Fish absolutely have a sense of smell, known as olfaction, which is one of their most sophisticated and relied-upon sensory systems in the aquatic environment. Water is an ideal medium for carrying dissolved chemical cues, making olfaction a primary means by which fish perceive their surroundings, often superseding the importance of sight or hearing. This highly sensitive ability guides nearly every aspect of their survival, from finding a meal in dark waters to navigating vast distances.
The Mechanism of Fish Olfaction
The mechanism fish use to smell is distinct from mammalian breathing, as their nostrils are not connected to the respiratory system. Fish utilize a pair of openings called nares, located on their snout, solely for chemoreception. Water flows into the incurrent naris, passes over specialized sensory tissue, and then exits through the excurrent naris.
Inside the nasal cavity, the water flows over a structure called the olfactory rosette, which is the functional center of the fish’s sense of smell. This rosette is a series of folded, leaf-like tissues called lamellae, which dramatically increase the surface area available for chemical detection. The number of lamellae varies widely between species, reflecting the relative importance of olfaction; for example, some catfish species possess a rosette that is exceptionally large relative to their body size.
The olfactory epithelium lining these lamellae contains millions of receptor neurons that directly contact the water. These neurons bind to dissolved molecules, such as amino acids or pheromones, triggering an electrical signal transmitted directly to the olfactory bulb of the forebrain. This process often relies on the fish’s own movement to push water through the nasal cavity, though some species employ ciliated cells or muscular contractions to actively pump water.
Olfactory Roles in Survival and Behavior
The acute sensitivity of the fish olfactory system is used for a multitude of life-sustaining behaviors. One fundamental role is foraging, where fish track chemical trails released by prey, such as dissolved free amino acids (DFAAs) that leak from aquatic invertebrates. Some species, particularly those living in dark or turbid conditions, exhibit extraordinary sensitivity; for instance, cavefish larvae can detect the amino acid alanine at concentrations as low as \(10^{-10}\) molar.
Olfaction is also indispensable for defense, primarily through the detection of chemical alarm cues known as Schreckstoff (German for “fear substance”). This alarm substance, composed of oligosaccharides like chondroitin-4-sulfate, is released when the skin of certain fish, such as minnows, is damaged by a predator. Upon detection, nearby conspecifics respond immediately with defensive behaviors like freezing, dashing, or tightening their school formation, increasing their collective survival odds.
Reproduction is heavily mediated by chemical signaling, as fish use pheromones to coordinate mating. These structurally diverse compounds, which include steroids and bile acids, act in two ways. They function as “releaser” pheromones, causing immediate behavioral changes like courtship displays, or as “primer” pheromones, which modulate the endocrine system to synchronize gamete maturation. For example, male-released pheromones in Mozambique tilapia can accelerate oocyte maturation in females.
The most recognized use of fish olfaction is the migratory navigation of anadromous species like salmon. Juvenile salmon imprint on the unique chemical signature of their natal stream water, a complex mixture of minerals and dissolved free amino acids. Years later, adults use this chemical memory to navigate from the ocean back to the exact tributary where they were born. This process of olfactory homing can be experimentally confirmed using synthetic odors like morpholine.
Environmental Threats to Fish Sense of Smell
The olfactory system’s direct exposure to the aquatic environment makes it highly susceptible to disruption from anthropogenic changes and pollutants. Ocean acidification (OA), caused by the ocean absorbing excess atmospheric carbon dioxide, is a pervasive threat that directly impacts olfaction. The resulting decrease in seawater pH can alter the conformation of odorant molecules and their receptor binding sites on the olfactory epithelium. This change significantly reduces the binding affinity between odorants and receptors, effectively raising the threshold required for a scent to be detected.
Over time, high carbon dioxide levels can also affect the excitability of olfactory neurons, impairing the transmission of chemical information to the brain. These physiological consequences can leave fish “olfactory-blind,” preventing them from discriminating between predator cues and habitat-selection signals.
Chemical pollutants pose an equally severe challenge, either by damaging sensory tissue or by masking natural scents. Heavy metals like copper and cadmium, even at low concentrations, can physically damage the olfactory epithelium, reducing electrical signals and disrupting behaviors such as predator avoidance. Pesticides and surfactants, such as 4-nonylphenol, can interfere with receptor function or alter the fish’s chemical profile, disrupting social recognition cues necessary for shoaling and mating.