The Pacific spiny dogfish shark recycles the toxic ammonia found in ocean water and converts it to useful urea, new research from the University of British Columbia suggests. The urea keeps the skin of the shark moist, thus preventing them from drying out in salty seawater.
The zoologists who carried out the study explained that animals usually eat protein to grow; however, sharks are special, as they require protein throughout the entire lives to replenish the urea in their tissues.
Urea is a non-toxic nitrogen-containing substance, one that humans excrete in their urine. The substance keeps fish from drying out in the highly saline marine environment.
According to the new research, dogfish sharks have the capacity to absorb the ammonia at surprisingly high rates through their gills, and the quantity a shark can absorb in order to synthesize it can even get to a third of the nitrogen the animals need for their diet.
Most fish are ammoniotelic, which means they produce ammonia as their major nitrogenous waste, and then excrete it across the gills in order to avoid death or cerebral dysfunction due to the buildup.
Sharks, which belong to the marine subclass of elasmobranchs, are among the exceptions to this rule, along with skates and rays.
"These animals are ureotelic, producing urea, which is a much less toxic molecule, as their major nitrogenous waste," the researchers wrote in their study.
In addition, the animals are ureosmotic, which means they also retain high levels of urea in their body fluids; this process allows the animals to avoid the need to drink seawater.
The researchers, study lead author Chris Wood and doctoral student Marina Giacomin, also pointed out that ammonia isn't simply assimilated into the animal's body through the means of a simple infusion, as it would be intuitive to believe. Instead, there is an entire biological process responsible for this assimilation.
As it turns out, the gas is most likely carried into the animals' tissues by proteins called Rhesus. These proteins are known for acting as gas channels and have already been proven in the scientific world to be responsible for the transportation of ammonia gas molecules between cellular membranes.
The dogfish shark could use their capacity to absorb ammonia in order to create urea stores, especially since the fish happen to go through extended periods of time without any food intakes whatsoever. This would make their protein supply perfect for maintaining a good urea level in their tissues, even at longer intervals.
The Pacific spiny dogfish is among the most popular species of sharks living in the Pacific, residing in the ocean's northern area. The gray-colored fish can develop up to 150 centimeters (nearly 5 feet) long.