Fish live in every part of our world except the extremely coldest regions and areas that lack sufficient oxygen, such as stagnant ponds. There are almost 30,000 known species of fish living today. Most fish live in bodies of water, such as ponds, rivers, lakes, and oceans. There are four kinds of fish: jawless, cartilaginous, lobe-finned, and ray-finned.
All fish have a similar body shape with a streamlined body for moving through the water. Fish come in all shapes and sizes; their weight varies from the small bristlemouth that only weighs 1 ounce to sharks that can weigh up to one ton. Light is absorbed through the water by the red blood cells in most fish; therefore most fish appear black.
However, some deep-sea species produce their own light. A few species of fish do not need oxygen because they breathe dissolved oxygen or organic compounds.
Fish are able to produce their own food because of an adaptive organ known as the liver that has many functions, including the production of bile which aids in digestion, detoxification, and the storage of energy-rich compounds. The digestive system is adapted for both carnivorous and herbivorous diets.
Each jawless fish has two barbels, one above each eye, which act like fingers. Cartilaginous fish have gills or five pairs of gill slits on each side of the body; lobe-finned fish have internal lungs and a swim bladder used for buoyancy control; ray-finned fishes’ respiratory system consists primarily of a primitive lung with a labyrinthine cavity beneath it.
The heart is composed of two chambers and pumps blood to the gills where it is oxygenated and then returns back to the heart. Many fish have an internal organ known as a swim bladder that adjusts and controls buoyancy by taking in gas when the fish is above water or releasing gas when the fish is underwater.
Some fishes, such as sharks, maintain their buoyancy by having oil-filled livers. A few species of fish use special organs such as anglerfish that can emit light for attracting prey or lures that look like small creatures.
Do fish see color?
Scientists have many answers to this question. One answer is that they can’t see all the colors humans can, but they do see some. Some species of tropical fish are even known to be able to see into two parts of the ultraviolet range where human eyes only perceive darkness.
Fish’s pupil shapes also affect their ability to determine different colors. For example, fisheye lenses (fish with pupils shaped like a fisheye lens) let them pick up on polarized light and make out other differences in contrast. Fish also don’t “see” what we call gray; they may not perceive it as an absence of color or take it for another hue entirely. So in a nutshell, yes but not in a way humans do!
Fish vision adaptations
What is also important to note is that fish see things differently than we do. Fish eyes are adapted for seeing underwater, and it’s not just the colors of light they perceive differently than humans – it’s how they process images and their surroundings relative to what we perceive. But some species of fish can see ultraviolet light – the same type of light we use in sunscreen – and this added perception may help them protect themselves from sunburn.
If you ever thought shielding your skin with zinc was a bit extreme (and wished there were another way), think again!
There are four common pupil shapes: round, vertical (or slit-like), diamond-shaped, and fisheye. Humans have round pupils; some nocturnal animals have vertical-slit pupils, and some deep-sea fish have diamond-shaped ones.
Fish with fisheye pupils (like goldfish and their domesticated cousins) can see clearly in almost every direction thanks to the pupil’s shape: its horizontal diameter is larger than the vertical diameter, which lets them pick up on polarized light – a type of light that has been scattered from the sun or from manmade sources such as water reflecting off roads, boats, etc. This means they can sense things most other species would miss, including their prey hiding just below the surface of the water.
Fish pupil shapes vary depending on whether a fish lives in water or air: round pupils work best underwater because vertical slits don’t let enough light enter the eye; however, a vertical slit works far better in the air because a round pupil lets too much light into the eye.
Fish vision limitations
What’s also important to note is that certain species of fish have specific types of blind spots. For example, sharks and some other predatory fish have small holes at the back of their retinas through which blood vessels pass from behind to in front of the retina.
This structure can block out considerable amounts of light from entering these kinds of fish’s eyes – whether from above or below – making it difficult for them to see objects directly below them or directly above them, where more sunlight would fall.
However, this type of vision comes with a big advantage: bioluminescent organisms, such as anglerfish and flashlight fish, use this hole to their advantage, luring prey with the “light” that comes out of their eyes.
Be aware that fish have more senses that go beyond eyesight. It’s also the last sense they employ to identify potential food sources. They can smell and feel waves in the water. They generally track nearby food sources with their other senses even when they utilize their eyesight, it’s typically dependent on movement rather than color.
This is why anglers suggest mixing your retrieve before you make a mistake with your fly selection. It’s all about placement and movement.
If fish come in to look at your fly only to abandon it then, it could be the color of your fly. If you’ve ever been fishing for crappie or trout before you’re aware that a red or a black one could be the key to determining the difference. Don’t be afraid to try the various choices.