Fascinating Facts About Fish: Are Fish Animals?

Despite fish being the oldest vertebrates on our planet, humans discover new species and learn interesting facts about them every year.

Animals are a kingdom of multicellular living organisms that can move and feed on ready-made organic substances. But do fish belong to this kingdom?

Are Fish Animals or Not?

Fish are aquatic vertebrates belonging to the superclass of jawed vertebrates, which is further divided into two classes: cartilaginous fish and bony fish. They are animals by all characteristics of the kingdom: they absorb oxygen dissolved in water and actively move using paired limbs—fins.

Currently, there are about 35,000 species of fish, and this number increases every year.

How Fish Differ from Other Animals

Despite the characteristics of the animal kingdom, fish differ significantly from other classes. Water is their only habitat.

They are cold-blooded creatures without hair and can only breathe in water. They also have organs not found in other classes: a swim bladder, lateral line, and gills. However, their eyes lack eyelids. Their reproductive method is also different: most fish lay eggs.

Interesting Facts About Fish

1. The largest fish in the world is the whale shark. A female, recorded in the Guinness World Records, was caught off the coast of India on May 8, 2001. She measured 18.8 meters in length.
2. The smallest fish in the world is the Paedocypris fish, which lives in the warm waters of the Great Barrier Reef. Females of this species are relatively large—about 40 mm, while males average around 7 mm in length.
3. The Peruvian anchovy is the most numerous species. Its annual catch reaches about 11 million tons, out of a total global catch of 90 million tons.
4. The age of fish is determined by the rings on their scales—similar to tree rings. These rings also form on their vertebrae.
5. The oldest living fish is the coelacanth. Its ancestors lived about 400 million years ago, even before the dinosaurs. The species was considered extinct until a coelacanth was caught near the Comoros Islands in 1938.
6. The saddest fish on Earth is the blobfish. Sometimes it is also considered the ugliest. In water, it resembles its relatives, but on land, it begins to melt and turns into a jelly-like mass. This is due to its deep-sea habitat, where water pressure is extremely high, forcing it to evolve.
7. The salpa maggiore is a completely transparent fish, allowing it to be invisible.
8. The most poisonous fish is the fugu or pufferfish. Besides containing the highly dangerous toxin tetrodotoxin (yet it is still eaten in Japan), the fugu has another feature—it inflates into a ball covered with sharp spines when threatened.

9. Whales, dolphins, orcas, and sperm whales are often mistaken for fish. However, these animals are mammals. They are only related to fish by their habitat.
10. The seahorse, on the other hand, is often not considered a fish, but it is. It is the slowest fish, moving no more than 1.5 meters per hour. Unfortunately, their population is declining due to being considered a delicacy, especially their eyes and liver. Uniquely, male seahorses are responsible for carrying and hatching the eggs.
11. The seahorse has a relative—the leafy seadragon, which lives in shallow waters off the coast of Australia. Its body is covered with numerous appendages that resemble colorful rags, earning it the nickname “ragfish.” These adaptations are necessary for camouflage.
12. About 96% of all fish are bony. Sharks, chimeras, and rays—representatives of the cartilaginous fish class—make up only 4%.
13. Sharks have placoid scales, also called dermal teeth. These scales resemble teeth and are directed toward the tail, reducing water friction and allowing sharks to move faster.
14. The longest-living fish is the Greenland shark, with an average lifespan of 150 years. In 2016, a specimen caught was estimated to be 392 years old, meaning it was born during the reign of Tsar Mikhail Fedorovich, the first Romanov ruler in Russia, and during the Thirty Years’ War in Europe.
15. The sailfish is the fastest fish on Earth, reaching speeds of up to 109 km/h—faster than a subway train.
16. The snailfish Pseudoliparis belyaevi lives at a depth of 8,336 meters. If compared to land mountains, it has practically climbed Mount Everest.
17. The greatest diversity of fish species is found in the longest river in the world—the Amazon, with about 2,400 species. In comparison, the Dnieper River has about 70 species.
18. The black swallower is the most voracious fish. It has underdeveloped muscles and no ribs, allowing it to swallow prey two or even three times its size.
19. The barreleye fish has a transparent head, which helps it track prey with its green tubular eyes.
20. The flying fish can glide over water for about 50 meters, and with a tailwind, it can cover 400 meters at speeds of around 60 km/h.
21. The tripod fish survives at depths of up to 4 kilometers, under thousands of tons of pressure. Swimming there is difficult, so it has elongated fin rays—about a meter long—that allow it to anchor to the seabed and withstand currents.
22. The heaviest bony fish is the ocean sunfish. A specimen caught in 1908 weighed 2,235 kg and was recorded in the Guinness World Records as the heaviest bony fish on Earth. The ocean sunfish is also the most prolific—it can lay up to 300 million eggs at once!
23. The climbing perch, or Anabas, is known for its ability to climb onto land and even trees, surviving out of water for up to eight hours. Its gills can absorb oxygen from moist air.

24. The most ferocious fish is the piranha. With its sharp teeth, it can tear apart any prey. In 1981, in Brazil, a shipwreck on the Amazon led to about 300 people being killed by piranhas.
25. The most expensive fish is the bluefin tuna. A specimen weighing 222 kg, caught off the coast of Japan in 2019, was sold for $1.76 million.