A compass jellyfish swimming near the ocean surface

Fascinating facts about jellyfish

1 minute read

Jellyfish have been drifting, floating and bobbing along in our ocean for over 500 million years - that's longer than dinosaurs, and a lot longer than us. We're sharing some facts you might not know about jellies...

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A barrel jellyfish | Image credit: Sam Morys

Jellyfish are 95% water

Surprisingly, jellyfish are mostly made up of water, not jelly. This is why they look so different in the water than out of it.

They don’t have brains, hearts, or lungs

Jellyfish are very simple organisms. Their bodies are made up of just three layers— the outer epidermis, a gelatinous middle layer called the mesoglea, and the inner gastrodermis which lines the gut cavity and helps jellyfish to digest food and absorb nutrients.

Jellyfish are a type of ‘cnidarian’

Cnidarians are jelly-like marine animals which have radical symmetry and sting their prey using cells on their tentacles called ‘cnidocytes’, which explode when stimulated! Other well known Cnidaria include sea anemones and coral.

Cnidarians are named after the Greek word for "sea nettle”

A blue jellyfish swimming in clear azure blue waters in Scotland. It appears ethereal with long thread-like tendrils coming from it's white bell-shaped body. It appears to be swimming towards the surface with its bell concave towards the surface.

A blue jellyfish found in Scotland | Image credit: Mark Harris

Jellyfish have hydrostatic skeletons

Rather than a skeleton made up of bones, jellyfish have bells containing fluid and circular muscles. If they want to move, jellyfish can propel themselves along by contracting their muscles and squirting water in the opposite direction from where they want to go.

Or to save themselves the effort of doing this, jellyfish can also simply drift along with the ocean’s currents.

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They have very short lifespans

Although some smaller species of jellyfish only live for a few hours, larger species like lion’s mane can live for a few years.

Jellyfish range from a few centimetres to over two metres in size

The Irukandji jellyfish can be as small as one to two centimetres with a bell only a few millimetres wide, while the largest species - lion’s mane - can grow up to two metres wide and have tentacles as long as 36 metres!

A stalked jellyfish | Image credit: Peter Bardsley

Some jellyfish can turn back their biological clock

Jellyfish begin as a polyp, before becoming an ‘ephyra’ and then an adult ‘medusa’. But, if injured or starved, or in water which is either too hot or cold, an adult Turritopsis dohrnii (also known as the immortal jellyfish) can drop to the seabed and become a polyp all over again.

Box jellyfish are highly advanced

Unlike most species of jellyfish, box jellies have eyes and can see, rather than only sense light. They have 24 eyes spread across all four sides of their bell, meaning that they have 360-degree vision.

Not only that, but box jellyfish can also move up to six metres per minute! The shelf-like shape of their bells allows them to catch and expel water more quickly than most jellies, which simply drift with the current.

The comb jelly is not actually a jellyfish

Though they look quite similar, comb jellies are not Cnidaria but actually from a phylum called Ctenophora, which roughly translates to "comb bearing". One distinction is that instead of having tentacles that sting prey, comb jellies have sticky cells to trap prey. This means you can't get stung by a comb jelly as they do not have stinging cells to do so.

A comb jelly | Image credit: Mar Kirkland

Some jellyfish trap their prey, while others trawl

Jellyfish spend a lot of their time seeking food. Some arrange their tentacles in a twisted, spiral pattern to trap prey. Others simply drag their tentacles behind them like a trawler net as they drift along the ocean.