Historically fish have existed for 500 million years and have been idolized as deities in certain religious groups, featured in some of the most popular artwork in history, made the center of countless stories in mythology, become household pets for both the middle and upper class, gained celebrity status in Hollywood (movies such as finding Nemo) and attracted millions of viewers at live aquarium shows and exhibits.
Fish can be found swimming throughout the worlds oceans and coastal shores from tropical climates near or at the equator to the northern/southern polar hemispheres.
Many species of fish are divided and characterized between the oceanic and freshwater ecosystems.
Unlike marine mammals most fish are cold-blooded which allows their bodies to adapt to various aquatic climate changes.
Several characteristics separate fish from other types of marine life.
For example fish do not breathe air; they use gills.
Gills are respiratory organs that aquatic life forms such as fish use to pull oxygen from water instead of breathing air.
Another characteristic that separates fish from marine mammals is the design and function of their spines.
Fish (in most cases) are designed with streamlined bodies to allow them to move quickly through the water and swim forward by moving their body from left to right, while marine mammals such as whales, dolphins and porpoises swim by moving their tail and flukes up and down.
Many fish have also developed exoskeletons or scales to aid in protecting them from predators.
Not all fish fit the stereotype explained above; some examples of different species of fish include starfish, jellyfish, shellfish, crayfish and others.
Each of these species carries its own unique physiological characteristics that differ very much from the traditional identity fish have become commonly known by.
As an entire species fish can differ drastically in terms of size, speed, diet, function and habitat.
Some examples include comparisons such as the sea-horse which moves at an average speed of 0.5 mph vs the sailfish which can reach speeds of 70 mph and the Dwarfgoby which measures in at 8-10 mm in length vs the whale shark which can reach a length of over 50 ft..
The length at which a fish can survive above water varies significantly from a few minutes up to several weeks depend on the species.
Fish such as the mudskipper are able to navigate on land as well as in the ocean and have the ability to pull oxygen from the air allowing it survive above water for extended periods of time, lasting up to several days.
Other fish are able to survive for weeks in damp environments without water by entering a state of hibernation until water returns.
Over many millennia different species have adapted a variety of mechanisms that are used to extract oxygen from the air from absorbing oxygen directly through the skin to the evolution of accessory breathing organs.
The ability to breathe air has proved very useful and allows fish to survive in oxygen depleted waters and murky swamps.
In terms of diet small fish can live on a diet primarily containing plants and small aquatic organisms while larger fish consume smaller fish, squid, crab, larvae and plankton.
Most fish have developed acute senses and are very sensitive to smell and taste.
Many species are also able to see in color and some can detect ultraviolet or polarized light.
Small receptors allow fish to detect subtle changes and vibrations in the current which can alert them of nearby fish as well as dangerous prey.
It is currently estimated that over 97% of fish reproduce externally by laying eggs as compared to the majority of marine mammals that carry their offspring internally (feeding their fetus nutrients through the placenta) until birth.
Many eggs can be laid during reproduction allowing the fish to bear lots of offspring while most marine mammals will only produce one (occasionally two) offspring every 1 – 5 years.