66: The Lion’s Mane Jellyfish

Time for another creature spotlight! Many of our spotlights so far have been on slightly obvious animals- reindeer, Arctic foxes, lynx, and Arctic terns, to name but a few. We’ve dipped out toes into the water with narwhals and phytoplankton too, but this week we are going to jump right in with an Arctic giant that is not so famous. This is the lion’s mane jellyfish. It’s bigger than you think, has odder babies than you think, and it may be one of the creatures that could survive a worst-case scenario climate and pollution apocalypse. Curious? Read on!

A lion’s mane jellyfish (Picture: W. Carter/Public Domain)

Growing up

Much like a tadpole grows into a frog, or a caterpillar into a butterfly, many jellyfish start life looking totally different than when they are adults. They mate externally, with eggs and sperm meeting out in the water (after all jellyfish don’t really have much of an inside). The female will carry the eggs in her tentacles until they hatch into their first phase of life, the planula larva. This larva doesn’t hang around long, it pretty much swims away from the mother and settles into a nice bit of seabed before starting on the next phase of life, the polyp.

Polyps are sort of cylindrical or vase shapes with their end anchored into the seabed. On their top they have tiny little tentacles, so look like some kind of little coral reef creature. The tentacles catch food and bring it into the cylinder which is the mouth and digestive system. Think of it basically as an upside down jellyfish with its head stuck in the mud. At this stage the polyp starts reproducing asexually, so cloning itself, creating little creatures called ephyrae. These break free and go on to the final jellyfish stage of life, the medusa stage. Medusa are the classic jellyfish we think of, with a bell shaped head and long tentacles. This is also the stage where the jellyfish can go on to reproduce sexually. For a lion’s mane jellyfish, the journey from larva to medusa only takes around 30-40 days, and its total lifespan is around a year.

The life cycle of a jellyfish (Image: teara.nz)


Lion’s mane jellyfish live in the boreal waters of the north, in the Atlantic, Pacific and Arctic oceans, although are known to come as far south as the English Channel. The ones in the south tend to be a little smaller, but the true Arctic lion’s mane jellyfish are whoppers. They are on average 30cm in length with a 50cm diameter bell (this being the main body above the tentacles). However, they have been found with an up to 2m wide bell and 36m long set of tentacles. That is longer than a blue whale, making them arguable the longest animals on the planet!

Their bell houses all their organs and their reproductive system. Attached to this are up to 1,200 tentacles, each of which have a powerful sting. These stings are usually not fatal to humans but can really hurt, especially as they still sting even if not attached to the jellyfish’s body, or when they have been on the shore of a beach for quite a while. I would say prod with care but… just don’t prod! Remedies to stings include vinegar and salt water, though it’s important to pay attention to symptoms in case you need proper medical attention.

The purpose of the sting is to hunt and protect. The lion’s mane jellyfish uses its stingy and also incredibly sticky tentacles as a net within which to catch prey. This can include small fish, crustaceans, and even other jellyfish. The prey is then transferred up to its mouth in the bell of the body. These jellies might seem terrifying, but they also have a beautiful side. They are bioluminescent, meaning they are able to create their own light, glowing when it gets dark.

Lion’s mane jellyfish like the travelling lifestyle, continually swimming and covering huge distances when ocean currents bring them along for a ride. They are pelagic, usually living in the top 20m of the water column, and are pretty solitary creatures, but during certain weather conditions, like storms, big swarms can form.

Oddly, this is one species that has benefitted from pollution, overfishing and global warming. These factors have removed many of their predators like seabirds, turtles and some large fish like the sun fish. Overfishing has also removed many large fish like tuna, which the jellies compete with for food, meaning they now have abundant prey. At the same time pollution and a warming climate has reduced the amount of oxygen in the oceans and made them more acidic. This is problematic to some species, but lion’s mane jellyfish actually prefer acidic low oxygen environments, so have benefitted. All these have come together to create more jellyfish swarms helping them to multiply even faster.

Curious what to do when you see a lion’s mane jellyfish on a beach? DO NOT DO THIS!!!!! Even when on land, or if some of their hundred of tentacles are detached from the bell of their body, they can still give a nasty sting. Vinegar or salt water can be used as remedies. (Picture: The Irish Post)


Sounds like jellyfish are taking over the world! What does this mean for us? Well, it can mean a bit of inconvenience. Whilst the lion’s mane jellyfish sting doesn’t generally kill, it can really hurt. As mentioned earlier the creature can continue to sting even when washed up and broken up into piece. In 2010 on a beach in New Hampshire, 150 people were stung by the broken up bits of a lion’s mane jellyfish, leading to mild pain in some and nausea, headaches, drowsiness, vomiting and swelling in others. Ouch…

Otherwise large numbers of jellyfish in general can wreak havoc on tech which needs ocean water to cool it down. Torness nuclear reactor in Scotland had to temporarily shut down in 2011 because huge swarms of jellyfish appeared at their cooling water filters. The filters ensure no seaweed and wildlife get sucked in, but the jellyfish started clogging up their surface meaning the power plant had to shut down the reactors in case the cooling water could no longer get in. This isn’t a one off occurrence, the same has happened at nuclear and coal power plants in Japan, Israel and Sweden. Whilst the jellies responsible for these specific pieces of havoc weren’t lion’s mane jellyfish, there’s no reason they couldn’t do the same thing.

Containers of jellyfish filtered out of the cooling water of a coal power station in Israel in 2011 (Picture: Reuters)

We often think that things like climate change and pollution are bad for wildlife. They most certainly are, to most of them, but nature has a way of getting around pretty much anything. If we continue in the direction we’re going with our overall poor stewardship of the earth, the pandas and rhinos might disappear, but fear not, at least the rats, cockroaches and lion’s mane jellyfish will survive. Indeed, they’re doing better than ever before! Lion’s mane jellyfish are fascinating creatures, the often overlooked glowing giants of the Arctic waters. To them, oblivious of our word, they are simply floating about, living their best life, and being quite successful at it.

For more info:

A rather dramatic but interesting video on the lifecycle of jellyfish:

A short video on some other ways climate change is encouraging the takeover of the jellies:

Music: kongano.com

Cover image: W. Carter/Public Domain

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