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

Albacore Tuna
The albacore is a very fast swimmer

Alligator Gar
The alligator gar has toxic eggs to protect against predators

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

Angelfish
There are 70 different species!

Arapaima
One of the largest freshwater fish

Baiji
Baijis use echolocation to find food in the Yangtze River.

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

Barb
There are over 2,000 known species!

Barracuda
Can grow to nearly 2 meters long!

Barramundi Fish
Scale rings indicate age

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

Black Marlin
Every black marlin is born as a female.

Blobfish
One of the ugliest creatures in existence!

Bluefin Tuna
The bluefin is one of the largest fish in the world

Bonito Fish
May eat squid or other small invertebrate ocean life

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

Butterfly Fish
There are more than 100 different species!

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

Catfish
There are nearly 3,000 different species!

Chimaera
Also called ghost shark

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

Cichlid
There are more than 2 000 known species!

Clownfish
Also known as the anemonefish!

Codfish
They eat other fish

Coelacanth
The coelacanth first evolved almost 400 million years ago.

Discus
One of the only schooling Cichlids!

Dragonfish
Dragonfish can emit red light from their eyes

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

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

Electric Eel
Can generate a 500 volt electric shock!

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

Fish
Respire through the gills on their heads!

Florida Gar
The Florida gar has toxic eggs to protect against predators

Flounder
A flat fish found in the Atlantic and Pacific!

Fluke Fish (summer flounder)
The chameleon of the seas!

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

Gar
Can grow to more than 3m long!

Goblin Shark
Goblin Shark have between 35 and 53 rows of teeth. Their teeth have specialized jobs.

Great White Shark
Can grow to more than 8 meters long!

Grey Reef Shark
One of the most common shark species!

Guppy
Also known as the Millionfish!

Hagfish
Can use slim to suffocate marine predators or escape capture

Hammerhead Shark
Found in coastal waters around the world!

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

Horn Shark
Endemic to the Californian coast!

Immortal Jellyfish
Excellent hitchhiker on long-trip cargo ships

Jellyfish
Have tentacles around their mouths!

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

Lamprey
Not related to the eel

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

Loach
Have sharp spines below their eyes

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

Lungfish
The lungfish first evolved almost 400 million years ago.

Manta Ray
Can grow up to 9m wide!

Megalodon
The largest shark to ever live!

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

Molly
Known for their calm and peaceful nature!

Monkfish
Called "The Poor Man's Lobster!"

Moray Eel
Can grow to nearly 2 meters in length!

Narwhal
Lives and hunts in the frigid Arctic!

Nurse Shark
Commonly found in Central American waters!

Paddlefish
Paddlefish have existed since the Cretaceous Period

Pike Fish
Apex freshwater predators with fearsome teeth!

Piranha
Generally found in fast-flowing streams!

Pufferfish
The second most poisonous creature in the world!

Rockfish
These fish can grow up to three feet long!

Salmon
Returns upstream every year to spawn

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

Sawfish
Sawfish teeth keep growing as the fish gets older

Scorpion Fish
There are more than 200 recognised species!

Sea Dragon
Inhabits tropical coastal waters of Australia!

Sea Slug
All sea slugs have both male and female sex organs

Sea Urchin
Can live for up to 200 years!

Seahorse
Males give birth to up to 1,000 offspring!

Shark
No shark species has any bones in their bodies

Shrimp
There are 2,000 different species worldwide!

Siamese Fighting Fish
Can live in low-oxygen environments!

Silver Dollar
Closely related to the Piranha

Skate Fish
More than 200 species exist!

Skipjack Tuna
The skipjack is the most commonly caught tuna in the world

Spiny Dogfish
Found in ocean waters worldwide!

Sponge
There are more than 9,000 known species!

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

Starfish
Has 2 stomachs to aid digestion!

Stingray
It's stinger is razor-sharp or serrated!

Sturgeon
Large species can swallow whole salmon

Sucker Fish
Commonly found throughout America!

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

Tang
Found around shallow coral reefs!

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

Tetra
Native to the freshwater streams of South America!

Tiger Shark
The fourth biggest species of shark in the world!

Toadfish
Can be heard out of water

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

Walleye Fish
Has great night vision

Whale Shark
The largest species of fish in the world!

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

Wolffish
The wolffish has impressive canines with a powerful bite force!

Wrasse
There are more than 500 different species!

Yellowfin Tuna
The yellowfin forms schools with other tuna species

Zebra Shark
Can get to be 30 years old in the wild!
List of Fish
- Albacore Tuna
- Alligator Gar
- Anchovies
- Angelfish
- Arapaima
- Baiji
- Banjo Catfish
- Barb
- Barracuda
- Barramundi Fish
- Basking Shark
- Beluga Sturgeon
- Black Marlin
- Blobfish
- Bluefin Tuna
- Bonito Fish
- Bowfin
- Butterfly Fish
- Carp
- Catfish
- Chimaera
- Chinese Paddlefish
- Cichlid
- Clownfish
- Codfish
- Coelacanth
- Discus
- Dragonfish
- Drum Fish
- Eel
- Electric Eel
- Fangtooth
- Fish
- Florida Gar
- Flounder
- Fluke Fish (summer flounder)
- Frilled Shark
- Gar
- Goblin Shark
- Great White Shark
- Grey Reef Shark
- Guppy
- Hagfish
- Hammerhead Shark
- Herring
- Horn Shark
- Immortal Jellyfish
- Jellyfish
- Krill
- Lamprey
- Lionfish
- Loach
- Longnose Gar
- Lungfish
- Manta Ray
- Megalodon
- Milkfish
- Molly
- Monkfish
- Moray Eel
- Narwhal
- Nurse Shark
- Paddlefish
- Pike Fish
- Piranha
- Pufferfish
- Rockfish
- Salmon
- Sardines
- Sawfish
- Scorpion Fish
- Sea Dragon
- Sea Slug
- Sea Urchin
- Seahorse
- Shark
- Shrimp
- Siamese Fighting Fish
- Silver Dollar
- Skate Fish
- Skipjack Tuna
- Spiny Dogfish
- Sponge
- Spotted Gar
- Starfish
- Stingray
- Sturgeon
- Sucker Fish
- Swai Fish
- Tang
- Tarpon
- Tetra
- Tiger Shark
- Toadfish
- Tuna
- Walleye Fish
- Whale Shark
- Wolf Eel
- Wolffish
- Wrasse
- Yellowfin Tuna
- Zebra Shark
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.