AMMONIA is as repulsive to most marine animals as it is to land-lubbing ones—and for good reason. It is extremely toxic. But there is an exception. Far from being repelled by ammonia, sharks are actually attracted to it. The longtime assumption has been that this is because it is a waste product, voided into the water by fish and other creatures, that signals the presence of potential prey. But Chris Wood and Marina Giacomin of the University of British Columbia, in Vancouver, think there may be more to it than this. As they describe in the Journal of Experimental Biology, they suspect that for sharks, ammonia is itself a useful resource.

All animals make ammonia. It is a compound of nitrogen and hydrogen produced by the breakdown of amino acids, the building blocks of proteins. Marine creatures can flush it directly into the sea (fish do so through their gills), since it is soluble in water. Land animals often add carbon and oxygen to convert it into urea, which is far less toxic, and store the result in solution in a bladder, for periodic evacuation. Sea creatures can make urea too, though—and in sharks this molecule, which they synthesise in their gills, plays a crucial role in stabilising the salinity of their tissues.

Dr Wood and Ms Giacomin knew from the work of others that sharks forced to swim in water containing unnaturally high concentrations of ammonia absorb the chemical into their gills, convert it into urea and then expel that urea back into the water. The presumption was that this was an anti-poisoning mechanism. That, though, is a slightly odd idea. In the wild, unconfined by an experimenter’s tank, it would surely be simpler and safer for a shark to swim away from the dangerous area and avoid the problem altogether. The two researchers therefore wondered if what had been seen in these previous experiments was really an accidental consequence of something else. Given urea’s role in shark salinity-stabilisation (a role which it does not play in other groups of fish), they wondered if the animals’ eagerness to find water with lots of ammonia in it was as much to do with replenishing their urea supplies as with locating prey. They therefore decided to run some experiments of their own.

To this end, they exposed ten Pacific spiny dogfish (a type of small shark easily maintained in the laboratory) to ammonia concentrations ranging from 100 micromoles per litre (µmol/l), a level commonly found in the wild, to 1,600 µmol/l, an unnaturally high level, while monitoring the water’s chemistry closely.

Whatever the initial level of ammonia, they found, that substance’s concentration began declining almost as soon as the sharks were put into the tank. The animals were, indeed, absorbing it. They were not, though, automatically excreting the resulting urea. Levels of this in the water rose only when the dogfish were exposed to ammonia concentrations of 800 µmol/l or more. And a closer look at the animals’ gills and blood confirmed that they were retaining urea.

All this makes perfect sense. The importance of urea to shark physiology means they have to make it from something. Amino-acid breakdown, the alternative source of its central element, nitrogen, requires otherwise-valuable proteins. Calculations performed by Dr Wood and Ms Giacomin suggest dogfish swimming in ammonia-rich waters would be able to scavenge from those waters almost a third of the nitrogen they need to make urea. That adds up to a tidy saving in protein. So, sharks may well be driven by appetite to swim towards places where their prey have been releasing large amounts of waste ammonia. But, contrary to past theories, the appetite that takes them there may really be for the waste itself.