Going hungry is not an uncommon experience for most species, and Chris Wood, from the University of British Columbia, Canada, says that many Pacific dogfish often have empty bellies. However, while most animals consume protein for growth and tissue repair, sharks also require protein to continually replenish urea in their tissues, which prevents the fish from drying out in seawater, but is constantly leached from their bodies. So when Wood and colleagues discovered that dogfish can absorb ammonia at exceptionally high concentrations (1000 μmol l−1) through their gills from the surrounding water for conversion into urea, he wondered whether the fish may be able to extract sufficient ammonia from seawater with lower ammonia concentrations (200 μmol l−1), more typical of those near the coast, to supplement their urea supply.
Immersing Pacific spiny dogfish that had been caught in Barkley Sound in seawater containing ammonia ranging from 100 to 1600 μmol l−1 over a 10 h period and plotting the animals’ ammonia uptake rates, Wood and Marina Giacomin realised that the plot had the characteristic shape of a biological process driven by active transport through protein channels, instead of simple diffusion into the body. And when the duo calculated the ammonia concentration at which the uptake rate was half of the maximum value, they ended up with a value of 379 μmol l−1, ‘indicative of the normal concentration range within which the system evolved to operate’, they say. The duo also compared the fish's ammonia uptake rates with the rate at which they lost urea to their surroundings, and it was clear that the fish were benefiting from a net influx of the gas. They also suggest that ammonia scavenged from the sea over the length of time that it would take a small (1 kg) shark to digest a fish meal could provide as much nitrogen as 9 g of digested muscle; enough to supplement one-third of the urea lost to the environment through the gills over the same period.
Intrigued by the various transport mechanisms that the fish may use to absorb ammonia through their gills, Wood and Giacomin tested several alternatives and concluded that the gas is likely carried by Rhesus proteins – channels that are known to carry ammonia gas molecules across cell membranes. The duo also suggests that the fish may be able to actively pump sufficient gas into their blood for urea production to tide them over until they catch their next fish dinner.
