Hundreds of thousands of years ago, long before man appeared on the earth, fish were already swimming in the oceans. At that time they were the most highly developed form of life in existence. Today fewer creatures on earth are as interesting and unique as fish. They are cold-blooded vertebrates, with a backbone and a jaw, and they have fins instead of limbs.
With a few exceptions, fish have survived in an environment completely different from ours – in water. Like all other living organisms, fish have survived by acquiring information about the environment through the senses – senses of sight, smell, taste and touch – and a few other special senses that help them live underwater.
Touch and the Lateral Line System
If you’ve ever watched fish swimming in an aquarium, you’ve probably noticed that they rarely bump into anything. They may be swimming directly toward the side of the aquarium, but at the last second they make an abrupt turn and swim merrily on their way. What you are observing is the result of a special sense organ called the lateral-line system, which provides fish with information about its external world.
The lateral line system is a collection of small sensory patches (neuromasts) underneath the scales on the skin, or just under the skin. They can be seen as a line of small pores that runs down the sides of a fish from head to tail. These pores are not restricted to the lateral line, however, but are also distributed all over the fish, particularly on the head.
The pores serve to detect pressure changes in the surrounding water. A fish sets up his own pressure wave in the water that is detected by other fish. He also sets up a pressure wave in front of himself, and when he swims near a rock or the wall of the aquarium, these pressure waves are distorted, and changes are quickly detected by the lateral line system, enabling the fish to swerve or to take other suitable action. It is this ability that allows a school of fish to change direction at the same time without bumping one another.
Not all neuromasts, however, come in contact with the water. Some are arranged linearly to form lateral lines to give the fish an actual sense of touch. Nerve endings throughout the skin react to the slightest pressure and change of temperature.
Sense of Smell and Taste
A rose by any other name would still smell as sweet – except underwater. But fish have a good sense of smell and often hunt by smell. In most fish, the organs of smell consist of two pouches, one on either side of the snout, which are lined with nerve tissue that is highly sensitive to odors from substances in the water. A nostril at the front of each pouch allows water to enter the pouch and pass over the tissue, then to leave the pouch through a nostril at the back. Unlike humans, however, there is no connection between the nostrils and the throat. Some fish, like salmon use their sense of smell when migrating or traveling from one place to another.
Just like in humans, taste and smell work together through nostrils and taste buds located in the mouth. However, many fish have taste buds located on the their heads, on the barbels and on the outside of the body. These taste buds have the ability to distinguish the difference between sweet, sour, salty and bitter.
We take sound for granted while moving through the blanket of air we call atmosphere. But we hear very little under water. Fish have ears, too, although you can’t see them. They are located within their bodies as well as in the lateral line system. Fish can hear sounds in the water and can probably hear sounds made on shore if they are loud enough.
What makes them able to hear underwater is that they have no outer ears or eardrums to receive sound vibrations. Sound vibrations are first transmitted from the water through the fish’s body to its internal ears, which are divided into two sections:
- An upper section (pars superior) – The pars superior is in turn subdivided into three fluid-filled semi-circular canals, which provide the fish with a sense of balance. The canals also have sensory hairs that detect the rotational acceleration of the fluid.
- A lower section (utriculus) – This is the section that provides the fish with the ability to hear. It contains two large otoliths (ear stones) that vibrate with sound and stimulate surrounding hair cells.
No matter how good your eyesight and how clear the water, the underwater world looks hazy, distorted and sometimes downright murky to humans. This is because when you’re under water, the cornea isn’t nearly as good at bringing light to a focus as it is on land. Fish have special adaptations that allow them to see underwater and at great depths, and some fish even see in color. Because of the way light is refracted in water, fish have a wide “cone of vision” of about 83 degrees. It is like looking up from the base of an imaginary funnel. As a fish goes deeper, his window to the outside world grows. Even more interesting, if the water surface is relatively smooth, a fish can look up and see a mirror-like image of the bottom. This allows him to be aware of either prey or predator beneath him.
Most fish, because they cannot turn their heads, can see to the right and to the left at the same time. This gives them all-round vision. To allow them to judge distances they have a small area in front in which they can focus on with both eyes.
Fish also lack eyelids. Since the role of eyelids in land animals is to keep the eyes moistened and to protect them from harsh sunlight, there doesn’t seem to be much need. Fish’s eyes are kept moistened by the flow of water and there is seldom any bright sunlight to hurt their eyes. However, in an aquarium, sudden bright light can frighten them or damage their eyes. It is best to introduce light gradually by turning on a room light first before turning on the aquarium light. Provide plenty of hiding places, too.
Some fish have unusual adaptations for life in the sea. Adult flatfish have both eyes on the same side of the head because they spend most of the time lying on the ocean floor. Some fish that live in total darkness in caves or on the ocean floor have eyes but are blind; others lack eyes completely.
In the weightless underwater world, human divers use equipment known as buoyancy compensator devices (BCDs), which allow them to maintain neutral buoyancy. These are vests that contain bladders of air that can be adjusted to allow the diver to go deeper into the water or rise to the surface.
Fish have their own built-in device called the swim bladder, which is a gas-filled sac in the abdomen that helps them to maintain buoyancy in the water. Like a BCD, the sac inflates if the fish needs to be more buoyant and deflates if the fish needs to be less buoyant. Goldfish and some other fish are members of the cyprinid (minnows and carp) family and are physostomous, which means there is an open connection between the esophagus and the swim bladder. The bladder is called a pneumocystic duct, and it allows additional adjustment of buoyancy by letting air out through the digestive tract.
It was believed that before fish evolved this buoyancy organ, they would have needed to swim constantly in order to maintain their depth. Many experts on fish evolution believe that the eventual development of the swim bladder allowed fish to swim slower and become more maneuverable and agile, and that these free swimming habits were accompanied by changes in body form and fin shapes, forms and function.
Our fish are really not so different from us – they see, taste, smell and hear – but they do it in an interesting underwater world. Understanding how your fish relate to their world will help you provide them with the best care.