Feeding through your gills and turning a toxicant into a resource: how the dogfish shark scavenges ammonia from its environment.

Nitrogen (N) appears to be a limiting dietary resource for elasmobranchs, required not only for protein growth but also for urea-based osmoregulation. Building on recent evidence that the toxicant ammonia can be taken up actively at the gills of the shark and made into the valuable osmolyte urea, we demonstrate that the uptake exhibits classic Michaelis-Menten saturation kinetics with an affinity constant (K