's positively beneficial, so they recycle as much urea as possible, pumping urea out of their urine back into their urea saturated bodies. But how wasn't clear. Although several teams had suggested ways that elasmobranchs could reabsorb urea, no one had come up with conclusive evidence for the mechanisms that allow the dogfish to pump urea back into its body against a concentration gradient. Knowing that a urea transporter protein had been identified in a spiny dogfish's kidney, Susumu Hyodo and colleagues from the University of Tokyo began searching for a similar protein in the kidney of a close relative, Triakis scyllia (p. 347).

Using the spiny dogfish's urea transporter, the team were able to identify a urea transporter protein in Triakis' kidney, and then make a specific antibody to the new protein, ready to probe the protein's distribution through the kidney. The transporter was found exclusively in the kidney's collecting tubule, which is surrounded by a complex arrangement of countercurrent tubules that produce a low urea environment around the collecting tubule, confirming that urea reuptake in the dogfish's kidney is by facilitative diffusion.

Hyodo, S., Katoh, F., Kaneko, T. and Takei, Y.(
). A facilitative urea transporter is localized in the renal collecting tubule of the dogfish Triakis scyllia.
J. Exp. Biol.