Sharks possess visible openings on their snouts, often mistaken for breathing organs. These external features, termed nares or nostrils, raise questions about their function. Understanding their purpose reveals a fascinating aspect of shark biology.
The Sole Purpose of Shark Nostrils
A shark’s nostrils serve an exclusive purpose: the detection of scents in water, a sense known as olfaction. These nares are distinct from the shark’s respiratory system and have no connection to the throat or other breathing organs. Unlike in humans, where nostrils link to the respiratory tract, a shark’s nostrils are blind-ended sacs dedicated solely to processing chemical cues. This design allows for a refined sense of smell.
The Mechanics of Underwater Smelling
As a shark moves through the water, water enters the forward-facing opening of each nare, passing over specialized sensory tissue. This internal structure contains numerous folds of skin called olfactory lamellae, which increase the surface area for detecting dissolved chemical signals. After passing over these sensitive areas, the water exits through a separate rear opening. This dual-nostril system enables “stereo smelling,” allowing the shark to determine the direction of a scent by sensing which nostril detects it first or more strongly. Sharks can detect minute traces of substances, such as one part of fish flesh in 10 billion parts of seawater, or a single drop of blood in an Olympic-sized swimming pool.
If Not for Breathing, Then How?
Sharks breathe using their gills to extract oxygen from the surrounding water. Water enters either through the shark’s mouth or, in some species, through small openings called spiracles located behind the eyes. This water then flows over the gill filaments, which are rich in blood vessels. Here, oxygen from the water diffuses into the shark’s bloodstream, while carbon dioxide is released from the blood into the water. The oxygen-depleted water then exits through multiple gill slits located on the sides of the shark’s head.
Some sharks, known as obligate ram ventilators, must continuously swim to force water over their gills, a process called ram ventilation. Other species can actively pump water over their gills using buccal muscles, a method known as buccal pumping, allowing them to breathe while stationary.