On land, scents guide our experiences, from the aroma of blooming flowers to the warning of smoke. Yet, a fundamental question remains: what does it smell like underwater? The aquatic environment presents a unique challenge to our understanding of olfaction, inviting curiosity about how senses operate in a world so different from our own.
How Olfaction Changes in Water
The way odor molecules behave in water differs significantly from their behavior in air. In an aquatic environment, water is much denser and more viscous than air, which affects how chemicals move. While odor molecules are still present and can dissolve in water, their diffusion rates are considerably slower compared to their movement through air. The slower diffusion in water means that chemical signals disperse more gradually and over shorter distances than airborne scents. Unlike in air where volatile compounds easily become gaseous, in water, chemicals must be water-soluble to be detected. This fundamental difference shapes the types of chemical cues available and how organisms perceive them in aquatic habitats.
Human Perception of Underwater Scents
Humans typically perceive very little distinct smell when submerged, even with a mask. Our olfactory system is primarily adapted for detecting airborne molecules, relying on inhalation to draw odorants into the nasal cavity where they can bind to receptors. When underwater, this process is hindered because water, rather than air, fills our nasal passages, preventing effective interaction between odor molecules and our chemoreceptors. What humans might occasionally perceive as a “smell” underwater could be a result of dissolved chemicals irritating mucous membranes or stimulating taste receptors. Our sense of taste and smell are closely linked, and some aquatic experiences might blur these sensory distinctions.
How Aquatic Life Detects Odors
Aquatic animals possess highly specialized chemoreceptors uniquely adapted to detect dissolved chemicals in water. Fish, for example, have a pair of nostrils, or nares, located on their snouts that draw water over a dense network of sensory cells. These cells are equipped to detect minute concentrations of water-soluble chemical stimulants. Unlike humans, fish do not use these nostrils for breathing; their olfactory system operates independently of their respiratory system.
They use this sense to locate food, identify and avoid predators, find mates, and even navigate vast distances. For instance, salmon can detect specific chemical cues that guide them back to their natal rivers for spawning. Similarly, sharks are known for their ability to detect trace amounts of blood in the water, guiding them towards potential prey.
Sources of Underwater Smells
A variety of natural processes contribute to the chemical signals, or “smells,” found in aquatic environments. Decaying organic matter, such as dead plants and animals, releases dissolved organic carbon and nutrients into the water column. This decomposition process can produce compounds like methane and carbon dioxide, which aquatic organisms can detect.
Algae and plankton are significant sources of distinct chemical cues. Many species release dimethyl sulfide (DMS), a volatile sulfur compound that contributes to the characteristic “smell of the ocean” that humans often perceive in the air near coastlines. Pheromones, which are chemical signals used for communication within a species, are also widespread in aquatic life. These chemical messages facilitate behaviors like attracting mates, signaling alarm, or identifying individuals.