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What Are the Basics of Fish Physiology?

Debra Barnhart
Debra Barnhart

Fish are largely distinguished from other animals by their adaptation to their environment, which is of course water. Fish physiology differs in some key ways from the physiology of animals that inhabit land. The major differences lie in how fish breathe, or obtain oxygen, and how they move through water. Fish have also made other remarkable physiological adjustments to their environment.

Like animals on land, fish need oxygen to survive. The problem is that water has only two percent of the amount of oxygen contained in air. In addition, oxygen levels decrease as water gets warmer, and polluted or stagnant water has less oxygen as well.

An Atlantic salmon fish.
An Atlantic salmon fish.

Fish get oxygen mainly through their gills. By opening and closing their mouths, fish move water over their gills, which are filled with thousands of tiny blood vessels that absorb oxygen and send it into to the bloodstream. A few fish can take in oxygen in other ways. For example, a tarpon — a large saltwater fish, can swim to the surface and take in a little extra oxygen when needed. The lungfish has gills but gets a lot of its oxygen by swallowing air that fills up a sac that is somewhat like a lung, thus its name.

Freshwater types of fish absorb water through their skin and gills.
Freshwater types of fish absorb water through their skin and gills.

How fish move through water is a pretty remarkable element of fish physiology as well. Since water is dense fish must be very strong in order to move through it. Fish have a lot of muscles that enable them to swim. Fins also allow fish to move forward and backward, and their strong tailfins help propel them through the water. The majority of fish have seven fins, but some have six or eight.

Fish absorb oxygen through their gills.
Fish absorb oxygen through their gills.

The swim bladder, or air bladder, is another important element of fish physiology. Fish remain buoyant and move up and down in water by decreasing or increasing the amount of air in their swim bladders. Some fish also use the swim bladder to intensify sound.

Fish physiology has adapted to the environment in other important ways. Most fish have a protective covering of scales made out of calcium, which protect fish them injury and sickness. Another important adaptation has to do with staying hydrated and maintaining a proper salt balance, which is a special challenge for saltwater fish. Ingesting too much salt is not good, so saltwater fish drink water and excrete salt through their urine and gills. Freshwater fish take in water through their gills and skin rather than drinking it.

Frequently Asked Questions

What are the key components of fish anatomy?

Fish anatomy is specialized for aquatic life. Key components include gills for oxygen exchange, fins for movement, a streamlined body for efficient swimming, and a swim bladder for buoyancy control. Their scales provide protection and help in osmoregulation. Internally, their organs are compact to fit within their streamlined bodies.

How do fish breathe underwater?

Fish breathe underwater through gills, which extract oxygen from water. Water enters the mouth, passes over the gill filaments, and exits through gill slits. Oxygen diffuses into the blood in the gill filaments, while carbon dioxide is expelled. This process is known as 'branchial respiration' and is highly efficient in extracting oxygen from water.

What is the function of a fish's swim bladder?

The swim bladder is a gas-filled organ that helps fish maintain buoyancy at different depths. By adjusting the gas volume within the bladder, fish can rise or sink in the water column without expending energy. This organ allows fish to remain stationary at a particular depth or to ascend/descend with minimal effort.

How do fish maintain their internal salt balance?

Fish maintain their internal salt balance through a process called osmoregulation. Freshwater fish constantly absorb water and excrete dilute urine to expel excess water while retaining salts. Saltwater fish, conversely, lose water osmotically and must drink seawater, actively excreting excess salts through specialized cells in their gills.

Can fish sense their environment, and if so, how?

Fish have a keen sense of their environment. They use their lateral line system, a series of canals and sensory cells on their sides, to detect vibrations and pressure changes in the water. Additionally, they have well-developed eyesight, taste, and olfactory senses to locate food, avoid predators, and navigate their aquatic habitats.

What adaptations do deep-sea fish have for survival in extreme environments?

Deep-sea fish possess remarkable adaptations for survival in extreme environments. They often have specialized eyes or bioluminescence for navigating the darkness, slow metabolisms to conserve energy, and high-pressure resistance. Some species have expandable stomachs to consume large prey, while others have reduced skeletal structures to withstand high pressure, according to marine biologists.

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    • An Atlantic salmon fish.
      By: Witold Krasowski
      An Atlantic salmon fish.
    • Freshwater types of fish absorb water through their skin and gills.
      By: Evgeniy Drogozhilov
      Freshwater types of fish absorb water through their skin and gills.
    • Fish absorb oxygen through their gills.
      By: nupsik284
      Fish absorb oxygen through their gills.