Fish
With an estimated 20,000 different species, there is an immense variety of fish. The variation in size is enormous: the tiny Paedocypris progenetica is only 8 millimetres long, and the whale shark can reach over 12 metres. Fish are adapted to live in many different habitats: trout and catfish can live in freshwater; seahorses and yellow tang fish live in coral reefs; and down on the deep, dark ocean floor are anglerfish and cookie cutter sharks.
How can animals switch sex?
About one in 50 fish species can change from male to female – or back again
These are the best omega oils that could slow ageing
From salmon to supplements, here’s everything you need to know about omega oils…
Can any animals see in infared?
While infrared light is invisible to most animals, there are some, including salmon, bullfrogs and snakes, that can see it
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What is the most dangerous fish to eat?
Pufferfish is considered the most dangerous fish to eat; it is poisonous, potentially fatal. But if sliced properly, it can be quite tasty
This bizarre optical illusion could teach us how animals think
By seeing which animals fall for a classic visual trick, scientists are uncovering how different brains make sense of the world
What's the weirdest way fish mate?
Note to self: never date an anglerfish
How mucus kept this air-breathing eel hidden from scientists for 30 years
The lungfish isn't like other fish... for one key reason.
This very goofy fish is a certified freak, according to science
The ocean sunfish is a funny-looking guy. But that isn't even the weirdest thing about him.
Blobfish: Facts about the ugliest animal in the world
Learn what the blobfish really looks like – and why it’s become an unlikely poster child for marine conservation.
Nature's weirdest: The black swallower fish
It's a plucky fish whose eyes are definitely not bigger than their stomach.
Octopuses are now punching fish – and we have the footage to prove it
The fisticuffs occur during cooperative hunting trips.
The mysterious sex life of eels: How scientists finally unveiled their (truly bizarre) life cycle
Long, silvery and mysterious, these endangered fish make a big journey to reproduce.
How mating pufferfish created one of the ocean’s greatest mysteries
Mysterious 'crop circles' found on the seabed have been discovered to be a nest built by the most unlikely of creatures.
Scientists finally work out where a starfish’s head is
New research from Stanford University sheds a light on the animal’s interesting body.
What is a sheepshead fish?
The sheepshead fish has human-like teeth helps it consume an omnivorous diet.
Shark week | How do shark teeth work?
There's a reason shark teeth are so prolific in the fossil record.
Why being fish-shaped is the ultimate energy hack of the ocean
The classic fish-shaped body — a squashed teardrop with a pointed nose and tapered tail — has evolved time and again.
What is a pelican eel?
Also known as the Gulper Eel, this deep-water oddity can expand its mouth to incredible proportions, acting like a huge net to scoop up prey.
What is a coelacanth?
This denizen of the deep has been here since the time of the dinosaurs.
Why can’t marine animals survive in fresh water?
It's all down to osmosis.
Why do animals migrate?
Many animals make long, treacherous journeys year after year, so why do they go to all that effort?
What is a wolf eel?
This voracious eater is not actually an eel...
Fish "chock-full" of antifreeze protein found in sub-zero Arctic waters
Snailfish produce proteins in their liver to prevents them from freezing in the ice-cold waters around Greenland, but their survival may soon be threatened by climate change.
What’s the environmental impact of cigarette butts?
Cigarette butts contain chemicals toxic to wildlife and create 845,000 tons of litter every year.