Chemical Messages Underwater
Animals detect the chemical cues that surround them using both their senses of smell (olfaction) and taste (gustation). Of these, olfaction is the more important, especially over long distance. Taste is used to assess things that are physically in contact with chemoreceptor cells. The underwater environment is a soup, full of chemical information about the habitat and about other animals, who is around, what they are doing, whether they are injured and so on. Aquatic animals constantly intercept these messages, choosing which they should respond to and which they should ignore.
Do fish have noses?
Not exactly -- but they can smell! Unlike many land animals, fish do not breathe through their nostrils, which are used solely for smell. Some species have two pairs of nostrils and actively pump water in through one pair and out through the other. As the water passes over the sensory receptors within the nostrils, the chemicals in the water flow generate nerve impulses, which may then be decoded by the olfactory bulbs situated in the front part of the brain.
How well can fish smell?
A good sense of smell is essential for many freshwater fish species because vision is often limited underwater by turbid, muddy water and shade from plants, such as lilies. Coral reefs, however, are characterized by extremely clear waters. Even so, reef fishes can and do detect chemical cues.The ability is strongest in those species that live with least light. Fish in the deep seas are able to detect concentrations of chemicals as weak as one part per quadrillion. An excellent sense of smell is common to many species of fish, especially nocturnal ones.
What do fish use smell for on the reef?
On the coral reef, moray eels hunt at night. They locate their prey by moving through the holes and interstices of the reef itself, homing in on victims by detecting their characteristic odor trails in the water.
Some daytime fish use their chemical senses to detect food. Goatfish, for example, use their barbels to find small invertebrates in the coral sand. These barbels are covered in sensitive chemosensory cells that allow the fish to taste its prey before it can see it. Often, fish that live along the substrate of the reef, including some blennies and gobies, also have these chemosensory cells along the undersides of their bodies.
How do fish use smell?
The ability offish to orientate towards (or away from) the source of an odor is called chemotaxis. Each animal produces its own odor; if a fish detects that of a prey animal, it is likely to try to use it to track down that animal.The process of discovering the source starts when the fish crosses the odor trail of the prey animal. Once the fish detects the presence of the smell, it circles to reconnect with the trail and then moves up the odor gradient, going from a weak smell to a stronger one, until it finds the source and devours it. As well as finding food, fish are able to navigate using their sense of smell. The salmon is the most famous of all fish species for doing this, but most fish are capable of detecting and moving towards the smell of a local, familiar habitat. Each habitat has its own, unique, chemical signature and this is what the fish respond to.This ability is extremely important -- fish that become familiar with a preferred area of their habitat know from experience where the best hiding places are located and where to find food.