Jellyfish: The smart stinging creatures drifting through our oceans

Jellyfish are invertebrates – animals that don’t have a backbone – and belong to a group called Cnidaria. Some of their closest relatives include colourful corals and anemones.  

“Jellyfish are gelatinous and jelly-like,” explains Miranda Lowe CBE, our Principal Curator of Crustacea and Cnidaria.

“They have a curved, bell-shaped body with tentacles hanging down from the underside. On this underside they also have a mouth and four oral arms, which are thicker than the tentacles.”

Jellyfish come in incredible shapes, colours and sizes.  Most jellyfishes’ bell-shaped bodies are between two centimetres and 40 centimetres across. However, some species can grow to more than two metres in diameter, with thin trailing tentacles that are even longer than that.

Jellyfish have radial symmetry, meaning that if you sliced a jellyfish down the middle, the parts would be symmetrical, like the segments of an orange.

These cnidarians are famous for their stinging abilities. The intensity of the sting varies by species.

Some jellyfish can also generate their own light, called bioluminescence – further adding to their otherworldly beauty.

Do jellyfish have brains?

Jellyfish don’t have bones, brains, hearts, teeth or blood.

However, what they do have is a network of nerves called a nerve net. This allows them to process information including light levels, temperature and chemical changes in the water around them.

How many species of jellyfish are there?

Jellyfish are sometimes called sea jellies. They belong to a group called Medusozoa which is divided into four classes: Hydrozoa, Scyphozoa, Cubozoa and Staurozoa.

Comb jellies are also sometimes included in the broad sea jellies category. But these animals aren’t cnidarians. Instead, they belong to group of organisms called Ctenophora.

Scyphozoa

There are approximately 200 described species of scyphozoans. Most are free-swimming species with the characteristic bell-shaped body, also called a medusa.

Cubozoa

Cubozoans are more commonly known as box jellyfish. They differ from scyphozoan jellyfish because of their distinctive cube-shaped medusae and their tentacles are arranged around each of the four bottom corners of their body.

There are around 45 species of box jellyfish and they include some of the deadliest venomous animals on Earth. They are also more active hunters than scyphozoans. The largest species, Chironex fleckeri, has been recorded swimming at around five centimetres per second. 

Hydrozoa

There are some 3,600 species of hydrozoans. These are typically smaller than scyphozoan jellyfish. The polyp stage of their lives is usually more conspicuous than the small adult jellies.

However, not all hydrozoans are considered jellyfish. For example, the famous Portuguese man o’ war is part of the class Hydrozoa but is known as a siphonophore. They are made up of groups of small animals living and functioning together as a colony with a mixture of polyp-like and jellyfish-like parts. 

Staurozoa

Staurozoans are also known as stalked jellyfish. They resemble their cousins the sea anemone or coral more closely than other jellyfish groups. Rather than drifting through the water, these jellies live attached to seaweed, rocks and other objects. 

How do jellyfish move?

Jellyfish often allow themselves to drift along in the water, but they can use their oral arms to pulse along in the ocean current if needed. But they are not strong swimmers. This is why jellyfish sometimes get washed up on the beach in large numbers after a storm.

What do jellyfish eat and how do they do it?

A jellyfish eats by stinging its prey and using the oral arms to waft the food towards its mouth – which is also its anus – and into its stomach.

Jellyfish eat small fish, fish larvae, shrimp, tiny crustaceans such as krill and copepods, small shrimp-like organisms called amphipods and tiny plants such as algae. They will sometimes eat other sea jellies.

They also forage food from other animals, notes Miranda. “They utilise the scraps of larger predators in the food chain such as sharks and turtles.”

Jellyfish are eaten by leatherback sea turtles, fish such as ocean sunfish and grey triggerfish, some seabirds such as fulmars, whale sharks and some species of crabs and whales.

They’re also eaten by people. Some species of jellyfish are considered a delicacy in countries in East Asia including Japan and China. 

In fact, jellyfish may become an increasingly common food source around the world, as overfishing is making some fishes harder to come by. 

But researchers have noted that peoples' willingness to include jellyfish in their diet is affected by a variety of factors, including age, gender and even travelling habits. Food neophobia – the reluctance to eat new foods – is another barrier, found across consumers in Italy, Canada and Latin America.

How do jellyfish sting?

“Jellyfish have stinging cells called nematocytes,” explains Dr Ronald Jenner, our Principal Researcher in venom evolution.

“They have thousands of these along their tentacles. The stinging cells contain a microscopic harpoon filled with toxins. When a jellyfish hunts, or wants to defend itself, these harpoons can be triggered to shoot out, penetrate the skin of the victim and inject toxins into them.”

Jellyfish stings can cause a burning pain, so people often reach for ice to soothe them. However, scientists have reported that using a heat pack or dunking the sting in water as hot as you can comfortably stand may actually be better. “It’s counterintuitive, but it can bring relief,” says Ronald.

But not all jellyfish sting and some jellyfish stings aren’t harmful to humans.

Some creatures take advantage of jellyfishes’ stings to protect themselves from predators. So-called ‘Medusa’ fish hang out among the tentacles of a jellyfish to nibble on scraps and avoid being eaten by bigger fish.

How do jellyfish reproduce?

Jellyfish change form significantly as they move through their life cycle.

“Jellyfish release eggs and sperm into the ocean where they get fertilised,” explains Miranda.

“The fertilised egg changes into a free-swimming larva. Then the larva matures into a polyp, which finds a hard surface such as rock to fix itself to. These polyps mature until they develop and bud off into a young free-swimming jellyfish.”

Interesting jellyfish species

Jellyfish are a fascinating group. Have you come across these species before?

Lion’s mane jellyfish 

The Lion’s mane jellyfish, Cyanea capillata, is the largest known jellyfish. They’re found in the Arctic, North Pacific and North Atlantic oceans. They’re often seen in waters around the UK and Ireland.

The lion's mane jellyfish's tentacles can reach over 30 metres in length – that’s longer than a blue whale.

Upside-down jellyfish

Upside-down jellyfish belong to the genus Cassiopea.

These clever jellyfish have algae living on their underside. They will often sit inverted in tropical shallows so their algae can photosynthesise. The by-products of photosynthesis provide most of Cassiopea’s food requirements.

Immortal jellyfish

The remarkable hydrozoan Turritopsis dohrnii is considered biologically immortal. It can revert from its mature adult form into an earlier stage of its life cycle and redevelop into a new mature jellyfish when conditions are more favourable.

Fried egg jellyfish

The fried egg jellyfish – Cotylorhiza tuberculata – is named for its distinctive domed, yolk-yellow body.

This jellyfish is mostly found in the Aegean, Adriatic and Mediterranean seas. It has rather short oral arms and feeds mainly on zooplankton and other jellyfish.

What are jellyfish blooms?

For the most part, jellyfish are solitary creatures, drifting through the ocean alone. However, sometimes huge masses of jellyfish, known as blooms, occur. These are the result of an explosion in a jellyfish population.

Human influence is often involved in this phenomenon. For instance, overfishing can deplete the ocean of a jellyfish’s natural predators, allowing them to reproduce in disproportionate quantities.

Chemical run-off that finds its way into the ocean can also provide nutrients that cause a boom in jellyfish populations.

How are jellyfish affected by climate change?

“Jellyfish are useful to monitor as a baseline indicator of what is going on in our oceans,” says Miranda. “Because they are 95% water, jellyfish are very sensitive to chemical imbalances. That’s why we all have to be mindful of what we are putting into the oceans.”

This includes plastic waste. Jellyfish consume tiny organisms, so they can be an entry point for microplastics into the ocean food chain. From jellyfish, microplastics find their way into bigger organisms such as turtles and tuna.

By cutting down on single-use plastics, we can all help to ensure jellyfish, and the animals that rely on them, survive in our oceans for millions of years to come.

Find out more by listening to episode six of the Our Broken Planet podcast.