One of the world facts is that the earth is 71% water and 29% land. Man and many other organisms that live on land are adapted to live on land. However, life on earth is not limited to those of us living on land. The water too contains living organism such as the fish. The water can either be marine (salty), brackish (not too salty) and freshwater which supports different organisms according to their respective adaptations. In this article, I will talk about Fish and how they are able to survive in water which we cannot.
What is fish?
Seahorse - CC0 Creative Commons: pixabay
An animal can be called a fish if they live in water, breathe with gills throughout their lives, have a backbone, and have appendages (think of hand) in the shape of fins. This sentence is the biological definition of a "true fish" , also known as "finfish". This definition distinguishes "true fish" from the definition in Fisheries and Aquaculture which defines fish to include any commercial animals harvested from the water. Fisheries' definition of fish includes mollusks like snails and oysters as fish.
Most fish are coldblooded (that is, their body temperature is controlled by the prevailing ambient temperature) except for some large fish like the white shark and tuna which can maintain a higher temperature than the surrounding.
Example of fish includes the common fish you know already and some unusual ones like the seahorse. (Seahorses are not horses that live in water. They are fish.)
What is not a fish?
Starfish - CC0 Creative Commons:pixabay
Most of the animals that people call fish are actually not fish. You cannot call an animal a fish because they live in water. The definition of "true fish" I gave earlier contains the checklist for what truly is a fish. "True fish" lives in water, breath with gills, has a backbone, and "hands" modified as fins.
So, a starfish is not a fish because it has the suffix "fish" attached to its name. They belong to a class of animals called Asteroidea. Also, a crayfish is a crustacean and a jelly fish belongs to the grouping called medusozoa. Both of them are not fish.
Some other aquatic animals that most people confuse for a fish are dolphins and whales. They are both aquatic mammals with the characteristics of mammals that live on land. Here is a video clip of a young whale calf feeding on its mother's mammary gland.
How so fish survive under water?
Just as various land organisms have learned to adapt to their environment, fish too have developed adaptations to survive in water. These adaptations can be anything from respiration, reproduction, locomotion or even how they obtain their food. Fast fish have enough speed to catch their prey as food while slow-moving fish have devised a way of camouflaging and waiting for their food.
I'll limit this post just on the adaptation of fish for respiration and buoyancy.
Adaptation for respiration
Fish like land animals require oxygen for respiration. Land animals have access to more oxygen and they make use of their lungs to absorb oxygen from the air. However, unlike the land animals, fish depends on the oxygen that has been dissolved in the water. This is called the dissolved oxygen (DO). Fish through the use of their gills are able to absorb the DO from the water. A fish Gill is situated at both sides of its head. The Gill is supported by the gill arch which bears the Gill rakers (preventing food particles from entering the gills) and the filaments. They are threadlike and well supplied with blood capillaries and have a large surface area. The fish through the gills are able to exchange gases (oxygen and carbon dioxide) during respiration. The fish constantly takes in a fresh supply of water into its mouth which bathes the gills where the DO is extracted and the water passes out through the Gill opening called operculum.
Some other fishes have accessory breathing apparatus that enable them to breathe in air directly from the atmosphere. Such adaptive features become useful for fish that lives in shallow water and muddy water with little DO. African catfish have an accessory respiratory organ which can absorb oxygen directly from the air which allows the fish to survive out of water for many hours or for many weeks in muddy marshes. ref. Another example is the African lungfish.
Adaptation for buoyancy
If you know how to swim, you may realize it requires some amount of effort to keep floating in the water. Fish require some effort to keep them floating in water to reduce the amount of energy expenditure required for swimming. Bony fish are able to achieve the buoyancy they need by the means of an organ known as the gas bladder or swim bladder. The fish control their body density by controlling the volume of air in their gas bladder with air. The fish body density reduces when the amount of air in the gas bladder is increased. Some other fish are able to maintain their buoyancy by storing up fats and oils in their body. This makes them maintain a fairly constant level of buoyancy. Cartilaginous fish like sharks don't have the gas bladder but they maintain their position in water through a dynamic lift.
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