Fish are vertebrates (vertebrates have backbones) that live in water.
They are one of the six main groups of animals — along with invertebrates, amphibians, reptiles, birds and mammals.
The combination of gills, fins and the fact that they live only in the water make fish different from all other animals.
Fish live in almost every available body of water, including rivers, lakes, oceans and streams. The species of fish determines whether it lives in saltwater or freshwater.
Fish are so abundant in the world’s oceans, lakes and rivers that new species are constantly being discovered.
There are over 30,000 known species of fish.
Fish come in many shapes and sizes.
The world’s largest fish is the whale shark, which can grow to 12 meters (40 feet) long and weigh an average of 19,000 kilograms (42,000 pounds).
The world’s smallest fish is Paedocypris progenetica, and was only recently discovered in the last decade. Scientists discovered it living in the forest swamps on the Indonesian island of Sumatra. Mature females measure just 7.9 millimeters (0.31 inch).
Greenland Sharks are fish species with the longest known lifespan. An examination of 28 specimens in one study published in 2016 determined by radiocarbon dating that the oldest of the animals that they sampled had lived for about 392 ± 120 years (a minimum of 272 years and a maximum of 512 years).
Koi, a Japanese species of fish, allegedly living up to 200 years, though generally not exceeding 50 – A specimen named Hanako was reportedly 226 years old upon her death.
A tiny coral reef-dwelling fish called the pygmy goby (Eviota sigillata) is the shortest-lived fish. The pygmy goby lives an average of 59 days, pipping the previous record holder, an African fish which lives for just over two-and-a-half months.
Fish live at various water depths with some fish preferring deep water and others staying close to the surface.
World’s deepest fish – believed to be a snailfish – found in the western Pacific Ocean at a depth of 8,145 meters (26,722 feet).
The black marlin [photo below] is the fastest fish in the world – able to swim at a speed of 129 km/h (80 mph), followed by the sailfish at 110 km/h (68 mph).
Fish diets vary by species. There are some carnivorous (meat eating) fish, some herbivorous (plant eating), and many omnivorous (both meat and plant eating) fish.
Freshwater fish do not actively drink water, but absorb the water through their skin and gills. On the other hand, saltwater fish do actively drink sea water. Their gills process the water and take out the salt.
Fish are cold-blooded with the exception of Tuna family, the Mackerel shark family and the opah or moonfish.
Fish can drown in water if there is not enough oxygen in the water. Fish require a consistent intake of oxygen to survive. Their gills absorb oxygen from water molecules as water passes through the gill membranes.
In most fish respiration takes place through gills. Lungfish, however, possess one or two lungs. The labyrinth fish have developed a special organ that allows them to take advantage of the oxygen of the air, but is not a true lung.
Not all fish have scales. While scales are a characteristic of most fish, some fish, such as clingfish, lampreys and catfish, do not have scales.
Fish have sleep-like periods where they have lowered response to stimuli, slowed physical activity, and reduced metabolism but they do not share the same changes in brain waves as humans do when they sleep.
Some fish species can make audible sounds like clicks or croaks but most fish communicate with body language. Certain species will flare out their gill plates the same way a cat would raise the hair on their back.
Fish can form schools containing millions of fish. They use their eyes and something called a lateral line to hold their places in the school. The lateral line is a row of pores running along the fish’s sides from head to tail. Special hairs in the pores sense changes in water pressure from the movements of other fish or predators.
The fish in the middle of a school control the school. The fish on the outside are guided by those in the middle.
The primary means of fish reproduction are egg laying and live bearing. Egg-laying fish release eggs that are later fertilized. Live-bearing fish internally fertilize the female, who carries the offspring within until they develop and are birthed.
Some fish species are known to change sex during the course of their lives.
Some Fish, like the Triggerfish, can swim backward.
Seahorses are the only fish that regularly swim upright.
On average, flying fish can glide 50 meters (160 feet), but have been known to glide as far as 400 meters (1,300 feet). And they can reach heights up to 6 meters (20 feet).
Fish do not have eyelids, with the exception of the shark.
Puffer fish are thought to be the world’s deadliest fish. Their poison is contained in their flesh and one puffer fish carries enough toxin to kill 30 people. Despite this, the meat of some species is considered a delicacy in Japan.
A robotic fish was accepted by other fish and became their leader during an experiment by New York University.
In three decades, the world’s oceans will contain more discarded plastic than fish when measured by weight, researchers say.
Fossil evidence suggests that fish have been on Earth for about 530 million years.
Fish feature prominently in art and literature, in movies such as Finding Nemo and books such as The Old Man and the Sea.
The fish is used often as a symbol by Christians to represent Jesus, or Christianity in general; the gospels also refer to “fishers of men” and feeding the multitude.
In the dhamma of Buddhism the fish symbolize happiness as they have complete freedom of movement in the water. Often drawn in the form of carp which are regarded in the Orient as sacred on account of their elegant beauty, size and life-span.