How do shark void their waste products? There is a common misconception that sharks do not excrete their urine but instead void it through the blood and gill system. While partially true, sharks, like other vertebrates also urinate.
Like many of their adaptations, sharks have an excretory system unique to the elasmobranchs. Sharks do have a urinary system, but they also have a mechanism to use waste electrolytes to maintain osmotic balance. Sharks possess a cloaca, a single opening for both the urinary and digestive tracts.
The urinary tract, including the kidneys, ureters, and urethra, leads to the urogenital sinus, which opens into the cloaca. Fecal matter, including waste products from the intestines (which feature a unique spiral valve to aid digestion), also exits through the cloaca. Like other vertebrates, sharks have kidneys and a urinary duct. The kidneys filter waste products from the circulatory system and excrete waste products in the urine. However, sharks do not have an external urethra that excretes their urine, but void through a common orifice called a cloaca.
Image Arun Divy Sharks
The cloaca serves for the elimination of both urinary and fecal wastes as well as providing an opening through which the shark or ray pups are born for live-bearing species.
Female sharks, like many other species of fish, possess two uteri, one on each side of their reproductive tract. The two uteri open into the postero-dorsal portion of the cloaca just ventral to the urinary papilla.
Urea allows sharks to osmoregulate in highly saline environments. When the environment changes (or the shark is stressed), urea can then be excreted through the shark’s gills or skin. This high urea concentration helps sharks achieve osmotic balance between the external environment and their tissues, meaning their internal salt concentration matches that of the surrounding seawater.
Sharks retain urea in their blood plasma as an adaptive strategy to keep the concentration of solutes in their blood in balance to the external seawater. Seawater has more salt (mostly NaCl), while shark blood plasma has more urea (CH4N2O), but overall it’s a similar concentration. This adaptation prevents them from losing fresh water constantly through their gills. Unlike other fishes, which constantly drink seawater, sharks don’t have to drink.
When sharks die, the urea in their tissues breaks down into ammonia. This leads to the characteristic ammonia smell and affects the taste of shark meat. Fishermen will often bleed the shark to eliminate this, however this can be cruel if the shark is still alive. One of the disincentives to shark bycatch (leading to shark finning) is the shark meat will spoil the high value meat of tuna or billfish targeted by commercial longliner and seiners.