In Murky Water, A Freshwater Fish Must Rely on Taste

For fish that seek their prey under cover of darkness or search among a silty, muddy substrate, vision is of little use in hunting. If they encounter food under these conditions, they need to be able to respond to it quickly, especially if that food has ideas of escaping. As a result, fish that hunt under these conditions make extensive use of their taste cells to provide information. Unlike the sense of smell, taste requires contact between the sensory cell -- a taste bud -- and the thing being tasted. Many species of fish have an excellent sense of taste. Although lacking a tongue in the sense that we understand it, they have plenty of taste buds and, unusually, these can be found not just in the mouth, but outside it.

Where can fish have taste buds?

The idea of tasting food while we eat is familiar to most of us, but some fish are able to taste their food before they eat it. Catfish, especially, use extra-oral taste buds; the body surface of some species maybe covered in as many as 200,000 of these sensory cells. By comparison, the human tongue has around 10,000 taste buds.

Many of the catfish's taste buds are found on their barbels, which extend far beyond their mouths to provide a kind of chemo-sensory radar all around the fish, detecting food hidden out of sight. If the barbels miss the food, then all is not lost -- the area around the mouth and indeed much of the body surface of the fish is covered in taste buds. Catfish and many other bottom-dwellers effectively swim along, constantly tasting for food, much like a swimming tongue.

Why do some species have whiskers?

These "whiskers," which tend to be sited around the mouth, are more correctly known as barbels. They vary considerably between different fish species but in many cases are covered with a layer of taste receptor cells -- taste buds. The whiskers of the zebra danio, for example, are barely visible, whilst the barbels of some catfish are far longer than the body of the fish itself, or are branched to provide comprehensive sensory coverage of tastes in the substrate. Even the modified feeler-like pelvic fins of some gourami species have a covering of sensory cells that may enable the fish to locate food.

Do fish have favorite flavors?

Just as with humans, fish have preferences for certain flavors, but these are hardwired into the animal's genes so fish within a species tend to have a preference for certain foods. But there are differences between fish species -- herbivorous fish have a taste for the carbohydrates, especially sugars, found in their food, whereas carnivores seek out the amino acids and metabolic compounds found in their common prey.

How else do fish use their taste buds?

As well as for foraging, fish also use their sensory systemss to detect minute changes in water chemistry. the ability to respond to slight differences in oxygen concentrations is important for a fish's metabolism, for
example. Also, in the modern industrial world, these sensory abilities enable fish to avoid the worst pollutants in the aquatic habitat.

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