Last Reviewed and Updated on January 29, 2023
Fish are a very diverse group of animals with many unique and unusual characteristics. Read through these facts about fish. We guarantee some of them will feel out of this world.
1. Most, but not all, fish are cold-blooded
Starting off this list of facts about fish is fascinating, demonstrating just how diverse fish are. Almost all fish are cold-blooded. Being cold-blooded was one part of the definition of what fish are. Until it was discovered (in 2015) that opah fish are warm-blooded. At the moment, they are the only known fully warm-blooded fish.
2. Fish have the most diversity of all vertebrates
With over 34.000 species described and many yet to be discovered, fish exhibit far greater species diversity than any other group of vertebrates (animals with a backbone).
3. Some fish have a stomach, and some don’t
Fish anatomy is interesting; not all fish are made the same or even similar, and their stomachs (or lack of) are just one of the examples. Lampreys, hagfishes, chimaeras, lungfishes, and some teleost fish have no stomachs. Their esophagus, a part of the digestive system that generally connects the throat to the stomach, leads directly into the intestine.
4. Most fish lay eggs, but some have live offspring
Most fish lay eggs, but quite a few species of fish give birth to live young. The majority of sharks give birth to live young, for example. Quite a few popular aquarium fish give birth to live young as well; they are known as livebearers. Some of the species include guppies, mollies, platies, and swordtails.
5. There are only two things that define a fish that all fish have in common; they live in water and have gills
Most fish have swim bladders, are cold-blooded, have fins and scales, and most have backbones, but this is not true for all species of fish.
There are only two things that are predominately in the domain of fish. All fish live or, at the very least, spend some of their time in the water, and all fish have gills (but some have lungs in addition to gills).
6. Most fish don’t have eyelids
Most fish don’t have eyelids, some fish have adipose eyelids (transparent eyelids covering a part or all of the eye), and sharks have eyelids; they don’t blink, though, and the eyelids don’t completely close.
7. Not all fish can swim backward
Swimming forwards is something wast majority of fish do most of their lives, and all for at least a portion of their lives, while swimming backward isn’t something all fish can do. Sharks aren’t capable of swimming backward.
Most fish won’t swim backward, though, unless it’s really necessary.
8. The giant whale shark is the biggest fish in the world
The whale shark (Rhincodon typus) is the largest fish in the world. Basking sharks and the great white shark follow it.
The largest non-shark fish is the giant oceanic manta ray, followed by southern sunfish.
9. Handfish walk along the seafloor
Rather than swimming, the handfish move around by walking on the sea floor. Their modified pectoral fins have the appearance of hands.
10. Seahorses, Sea dragons, and Pipefish are the only known animals where the male gives birth
Seahorses and their close relatives, the sea dragons, and pipefish are the only known animals where the male gives birth to their young.
The female of these species transfers her eggs to the male’s abdominal pouch; the eggs get fertilized there. The males incubate the eggs until young fish are ready to emerge into the world.
11. If they are left in the dark for too long, goldfish lose their color
Goldfish have a special pigment in their skin, and the pigment reacts to light. They lose their gold color if they don’t get enough light exposure.
12. All clownfish are born male
All clownfish are born as males. They can change their sex; however, they only do so when a dominant female is gone and there is a need for a new one. Once they change their sex, the change is irreversible.
They are one of the most known fish species with this ability, but over 400 fish species have the same or similar ability.
13. Some sharks would die if they stopped swimming
Some, but not all, shark species need to be in constant motion to keep the oxygen-rich water running through their gills. If they stopped, they wouldn’t be able to breathe through their gills.
Species like the great shark, whale shark, and hammerheads are a few that need to swim at all times.
14. The mudskipper mates and lays eggs on land
These fish can move efficiently both on land and in water. When on land, they breathe using water that is trapped inside their gill chambers. They also breathe through their skin as long as the skin is wet.
These fish spend a lot of their time on land (some speculate it could even be three-thirds of their lives); they also mate and lay their eggs on land,
15. Not all fish breathe through gills alone; lungfish also have lungs
Lungfish are unique freshwater fish that are best known for their lungs. Like all other fish, lungfish also have gills.
The lungs of lungfish are a complex structure; they are subdivided into smaller air sacks which maximize the surface area for gas exchange. Most lungfish have two lungs, and Australian lungfish has one.
Australian lungfish is the only one that can breathe through its gills without the need for air from the lungs; all other species of lungfish don’t get sufficient amounts of oxygen through their gills alone; they need their lungs to breathe – they have to breathe the air above water to survive.
16. Hagfish are one of the slimiest animals in the world
Hagfish are eel-shaped fish that are capable of producing insane amounts of slime. They produce this slime as a defense mechanism; if caught by a predator, a hagfish can quickly produce a large amount of slime and escape.
17. Croakers are the loudest fish
As their name suggests, fish from the family Sciaenidae, commonly called croaker fish or drums, make throbbing or drumming sounds.
Gulf Corvina and Atlantic corkers might be the loudest from this fish family.
When they are in a collective, their mating sounds can be heard from above the water.
18. Fish have a relatively small brain
Fish have a small brain-to-body ratio compared to other vertebrates (animals with backbones). That said, some fish species demonstrate good cognitive abilities; some studies show many fish’s cognitive abilities exceed that of many vertebrates.
19. Fish in the Antarctic have antifreeze proteins
To survive, Antarctic fish had to adapt. They developed proteins that act as an antifreeze, allowing the fish to avoid freezing in their habitat.
20. Seahorses are the slowest swimming fish
Seahorses, specifically the dwarf seahorse, according to the Guinness World Records, are the slowest-moving fish of them all.
21. Sailfish and marlin are the fastest fish in the sea
Sailfish and marlin are the fastest fish in the sea. Species from these fish groups compete for the title of the fastest fish in the world, with some studies placing one group at the top and others at the other.
22. The six-eyed spookfish has six eyes
Not all fish have a single pair of eyes. The six-eyed spookfish has the most eyes; as its name suggests, it has three pairs of eyes.
Fish with four eyes can be found in the ocean as well.
23. The American shad fish can have well over 1000 bones
Fish, on average, have 150 bones, some species have less, and some have more. But the American shad is an absolute winner as it can have over 1000 bones (usually between 800 and 1000), with claims the number can be as high as 3000.
24. Pacific lingcod has about 550 teeth
Fish come in all shapes and sizes and with a wide range of number of teeth, from zero to hundreds. Pacific lingcod has 555 teeth, making it one of the world’s toothiest animals.
25. Hagfish have four hearts
Most fish have one simple 2-chambered heart. Hagfish aren’t one of those; they have four hearts; one serves as the main pump, and the other three are accessory pumps.
26. Some fish are venomous, and some fish are poisonous
There is a difference between being venomous and being poisonous. Venomous is when the toxin is injected into you; so venomous fish are those that have the means to inject venom into bodies of other fish and animals, be it spikes, barbs or stingers. Poisonous is when the toxin gets into your body either by inhalation, absorption through the skin, or when you eat the venomous animal.
27. Tiger pufferfish is one of the most poisonous fish (but people still eat them)
This fish contains lethal amounts of the poison tetrodotoxin, a strong neurotoxin, in their internal organs, especially the liver and the ovaries. It is considered a delicacy in Japan. The consumption of liver and ovaries is forbidden, and only specially licensed chefs can prepare and sell fugu (the Japanese name for these fish).
Several people die every year eating these fish as people underestimate the amount of poison in the consumed parts of the fish.
28. The reef stonefish is the most venomous fish
The reef stonefish is considered to be the most venomous fish in the world. It has 13 spines on its dorsal area, each of which has two venom sacs. The spines are sturdy and sharp and can pierce through boot soles. It doesn’t take much venom for the dose to be fatal; it only requires 18mg of venom, which an average reef stonefish can release with six of its thirteen spines.
29. When it comes to sea animals, sharks aren’t responsible for most deaths in humans
Sharks are one of the most feared animals on the planet, and they should be as many are dangerous and can attack a human. Humans are pretty good at avoiding them, and the average number of fatalities per year is five for all shark species (with the trend generally decreasing).
The deadliest animals in the sea, although not fish, are jellyfish, with up to 100 fatalities per year.
30. The white-spotted pufferfish has a complex courtship display
This fish is known for its unique and complex courtship display. This involves creating large geometric circles (nests) in the sand that attract females.
The females will evaluate the structure of the nests, and it will play a huge role in choosing their mates.
Their nests were first spotted in 1995, but it was only in 2013 that it was discovered that these fish are the artists behind them.
31. Electric fish can generate electric fields
An electric fish is any fish capable of generating an electric field.
32. Some sharks are bioluminescent
There are many bioluminescent creatures in the sea, but it may come as a surprise that some sharks are also bioluminescent.
The kitefin shark is the largest known bioluminescent vertebrate in the world.
33. Flying fish can glide up to 650 feet / 200 m
Many fish can leap out of the water, but none can glide as far as flying fish can.
34. Amazon molly is an all-female species of fish, but they need a male to reproduce
This one is certainly a contender for one of the weirdest facts about fish. All male or all female at birth species isn’t something super uncommon in fish; like previously mentioned clownfish some fish species have the ability to change their sex during the course of their life.
But this is not the case with Amazon moly. Amazon mollies are born female and don’t change their sex during their life. They do, however, need a male to reproduce. These fish mate with males of other closely related species.
35. Female sunfish release up to 300 million eggs
More than any other vertebrae.
36. In some species of angler fish, when mating, the male merges with the female (and stays merged forever)
Talk about mating for life. These fish fuse their bodies to mate. The male bites the female and latches onto her until his body fuses with hers.
37. Lungfish are more closely related to tetrapods than to other fish
Lungfish, the odd fish species with lungs and gills, are more closely related to tetrapods (four-limbed vertebrate animals) than other fish.
38. Red-lipped batfish look like frogs with red lipstick
One of the most unique-looking fish out there, for sure.
39. Most abundant fish is the bristlemouth
With estimates ranging from trillions to quadrillions, this fish is the most abundant fish in the world.
40. The number of scales a fish has doesn’t change (or increase) throughout its life
The number of scales fish are born with stays the same throughout their lives. They may lose some due to damage or disease.
41. A group of fish is called a school or a shoal
The sizes of schools range from just a couple of fish to thousands of fish in a single school.
There is a difference between a school and a shoal; a shoal is less organized and can temporarily consist of different fish, while a school is a single species group.
42. Greenland sharks have the longest lifespan of all fish
It is unknown what the maximum life span of Greenland sharks is; some speculate it could even be about 500 years. Even if they don’t reach this impressive milestone, they still have the longest lifespan of all fish.
The longest recorded lifespan was 392 years.
43. The shortest lifespan seven-figure pygmy goby
With its lifespan of only about 59 days, the seven-figure pygmy, also known as the sign eviota or coral reef pygmy goby, has the shortest lifespan of known fish.
44. Some animals with the name fish in their names aren’t fish at all
Penis fish, jellyfish, and starfish aren’t fish at all.
45. Fish can “drown” too
They don’t drown in the same way that non-water animals and aquatic mammals can, but fish can also suffocate in water. This can happen when fresh water doesn’t contain enough dissolved oxygen.
46. Fangtooth fish has the largest teeth-to-body ratio of all fish
These harmless fish have the largest teeth considering the size of their body.
47. Saltwater fish need to drink water
One of the least expected facts about fish is the fact they need to drink water. Saltwater fish need to drink water to stay hydrated. The salt concentration in saltwater fish is lower than in the surrounding water, so water leaves their bodies through osmosis. With freshwater fish, things are the opposite; they have a higher concentration of salt in their bodies, so they get water through osmosis (and have to pee more to get rid of excess water).
48. A fish school is highly structured
Each fish is an individual, yet they act as a single organism when they are in schools, even those with thousands of individuals. What’s even more fascinating is that schools don’t have a leader. Every fish in the school needs to coordinate and react to the movements of other nearby fish.
How the movements are coordinated and decided upon is still up for debate.
49. Fish can (rarely) rain down from the sky
The lluvia de peces, also known as aguacero de pescadois a phenomenon occurring yearly for more than a century in Yoro, Honduras. It is a peculiar phenomenon in which fish are said to fall from the sky, and this occurs up to four times a year.
The explanation is most likely meteorological; the phenomenon is most likely caused to be a result of intense winds and waterspouts (vortex) forming on the body of water, grabbing fish from the water and making the fish “rain from the sky.” This is the case with other incidences of raining fish, although the “raining fish” in Yoro could also have other explanations.
50. Many fish species still remain undiscovered
One of the coolest facts about fish is that many fish species are still undiscovered. The oceans are still the least explored parts of our planet, and new species of fish are discovered all the time.
If you liked this list of facts about fish, you might also like this list of the 100 weirdest facts about animals.