By Jessie M. Sanders, DVM, CertAqV
Living in an underwater environment is not without its challenges. Water is significantly denser than air, and fish have adapted in various ways to cope with the pressures of being underwater. Fish need to pick up on the smallest changes in their environment, so many of the adaptations help fish to sense the world around them. Their eyes, nares, and specialized lateral line organ are their primary sensory organs.
Fish Eyes: How Fish See the World Around Them
Fish eyes are very similar to mammal eyes except that they have adapted to working well underwater. If you have ever swum in a pool and opened your eyes underwater, you may have noticed that you can see okay, but not with the same definition as in air. Fish eyes differ in the fact that they have a round lens, unlike our ovoid one, and focus by moving the lens forward and backwards, rather than constricting a pupil. Shape and color of eyes in fish vary widely between species depending on their feeding and lifestyle. Predatory fish can make quick changes in their focus to see potential prey, whereas bottom-feeding scavengers are slow to focus since they only have to focus on the bottom substrate.
Fish Nares: How a Fish’s Nose Works
The nares of fish are designed to pick up chemical differences in the surrounding environment. Even though fish do not have a true nose, they have superb olfactory senses. Fish use their sense of smell for feeding, reproduction, migration, and for knowing when another fish is in distress. When adding different treatments to your tank or pond, fish will often react to the smell of the chemical first and try to alter their behavior, usually by swimming away.
Fish kept in captivity that have lost their eyesight can rely on their nose to sniff out their food. Like the wide variety in vision, a fishes’ sense of smell differs between fish species.
The Lateral Line
The most unique adaptation of fish to sense their underwater environment is their lateral line. If you have ever looked at the side of a fish, running about midline on either side is a row of spots. Different species have developed varying color patterns, making some easier to see than others. In scaleless fish, such as catfish, the spots are all connected and easy to see. These spots make up the lateral line organ.
Each of these spots are pores containing a sensory structure called a neuromast. A neuromast is made up of a hair cell within a small dome, or cupula. These pores are connected to the external watery environment and vibrate with changes in the flow and vibrations around the fish. This amazing organ is found in all teleost (ray-finned) fish species and can be used in varying ways depending on the fish’s behavior and lifestyle. Fish can use the information they get from their lateral line for finding prey, avoiding predators, schooling as a group, and communication. Fish in tanks and ponds can distinguish between different caretakers’ footfall vibrations as they approach, especially with the added incentive of food. And when all other senses are cut off, the lateral line system can aid fish, allowing them to survive in harsh conditions.
The Ampullae of Lorenzini: How Fish Sense Temperature and Electric Fields in the Water
Even more specialized are the ampullae of Lorenzini, found in sharks and other cartilaginous fish. These pores found around the nose, mouth, and eyes are used to sense weak electronic fields underwater. (See the Lorenzi pores on a shark’s snout here. Each pore connects the water to sensing cells surrounded by a gel substance that conducts electrical signals to the shark’s brain. Using this organ, a shark can detect prey that it cannot see, smell, or sense in any other way.
Fish are amazing animals that have thrived underwater for millennia. With the help of their specialized senses, they have perfectly adapted to interpret and react to the world beneath the sea, just as we have above.
Image of Ampullae of Lorenzi on a Shark’s Snout courtesy of Wikimedia Commons
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