Sharks do have external openings on their snout, but they are fundamentally different from the nostrils of air-breathing mammals. These paired openings are known as nares, which are dedicated sensory organs operating solely within the aquatic environment to process chemical information. The structure and function of the nares highlight the shark’s success as an apex predator.
The Anatomical Reality: Nares, Not Nostrils
The openings on a shark’s snout are correctly termed nares, the scientific designation for nasal openings in fish. These paired structures are positioned on the underside of the snout. Unlike mammalian nostrils, which connect to the respiratory system, a shark’s nares are blind-ended sacs. Water enters one opening, passes over the sensory tissue, and then exits through a separate opening.
Each nare features two distinct apertures: an incurrent opening for water intake and an excurrent opening for outflow, often separated by a nasal flap. This arrangement ensures a continuous, one-way flow of water over the internal sensory surfaces. The structure is a highly refined chemoreception device, designed to sample the chemical composition of the surrounding water.
Function: Olfaction, Not Respiration
The primary purpose of the shark’s nares is olfaction, or the sense of smell; they play no role in breathing. This separation of function differs from air-breathing vertebrates, whose nostrils serve the dual purpose of smelling and respiration. Sharks, as fish, obtain oxygen from the water through their gills.
Water is continually drawn over the gill filaments, where dissolved oxygen is extracted into the bloodstream. Some species, like the Great White, use ram ventilation, forcing water over the gills by swimming constantly. Other, more sedentary species use buccal pumping, actively drawing water into their mouths and forcing it across the gills. The nares are completely separate from this respiratory process, focusing solely on chemical signal detection.
How Sharks Process Scents
The internal structure of the nares is engineered for chemical detection. Once water enters the incurrent opening, it flows into a specialized chamber called the olfactory sac. Within this sac is a complex arrangement of tissue known as the olfactory lamellae.
These lamellae are highly folded, plate-like structures that greatly increase the surface area available for chemical reception. Dissolved odor molecules in the water bind to neuro-sensory cells lining the lamellae, triggering an electrical signal. This signal is transmitted directly to the olfactory bulb and the large olfactory lobe in the forebrain for processing. The constant flow of water ensures the shark is continuously sampling its environment for new scents.
The Sensitivity of Shark Smell
The sophisticated anatomy of the nares results in an extremely acute sense of smell, a major tool for hunting and survival. The number of olfactory lamellae provides an enormous sensory surface area, allowing sharks to detect incredibly dilute concentrations of compounds. For example, some shark species can detect fish extracts at concentrations as low as one part in ten billion.
This remarkable sensitivity is often cited in terms of detecting blood, with some reports suggesting they can sense it at one part per million in water. The highly developed olfactory system is used for locating prey, navigating by sensing chemical gradients, and finding mates through pheromones. The dual nares also allow for directional smelling, enabling the shark to compare scent strength between the left and right sides to pinpoint the odor source.