What the Fish? Episode 2: Smells Like Freshwater Eels
Fishes have the five major human senses
Fishes use the same five major senses that all humans have: hearing, sight, smell, taste, and touch. But for fishes, all of these senses differ somewhat from our normal day-to-day experience.
Quite simply, living in a liquid environment is a very different thing than living in a gas (air) environment. Think about the difference between smell and taste. At some level tasting is like smelling wet things. How different are these senses when you are already wet or underwater? From an evolutionary or anatomical perspective, they do have fundamental, different origins and innervations, but because of their aquatic lifestyle these senses have more overlap in fishes when compared to humans. Given this similarity, one of the most striking differences is that fishes actually cover various parts of their bodies (ranging from their skin to specialized barbels, whiskers, or fin rays) with taste buds rather than just focusing on the tongue like we do. The whiskers of a catfish, like the one shown here, allow these fishes to taste the mud that they are digging around in. Would you want to drag your tongue around in the mud? We wouldn’t either.
Did you say seven senses?
Humans actually have more than five senses. For example, we have sensors for balance, temperature, and pain, but the five main senses dominate our daily lives and take up more relative sensory area in our brains. Fishes have two other major senses that are not found among the senses we experience: electroreception and mechanoreception (or distance touch). Electroreception is less common among fishes, but it is comparatively easy to grasp. This electro-sensitive system is much like a beach comber searching a sandy beach for valuable metals. Fishes use this system for a variety of reasons, but many fishes use this sense for hunting or gathering. A hammerhead shark or paddlefish will move the enlarged regions of their heads to search for small electrical signals in the water coming from animals respiring or moving. Specifically, fishes respiring underwater produce a small ionic charge that will stimulate electroreceptors, which allows a predator to find a sand dab or sea robin buried under the sand.
Mechanoreception is the sensory system that allows fishes to school, fishes to measure the surrounding current to hold their position in a moving stream, and fishes in the dark (e.g., deep-sea or caves) to find cave walls or rocky outcroppings. This is carried out by particular hair cells that are housed in a series of tubed scales along the side of a fish, found on the surface “pit organs” that cover the skin of some fishes, and distributed within bony canals in the head of a fish. These specialized hair cells or neuromasts are stimulated (bent/displaced) by the change in motion of water over the structures. This bending of the hairs in particular directions tells the fish that their schooling partners are changing direction or that a shark is quickly approaching, hence its common name of distance touch. If you are like us, you wish that you had these other wonderful vertebrate senses; but alas, they only work when you live underwater…