The sense of smell is an organism's ability to detect chemical cues in the environment. Smells are especially useful in water, where murky conditions make vision unreliable. Fish use smells for finding food and mates, and for identifying danger. Unlike those of other vertebrates, fishes' nostrils do not do double duty as both organs of smell and openings for breathing: they are used exclusively for smell. The sophistication of fishes' olfactory (smelling) organs varies greatly, but there is a basic design. Each nostril is populated by layers of specialized cells composing the olfactory epithelium (lining), which is folded upon itself to save space. Thousands of tiny cilia (hair-like structures on the cells) pulse in sequence to propel water into and out of the sense organ. Signals from the epithelium are sent to the olfactory part of the brain.

One of a fish's most useful adaptations is the production of an "alarm chemical" in the presence of danger, such as a predatory fish or a spear fisherman. We owe it to Austrian biologist Karl von Frisch (1886-1982) for discovering this phenomenon in the world of fish senses. When he accidentally injured one of his captive minnows, von Frisch noticed that other fishes in the tank began darting back and forth and freezing in place-classic predator-evading behavior. Experiments by von Frisch and others showed that injured minnows (among other fish species) release a pheromone-a secreted or excreted chemical that triggers a social response in members of the same species. Detecting this particular pheromone causes agitated reactions by the minnows. Von Frisch coined the term schreckstoff for these pheromones.

The cells that release schreckstoff are located in the skin, and they are fragile enough that they will rupture and release the substance if a fish is placed on moist paper. And it is potent: a thousandth of a milligram of chopped skin is enough to elicit a fright reaction from another fish in a 14-liter aquarium. That is like chopping a piece of candy into 20 million pieces, dropping one piece into a sink full of water, and then trying to taste the sweetness. Schreckstoff must have evolved long ago, for it is produced by several families of bony fishes. As a freely available signal, schreckstoff acts like a fire alarm that can be used by other nearby fishes, including different species that may also recognize it Fathead minnows, when they smell the body waste of northern pikes that have fed on other fathead minnows or on brook sticklebacks-both of which produce schreckstoff in their skin-immediately flee to hiding places or form tighter swimming groups. But if the pikes have fed only on swordtail fishes-which do not produce schreckstoff-the minnows show no signs of fear. Thus, it is not the smell of the pike that minnows are reacting to-instead they detect and react to the schreckstoff from the pike's victims. It is probably due to olfactory skills like the minnow's that pikes refrain from eliminating waste in their own hunting grounds.

The schreckstoff reaction illustrates how fishes can extract subtle clues from waterborne chemicals. But schreckstoff is not the only way to detect a fish foe by fragrance. There is the old-fashioned way of simply recognizing the smell of the predator. Juvenile lemon sharks react to the odor of American crocodiles, who sometimes prey on them. For an Atlantic salmon, it depends on what its predator has been eating. In a study conducted at Swansea University in Wales, juvenile salmon that had never been exposed to predators were presented with water containing traces of body waste from one of their natural enemies, the Eurasian otter. The salmon showed a fear response only if the otter had been dining on salmon. In those cases they fled the smell and then remained still, and they breathed faster. Salmon exposed to blank water or to body waste from otters on a nonsalmon diet were unfazed. The scientist concluded that Atlantic salmon apparently do not innately recognize otters as a threat-they perceive them as a danger only if salmon is on the menu. This generalized mode of predator detection works well because it does not require learning the smell of different predators.