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 32,000 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 the bony fishes.

Regardless, all fish have some characteristics in common and 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.

Fish Exceptions

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-limted 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.

Fish Pregnancy

  • 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.

Types of Fish

A Albacore Tuna
Albacore Tuna

The albacore is a very fast swimmer

A Alligator Gar
Alligator Gar

The alligator gar has toxic eggs to protect against predators

A Anchovies

November 12th is celebrated as National Pizza with the Works Except Anchovies Day

A Angelfish

There are 70 different species!

A Arapaima

One of the largest freshwater fish

A Baiji

Baijis use echolocation to find food in the Yangtze River.

A Banjo Catfish
Banjo Catfish

The banjo catfish is extremely shy and known for hiding from onlookers.

A Barb

There are over 2,000 known species!

A Barracuda

Can grow to nearly 2 meters long!

A Barramundi Fish
Barramundi Fish

Scale rings indicate age

A Beluga Sturgeon
Beluga Sturgeon

The beluga sturgeon is one of the largest bony fish in the world!

A Black Marlin
Black Marlin

Every black marlin is born as a female.

A Blobfish

One of the ugliest creatures in existence!

A Blue Shark
Blue Shark

Blue sharks can have up to 135 pups at a time.

A Bluefin Tuna
Bluefin Tuna

The bluefin is one of the largest fish in the world

A Bonito Fish
Bonito Fish

May eat squid or other small invertebrate ocean life

A Bonnethead Shark
Bonnethead Shark

Bonnetheads are the only hammerhead sharks that use their pectoral fins to swim.

A Bowfin

The bowfin is a primitive fish that first evolved in the Jurassic

A Butterfly Fish
Butterfly Fish

There are more than 100 different species!

A Carp

The carp is one of the most popular pond fishes in the world

A Catfish

There are nearly 3,000 different species!

A Chimaera

Also called ghost shark

A Chinese Paddlefish
Chinese Paddlefish

The Chinese paddlefish is one the largest freshwater fish in the world

A Cichlid

There are more than 2 000 known species!

A Clownfish

Also known as the anemonefish!

A Codfish

They eat other fish

A Coelacanth

The coelacanth first evolved almost 400 million years ago.

A Cookiecutter Shark
Cookiecutter Shark

The cookiecutter shark takes its name because it leaves a cookie-shaped bite hole in its prey.

A Discus

One of the only schooling Cichlids!

A Dragonfish

Dragonfish can emit red light from their eyes

A Drum Fish
Drum Fish

The drum fish makes a croaking sound with its swimming bladder!

A Eel

Eels can be a mere few inches long to 13 feet!

A Electric Eel
Electric Eel

Can generate a 500 volt electric shock!

A Fangtooth

Has the largest teeth compared to body size of any known fish

A Fish

Respire through the gills on their heads!

A Florida Gar
Florida Gar

The Florida gar has toxic eggs to protect against predators

A Flounder

A flat fish found in the Atlantic and Pacific!

A Fluke Fish (summer flounder)
Fluke Fish (summer flounder)

The chameleon of the seas!

A Flying Fish
Flying Fish

Can glide in the air for hundreds of feet

A Frilled Shark
Frilled Shark

Frilled Sharks got their name from the six rows of gills on their throat that look like ruffled collars.

A Gar

Can grow to more than 3m long!

A Garden Eel
Garden Eel

Garden eel colonies are made up of hundreds to thousands of individuals.

A Goblin Shark
Goblin Shark

Goblin Sharks are called a living fossil because their family, Mitsukurinidae, can be traced back 125 million years.

A Great White Shark
Great White Shark

Can grow to more than 8 meters long!

A Greenland Shark
Greenland Shark

This shark has the longest lifespan of any vertebrate.

A Grey Reef Shark
Grey Reef Shark

One of the most common shark species!

A Guppy

Also known as the Millionfish!

A Haddock

The haddock is very popular in both recreational and commercial fishing

A Hagfish

Can use slime to suffocate marine predators or escape capture

A Hammerhead Shark
Hammerhead Shark

Found in coastal waters around the world!

A Herring

People enjoy the taste of the oily fish in many different ways including pickled, smoked, salted, dried and fermented.

A Horn Shark
Horn Shark

Endemic to the Californian coast!

A Immortal Jellyfish
Immortal Jellyfish

Excellent hitchhiker on long-trip cargo ships

A Jellyfish

Have tentacles around their mouths!

A Krill

The krill is perhaps the most important animal in the marine ecosystem!

A Lamprey

Not related to the eel

A Lionfish

Females can release up to 15,000 eggs at a time!

A Loach

Have sharp spines below their eyes

A Longnose Gar
Longnose Gar

The longnose gar species of the gar family has potentially existed for 100 million years.

A Lungfish

The lungfish first evolved almost 400 million years ago.

A Manta Ray
Manta Ray

Can grow up to 9m wide!

A Megalodon

The largest shark to ever live!

A Milkfish

Females lay up to 5 million eggs at one time in warm, shallow and salty waters

A Molly

Known for their calm and peaceful nature!

A Monkfish

Called "The Poor Man's Lobster!"

A Moray Eel
Moray Eel

Can grow to nearly 2 meters in length!

A Nurse Shark
Nurse Shark

Commonly found in Central American waters!

A Paddlefish

Paddlefish have existed since the Cretaceous Period

A Pike Fish
Pike Fish

Apex freshwater predators with fearsome teeth!

A Piranha

Generally found in fast-flowing streams!

A Pufferfish

The second most poisonous creature in the world!

A Rockfish

These fish can grow up to three feet long!

A Salmon

Returns upstream every year to spawn

A Salmon Shark
Salmon Shark

Salmon sharks are related to Great Whites.

A Sand Tiger Shark
Sand Tiger Shark

The sand tiger is the shark most commonly seen in aquariums.

A Sardines

Schools of sardines can be miles long and are often visible from an airplane

A Sawfish

Sawfish teeth keep growing as the fish gets older

A Scorpion Fish
Scorpion Fish

There are more than 200 recognised species!

A Sea Dragon
Sea Dragon

Inhabits tropical coastal waters of Australia!

A Sea Slug
Sea Slug

All sea slugs have both male and female sex organs

A Sea Urchin
Sea Urchin

Can live for up to 200 years!

A Seahorse

Males give birth to up to 1,000 offspring!

A Shark

No shark species has any bones in their bodies

A Siamese Fighting Fish
Siamese Fighting Fish

Can live in low-oxygen environments!

A Silver Dollar
Silver Dollar

Closely related to the Piranha

A Sixgill shark
Sixgill shark

The sixgill shark has six pairs of gills instead of the normal five

A Skate Fish
Skate Fish

More than 200 species exist!

A Skipjack Tuna
Skipjack Tuna

The skipjack is the most commonly caught tuna in the world

A Sleeper Shark
Sleeper Shark

The Greenland shark is one of the longest living vertebrates in the world.

A Spinner Shark
Spinner Shark

Can have up to 20 babies

A Spiny Dogfish
Spiny Dogfish

Found in ocean waters worldwide!

A Sponge

There are more than 9,000 known species!

A Spotted Gar
Spotted Gar

They are commonly mistaken as logs in the water due to their cylindrical body.

A Starfish

Has 2 stomachs to aid digestion!

A Stingray

It's stinger is razor-sharp or serrated!

A Sturgeon

Large species can swallow whole salmon

A Sucker Fish
Sucker Fish

Commonly found throughout America!

A Swai Fish
Swai Fish

The edges of an iridescent shark's fins have a signature glow

A Tang

Found around shallow coral reefs!

A Tarpon

Its genus dates back to the Cretaceous period – 113 million years ago

A Tetra

Native to the freshwater streams of South America!

A Toadfish

Can be heard out of water

A Tuna

The tuna has a sleek body that enables it to swim quickly through the water

A Walking Catfish
Walking Catfish

The walking catfish can move on land while breathing air

A Walleye Fish
Walleye Fish

Has great night vision

A Whale Shark
Whale Shark

The largest species of fish in the world!

A Wolf Eel
Wolf Eel

Wolf Eels may become tame and interact with human in areas where people frequently dive.

A Wolffish

The wolffish has impressive canines with a powerful bite force!

A Wrasse

There are more than 500 different species!

A Yellowfin Tuna
Yellowfin Tuna

The yellowfin forms schools with other tuna species

A Zebra Shark
Zebra Shark

Can get to be 30 years old in the wild!

List of Fish

Fish 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.

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.