Fish in Their World
Fish are among the most diverse of all animals. They are found in a huge variety of shapes, sizes, and colors, each perfectly fitted to the role they fill within their habitat, enabling them not only to survive, but to prosper.
The way in which all animals, including fish, perceive their environment is through their sensory system. Their senses -- vision, olfaction (smell), gustation (taste), and pressure detection -- are shaped by the world in which they live and produce an impression of their surroundings.
How are fish adapted to their environment?
Fish are one of the most successful groups of animals on the planet. There are over 25,000 species of fish occupying every imaginable habitat, from inhospitable, sulphurous thermal springs to temporary pools in the tropics that fill and dry up on an annual cycle. In each case, the fish are adapted to meet the demands of their habitat. For example, a fish living in the turbid, muddy waters so typical of many South American and African rivers may rely less on vision than on its other senses, such as smell and taste. In fact, some species living in these conditions, such as the mormyrids, or elephant-nosed fishes, use electroreception -- a kind of inbuilt radar system -- to find their prey and to interact with others.
How do fish use the information they gather?
Like all animals, fish are continually gathering information about their environment. This information is received and decoded by the fish's brain, which then transmits a response. So if a nocturnal predator approaches under cover of darkness, any surrounding prey fish may detect a large pressure wave using their lateral line. This information will be transmitted to the brain via the spinal column. In response, the brain will then stimulate muscles to contract to cause the fish to swim away from the direction of the pressure wave and therefore the predator that produced it At the same time, the fish may gather other information about the predator -- it may smell it, for instance, or hear if any small stones are displaced by it as it moves from concealment.
Although the fish's brain receives all this information, the fish does not have to "think" about swimming away, the movement away from the predator is a simple response.
What gets a fish's attention?
Certain stimuli may gain the attention of a fish more quickly than others, so a small animal swimming actively through the water column might attract the attentions of a hungry fish more than if it hid in the substrate. Similarly, a faint smell of food may cause a fish to try to orientate itself towards the source of the smell, whereas once it closes in on the smell, the signal becomes stronger and stimulates the fish to switch into an intense feeding state. At this point it may be far less able to react to other signals. For this reason, the predators of guppies often time their attacks to when their prey has itself found food.