Fish are aquatic vertebrates. They usually have gills, paired fins, a long body covered with scales, and tend to be cold-blooded. “Fish” is a term used to refer to lampreys, sharks, coelacanths, and ray-finned fishes, but is not a taxonomic group, which is a clade or group containing a common ancestor and all its descendants. Instead, there are 3 main classes, groups, or types of fish: bony fish (Osteichthyes), jawless fish (Agnatha), and cartilaginous fish (Chondrichthyes). Fish are the most diverse group among the vertebrates, with over 33,000 different types of fish species.
No one really knows how many different types of fish exist in the world, more are being discovered constantly. We may soon have over 35,000, or even 40,000 known species!
5 Fish Characteristics
There are three superclasses into which fish are grouped: Bony fish (Osteichthyes), jawless fish (Agnatha), and cartilaginous fish (Chondrichthyes). Ray-finned fishes are of the class Actinopterygii, while lobe-finned fishes are of the class Sarcopterygii. Both are clades of bony fishes.
Regardless, all fish have some characteristics in common that distinguish them from other animals.
- Cold-bloodedness: All fish are ectothermic or cold-blooded, meaning they cannot regulate their internal body temperature. Even warm-blooded fish such as tuna and mackerel sharks have only “regional endothermy” or warm-bloodedness limited to certain areas.
- Water habitat: All fish live in bodies of water, whether it is freshwater or saltwater. However, not all creatures that live in water are fish.
- Gills to breathe: Fish have gills throughout their life cycle. As with the water habitat, although all fish have gills, not all creatures with gills are fish.
- Swim bladders: Specialized organs fill with air to keep the fish afloat and in some species help them survive with low oxygen levels. They also help fish sleep and are sensitive enough to detect the movement of food and predators.
- Fins for movement: Most common are a tail fin, a pair of side fins, a dorsal fin and an anal fin. Variations exist but they all provide motion, maneuverability and stability.
There are several exceptions to the common definition of a fish. For example, hagfish don’t have scales and aren’t true vertebrates (or are considered primitive vertebrates); mudskippers are amphibious fish that can live outside water; lungfish use lungs instead of gills to breathe; lampreys lack paired fins, and tuna are warm-blooded.
Also, not all fish groups come from fish lineages. The superclass Tetrapoda of the four-listed animals is considered to be a group within Sarcopterygii, and includes amphibians, reptiles, birds, and mammals. Hence, Sarcopterygii includes both lobe-finned fishes and tetrapods.
Finally, not all aquatic creatures which resemble fish are considered fish. Whales, dolphins, and porpoises are aquatic mammals, for example.
- There are two main types of fish pregnancies, ovoviviparity or aplacental viviparity (egg-bearing) and viviparous (embryo-bearing). Both are considered to be live-bearing.
- Ovoviparity has the eggs develop and hatch internally, with the young being born live. It can express either ovuliparity (external fertilization of eggs and zygote development), oviparity (internal fertilization of eggs and external development of zygotes as eggs with yolks), or ovoviviparity (internal fertilization of eggs and internal development of embryos with yolks). Stingrays, seahorses and some shark species are ovoviviparous. So are guppies, mollies, swordtails, halfbeaks and platies.
- Viviparity has embryos develop internally before being born live. It can express either histotrophic (“tissue-eating”) viviparity (mother provides no nutrition and embryos eat their unborn siblings or mother’s unfertilized eggs) or hemotrophic (“blood-eating) viviparity (mother provides nutrition, usually through a placenta). Many shark species are viviparous.
You can read about some types of fish that are extinct.
Different Types of Fish:
It's one of the most commonly eaten fish in the world
The albacore is a very fast swimmer
The alligator gar has toxic eggs to protect against predators
November 12th is celebrated as National Pizza with the Works Except Anchovies Day
There are 70 different species!
The anglerfish has a glowing lure on its head to attract unsuspecting prey
One of the largest freshwater fish
The male Asian arowana raises the eggs in its mouth
These fish are known for their ability to leap and fight when hooked.
This small fish fetches a high price tag, with individuals selling from $1,000 to $5,000.
Baijis use echolocation to find food in the Yangtze River.
Named for the yellow body and brown spots that make it look like a banana.
The banjo catfish is extremely shy and known for hiding from onlookers.
There are over 1768 known species!
Can grow to nearly 2 meters long!
The batfish has a lure on its head to attract prey
The beluga sturgeon is one of the largest bony fish in the world!
Can live in low-oxygen environments!
Every black marlin is born as a female.
The tiny bladefin basslet belongs to the same subfamily as the giant grouper, Epinephelinae.
One of the ugliest creatures in existence!
It's a strong fighter when caught on a fishing line
Can live safely with many types of fish.
Blue sharks can have up to 135 pups at a time.
The bluefin is one of the largest fish in the world
The world record for longest bluegill is 15 inches.
May eat squid or other small invertebrate ocean life
Bonnetheads are the only hammerhead sharks that use their pectoral fins to swim.
The bowfin is a primitive fish that first evolved in the Jurassic
Can release a toxin from its skin
The bull trout is not actually a trout, but a member of the char family.
There are more than 100 different species!
The carp is one of the most popular pond fishes in the world
There are nearly 3,000 different species!
The Chinese paddlefish is one the largest freshwater fish in the world
The Chinook salmon undertakes a long migration for the spawning season
There are more than 2 000 known species!
Also known as the anemonefish!
It has teeth not only in its jaws but in its tongue and the roof of its mouth
The coelacanth first evolved almost 400 million years ago.
The European Conger ( Conger conger) can weigh as much as an adult human!
The cookiecutter shark takes its name because it leaves a cookie-shaped bite hole in its prey.
The crappie is one of the most popular freshwater fish in North America.
These fish make a popular choice for aquarium hobbyists due to their hardy nature.
One of the only schooling Cichlids!
Dragonfish can emit red light from their eyes
The drum fish makes a croaking sound with its swimming bladder!
Eels can be a mere few inches long to 13 feet!
The electric catfish can discharge an electric shock up to 450 volts
Despite its powerful shock, electric eels have terrible vision.
Its system can’t metabolize wax esters, which can lead to unpleasantness for diners.
Has the largest teeth compared to body size of any known fish
Fire Eels are not true eels.
Respire through the gills on their heads!
The Florida gar has toxic eggs to protect against predators
A flat fish found in the Atlantic and Pacific!
The Flowerhorn fish is an artificial species; it does not exist naturally
The chameleon of the seas!
Can glide in the air for hundreds of feet
The football fish is named after its unusual round or oblong shape
Freshwater eels are actually catadromous, meaning they migrate to saltwater to spawn
The freshwater jellyfish is native to China but is now found all over the world
Frilled Sharks got their name from the six rows of gills on their throat that look like ruffled collars.
The frogfish can change colors, but it takes several weeks to do so
Galapagos sharks are cannibalistic and sometimes eat their young, so the pups stay away from the adults in shallow water.
Can grow to more than 3m long!
Garden eel colonies are made up of hundreds to thousands of individuals.
You can see its heart beating
Goblin Sharks are called a living fossil because their family, Mitsukurinidae, can be traced back 125 million years.
Goldfish and common carp can mate and produce offspring
Featured in River Monsters TV series
The goonch catfish, or giant devil catfish, is one of the most fierce freshwater fish.
The grass carp is considered to be a natural weed control agent.
Can grow to more than 8 meters long!
This shark has the longest lifespan of any vertebrate.
One of the most common shark species!
Also known as the Millionfish!
The haddock is very popular in both recreational and commercial fishing
Can use slime to suffocate marine predators or escape capture
The word "halibut" is comes from haly meaning "holy" and butte meaning flat fish due to its popularity on Catholic holy days.
Found in coastal waters around the world!
The hardhead catfish has a sharp spine near its fin to inject venom
People enjoy the taste of the oily fish in many different ways including pickled, smoked, salted, dried and fermented.
Endemic to the Californian coast!
Excellent hitchhiker on long-trip cargo ships
Have tentacles around their mouths!
During spawning the look of the male changes. Among other things, he grows a beak called a kype that bears fangs.
In Japanese, the word koi sounds like the word for love. So the fish is a symbol of love among other good things.
The krill is perhaps the most important animal in the marine ecosystem!
Its skeleton is part cartilage and part bone.
Must be in temperatures of 78 degrees Fahrenheit to breed
Females can release up to 15,000 eggs at a time!
The lizardfish can camouflage itself against the sandy bottom to avoid predators.
Have sharp spines below their eyes
The longnose gar species of the gar family has potentially existed for 100 million years.
The lumpfish have sticky suction cups on their fins
The lungfish first evolved almost 400 million years ago.
It's called the rabbit of the ocean because it multiplies so quickly.
All masked angelfish are female until sometime after sexual maturity, at which point some become male.
The largest shark to ever live!
Swims with its mouth open to capture prey
The Mekong giant catfish is the largest purely freshwater fish in the world
Females lay up to 5 million eggs at one time in warm, shallow and salty waters
The mojarra's protruding mouth allows it to sift along the seabed for food
The ocean sunfish is the biggest bony fish in the world
Known for their calm and peaceful nature!
Called "The Poor Man's Lobster!"
Moon Jellies are bioluminescent, so they glow in the dark! They can also de-age!
Sometimes, groupers invite moray eels to help them hunt!
no stomach to digest food
The largest recorded specimen ever caught was 17" long
Commonly found in Central American waters!
The Oscar fish has teeth in its throat!
The oyster toadfish can produce poison to protect itself
Paddlefish have existed since the Cretaceous Period
The parrotfish can change from female to male at some point in its life.
The peppermint angelfish was only first described in 1992.
Apex freshwater predators with fearsome teeth!
The male pipefish has the ability to carry fertilized eggs with him
Generally found in fast-flowing streams!
The male broods the eggs and baby fish in his mouth.
The porbeagle is one of the few sharks that jumps out of the water
The second most poisonous creature in the world!
The rainbow shark has been genetically modified to glow in the dark
Despite its weird looks, the red-lipped batfish is harmless to humans
One of three giant catfish species
Grey reef sharks can give birth without males
These fish can grow up to three feet long!
Returns upstream every year to spawn
Salmon sharks are related to Great Whites.
The sand tiger is the shark most commonly seen in aquariums.
Schools of sardines can be miles long and are often visible from an airplane
Sawfish teeth keep growing as the fish gets older
There are more than 200 recognised species!
Its skull bones can compress so the fish can fit in narrow spaces
Inhabits tropical coastal waters of Australia!
All sea slugs have both male and female sex organs
Can live for up to 200 years!
Males give birth to up to 1,000 offspring!
No shark species has any bones in their bodies
Closely related to the Piranha
The sixgill shark has six pairs of gills instead of the normal five
More than 200 species exist!
The skipjack is the most commonly caught tuna in the world
The Greenland shark is one of the longest living vertebrates in the world.
Males change into females after the spawning season
Called "red salmon" because their skin turns bright red to dirty red during spawning season
Found in ocean waters worldwide!
There are more than 9,000 known species!
They are commonly mistaken as logs in the water due to their cylindrical body.
Uses vibrations to communicate with other fish
Has 2 stomachs to aid digestion!
Uses an electric shock to stun its prey!
Steelhead live in freshwater rivers and streams for 1 to 2 years before migrating into the ocean
It's stinger is razor-sharp or serrated!
Large species can swallow whole salmon
Commonly found throughout America!
Paracanthurus hepatus, the palette surgeonfish or bluetang, is the only member of its genus
The edges of an iridescent shark's fins have a signature glow
Found around shallow coral reefs!
Its genus dates back to the Cretaceous period – 113 million years ago
Swallows food, much of it larger than them, whole
Native to the freshwater streams of South America!
They like to burrow into aquarium sand.
Can be heard out of water
They don’t have scales for their first month of life!
The tuna has a sleek body that enables it to swim quickly through the water
Most data on these fish is based on observations near Japan.
Wahoo can change colors when they're excited and while they hunt
The walking catfish can move on land while breathing air
The wels catfish is among the largest freshwater fish in the world.
The largest species of fish in the world!
"Whiting" can refer to certain other species of ray-finned fish
Wolf Eels may become tame and interact with human in areas where people frequently dive.
The wolffish has impressive canines with a powerful bite force!
There are more than 500 different species!
The yellowfin forms schools with other tuna species
The zebra pleco is a bottom feeder with a sucker mouth.
Can get to be 30 years old in the wild!
List of Fish
Fish: Different Types, Definition, Photos, and More FAQs (Frequently Asked Questions)
What are the 3 types of fish?
Bony fish, jawless fish and cartilaginous fish.
What's the most famous fish?
The most famous fish is the coelacanth, which has four lobed fins resembling limbs. It is one of the world’s most ancient fish species. Its name means “hollow spine” and comes from the Greek words koilos (hollow) and akantha (spine).
Coelacanth also refers to the order Coelacanthiformes. which comes from the clade Sarcopterygii and subclass Actinistia. It includes two species in the genus Latimeria: the West Indian Ocean coelacanth (Latimeria chalumnae) and the Indonesian coelacanth (Latimeria menadoensis).
Can fish feel pain?
Yes, fish can feel pain, but it is different from the expression of pain from humans. It is difficult to test fish for pain except by looking for unusual behavior and physiological responses in reaction to certain stimuli.
What do fish eat?
Fish can be predatory, foraging or filter-feeding. Their diets can be carnivorous or omnivorous depending on the species and can include prey from zooplankton to invertebrates, crustaceans, annelids and smaller fish.
Discover a fish with human-like teeth here!
How do fish breathe?
Fish usually breathe through gills, which filters oxygen through water. However, some fish breathe using different means. Lungfish have lungs and mudskippers can breathe through wet skin and the lining of their mouth and throat.
What are the smallest and largest fish?
The smallest fish is the cyprinid fish (8mm) and the largest fish is the whale shark (12m).
What is the difference between "fish" and "fishes"?
“Fish” refers to the singular and one species or to the plural within context. “Fishes” refers to the plural, especially when talking about more than one species of fish.