There are so many things we don't know about the deep blue sea, so here are a few astonishing facts about fish to amaze you.



The largest fish in the sea is the whale shark which can reach 15 metres in length. That’s longer than two giraffes stacked on top of each other. They are completely harmless though and feed on plankton and small fish that they catch in their huge mouths.

The smallest fish is the Philippine goby that just over half a centimetre when fully grown. It’s almost completely see-through and has two rows of teeth in each jaw.

Blackfin barracuda swimming in circles, Malaysia

Blackfin barracuda swimming in circles, Malaysia


There are some fish that can walk on land. In Hawaii, gobies are able to climb waterfalls as high as 122 metres using a special disc that helps them stick to vertical rocks.

Some fish have a swim bladder which helps them to control their buoyancy. The more gas that fills this thin-walled sac, the lighter the fish becomes and the higher it floats in the water. Some scientists think this may be the evolutionary precursor to lungs.

Flying Fish

The South American marbled hatchetfish are the only fish that can achieve powered flight. Unlike other flying fish, which simply glide through the air, the marbled hatchetfish can fly by flapping their pectoral fins very rapidly, making a buzzing noise.

White marlin

White marlin


The sailfish is the fastest fish in the sea, clocked going just over 110 kilometres per hour. Other closely related super-fish include swordfish and marlin, all using their streamlined form and fast-twitch muscle fibres to chase down fish - bad news for sardines.


Weedy seadragons, related to seahorses, have an unusual approach to parenting. Bizarrely, the eggs are laid onto the tail of the male. Two months later, the young weedy seadragons hatch and the father’s job is done and the offspring swim off.

Quick Facts

  • Every time an animal uses a muscle it gives off a tiny electrical signal. Certain types of fish like rays and sharks can sense this electrical signal and use this to find and catch hidden prey.
  • The weird-looking (and equally wierd-named) sarcastic fringehead fish get into a lot of fights protecting their territory. When they battle with a fellow fringehead their heads appear to turn inside out as they lock lips with each other.
  • Convict fish are amazing builders who create complex tunnels under the sea bed for their families to live in. The adults stay underground while the children leave the burrow in swarms to collect food to bring back for their parents.