Why so many gosh darn jellyfish?

Mediterranean resorts are having to ban swimming because of plagues of jellyfish, which scientists blame on a complex cocktail of human impacts, from climate change to overfishing.

Few things evoke the idea of a relaxing vacation than a dip in the calm waters of the Mediterranean — especially for northern Europeans. But idyllic tourist spots such as southern Spain are increasingly having to prohibit bathing due to plagues of dangerous jellyfish.

Nature and Environment | 12.06.2018

Experts say jellyfish aren't just an inconvenience for swimmers. They are evidence of a perfect storm of human impacts destabilizing marine ecosystems.

Climate change, unsustainable fishing practices and agricultural chemicals are all suspects in the explosion in jellyfish numbers. But a lack of scientific knowledge about these alien-looking creatures and their complex biology means pinning down the exact cause is a complex business.

The many differences between thousands of species of jellyfish make it all the more challenging for researchers to pin down clear data.

Nature and Environment | 09.06.2018

"To say that all jellyfish are the same is like saying that a lion and a rhino are the same," Damien Haberlin, post-doctoral researcher at the Centre for Marine and Renewable Energy in Ireland, told DW.

Nature and Environment

No brain? No problem!

Jellyfish have been floating around the Earth's oceans for 500 million years now - without a brain to guide them. Jellyfish use their sophisticated nervous system which immediately translates outside impulses into action. That's why this rhizostome jellyfish and its relatives don't need a brain to process information.

Nature and Environment

Medusa of the seas

Jellyfish live in the sea. But the name is misleading - they're not actually fish. They're members of the cnidaria phylum and are related to corals and anemones. They're also classified as medusozoa - with the tentacles floating around their bodies, they look a little like the Greek monster Medusa, who had living snakes instead of hair on her head.

Nature and Environment

Umbrella with tentacles

A jellyfish body contains up to 99 percent water. Human bodies only contain around 63 percent water. A big part of the jellyfish is its umbrella-shaped bell. Attached to that is the manubrium, through which the animal takes up nutrients, and hundreds of tentacles. With some jellyfish, the tentacles can be a couple of meters long. The animals use them to feel their way around and to hunt prey.

Nature and Environment

Giant jellyfish

Most jellyfish are white or transparent. There are also some exceptional jellyfish species out there, though. The Asian Nomura's jellyfish isn't especially colorful, but it's huge: it has a diameter of up to two meters (6.5 feet) and can weigh more than 200 kilograms (440 pounds).

Nature and Environment

Follow the current

Scientists consider jellyfish plankton because they're swept along by the current of the sea. The jellyfish isn't great at getting anywhere by itself. It propels itself forward by constricting and relaxing its bell, achieving speeds of up to 10 kilometers per hour (6 mph). Even bugs walk faster.

Nature and Environment

Pretty and poisonous

Jellyfish might look graceful floating through water like squishy goasts, but some of them have extremely dangerous tentacles - like this lion's mane jellyfish. Their tentacles are covered in nematocysts. The animal injects the stinging cells into its prey and kills them with the toxic injection. Plankton, algae, small crabs and fish larvae are all on the menu.

Nature and Environment

Burns like fire

For humans, encounters with the lion's mane jellyfish are very painful: after a sting, our skin burns and develops red welts. At least there's no lethal danger - which can't be said for encounters with the box jellyfish, or sea wasp. This species is at home along the northern and eastern coasts of Australia and in the western Pacific. Its toxin is among the strongest in the animal kingdom.

Nature and Environment

Colorful special effects

What else can jellyfish do? Tons of things! Pelagia noctiluca for example starts glowing as soon as its triggered mechanically, for example by water turbulence. This ability to create light, either alone or with the help of bacteria, is called bioluminescence. Incredible, isn't it?

Nature and Environment

Sophisticated life cycle

The sexual and asexual reproduction of jellyfish alternates from generation to generation. When jellyfish have produced sexual cells, these cells merge and create a larva type that attaches itself to the seafloor. A polyp emerges from this, and later several new jellyfish emerge from the polyp.

Nature and Environment

The sleepyhead of the sea

Jellyfish have no brain and no heart. But they do sleep. Surprised? Researchers at the California Institute of Technology have found that the upside-down jellyfish, Cassiopeia - which spends most of its time on the seabed - shows signs of sleep at night. How is that? Well, their pulse drops when they nap. And when they are disturbed, it takes them a while to wake up - just as with humans.

Nature and Environment

Jellyfish carpaccio

Beach towns often have to deal with jellyfish invasions. Bioligists believe this is due to overfishing and the decline of sea turtles and jellyfish-eating fish. But the squishy sea-dweller is also gaining popularity as a delicacy on restaurant tables. It has no natural aroma, which makes it the perfect flavor carrier.

Nature and Environment

Stranded

If you find a sad blob like this on the beach, it's most likely a jellyfish out of its natural habitat. If you want to do a good deed, grab some gloves for protection and deliver the animal back into the sea. Don't touch it with your bare hands, don't step on it and don't toss it onto your unsuspecting girlfriend while she's sunbathing.

Jellyfish invasions in coastal areas aren't an entirely new phenomenon. But two decades ago they were an occasional event. Now, they happen almost every year, Haberlin said.

Capitalizing on climate change?

Scientists agree that warmer sea temperatures are opening up new areas where jellyfish can reproduce, and increasing the availability of their favorite food — plankton. But it's not a uniform picture. 

Earlier this year, beaches on the Spanish island of Mallorca and the seaside resort of Benidorm were closed due to a plague of the Atlantic Portuguese man o' war — not exactly a jellyfish, but a siphonophore, a group of animals closely-related to jellyfish.

Qualle Portugiesische Galeere

The Portuguese man o' war plague has caused panic among tourists in the Mediterranean coastal areas

Experts suspect their arrival on the Mediterranean coast — which dates back to 2010 — is due to strong wind currents from the Atlantic Ocean, but these climatic conditions are not directly related to climate change.

And not all jellyfish thrive in warm waters. The world's largest jellyfish — the lion's mane jellyfish, which has a powerful sting and is often found around the UK and Ireland — prefers cooler waters. 

Related Subjects

Climate change has complex effects. For instance, it's blamed for lower levels of rainfall in southern Europe. That means the amount of fresh water flowing from rivers into the sea decreases, as does the difference between salinity levels in coastal waters and the open sea, where jellyfish mainly reproduce. This allows jellyfish to reach the shore with far less effort, often simply driven by the wind.

Feeding on pollution

A growing number of dams and inefficient river management exacerbate the situation, further reducing the amount of fresh water reaching the sea, José Luis García Varas, head of the marine program at WWF Spain, told DW.  

Ohrenquallen im Roten Meer

Jellyfish are part of complex ecosystems that have been sent out of whack by human activity

And the water that does make it into the Med is often highly polluted. High levels of nitrates and phosphates from agriculture ultimately end up in the oceans. Nitrates are key to the growth of phytoplankton, and even sewage from coastal developments can feed them up, meaning, in turn, more food for jellyfish.

At the same time, overfishing reduces the number of marine species that make life hard for jellyfish — a fact the Spanish environment ministry acknowledges.

Fewer predators, less competition

Unsustainable fishing practices, for example those that result in bycatch — fish caught unintentionally alongside the desired catch — remove small fish from the oceans, meaning jellyfish face less competition over food, Haberlin explains.

Haberlin says the complexity of the ecosystem means it's difficult to prove unequivocally that this advantage leads to an explosion in jellyfish numbers, but studies suggest it could be a factor. But there is little doubt that overfishing has drastically depleted the jellyfish's natural predators — animals like sea turtles, swordfish, ocean sunfish and Atlantic bluefin tuna.

Watch video 03:35
Now live
03:35 mins.
Eco-at-Africa | 04.05.2018

The deadly Mediterranean

The Mediterranean is the most important bluefin tuna fishery in the world but the species is endangered, WWF says, and more than 25,000 sea turtles are caught along its coasts every year, even though they are protected species.

And our impact on the marine ecosystem doesn't stop there. Even maritime traffic could be a factor, as it sometimes inadvertently transports species, including jellyfish, from one part of the world to the other, García Varas says.

Rooting out the causes

Experts stress that to really understand how all these factors fit together, we need more accurate data on jellyfish behavior and what makes them thrive. But we don't have to wait for a complete picture to act on factors we know are upsetting the Mediterranean's delicate ecosystems. 

"We have to work on the root causes that generate these changes to restore the balance in the oceans," García Varas says.

What is clear is that exploding numbers of the wobbly stingers along Mediterranean coasts are sign of the poor health of our oceans. And finding yourself skin-to-tentacle with one could be enough to ruin your vacation.

But they aren't just a worry for tourists. Ironically, jellyfish could bite the hand that feeds them, as people desert jellyfish-infested beaches, and fishermen in some parts of the world have already lost out economically as jellyfish clog their nets and gobble up fish eggs and larvae.

Fried egg jellyfish

With such a culinary name and its astonishing appearance, sea lovers probably do not need any further argument to be amazed. To top it off, it seems that this jellyfish's sting has very little or no effect on humans. In contrast to most jellyfish, this one can move on its own, and, surprisingly, it is one of the most common jellyfish in the Mediterranean Sea.

A vast fauna down there

When you feel something unfamiliar touching your toes, it might be comforting to think it's just plants - but that's not always the case. One of the plant-like creatures living on the bottom of the sea is this yellow cluster anemone, whose polyps can have up to 36 tentacles each. In the Mediterranean, this species often appears in dense agglomerations close to sponges, tunicates and algae.

Look but don't touch

Although it's very attractive for divers, this marine worm of about 15 centimeters - which can reach up to 35 centimeters - does not like to be disturbed. If so, its bristles can penetrate flesh and produce an intense irritation to the skin. Its vivid combination of colors is a warning for those in the know, but in case the message is still unclear, its name spells it out: bearded fireworm.

Flying under the water

The 'wings' of the flying gurnard are, in reality, very large pectoral fins, which enable the fish to glide above the water for short distances. Disappointingly, the flying gurnard does not fly as such; it is mainly a bottom-dwelling fish and only extend its fins when troubled or in need of scaring predators. This produces a beautiful spectacle of bright and fluorescent colors.

Would you guess that?

This three-centimeter flatworm is one of the enigmatic wonders that nature has to offer us. Its oval, flat body has a blue tone, a yellow central longitudinal band and dozens of white stripes - it can also appear in pink. Two tentacles on its head are reminiscent of a small mammal's ears or to a snail's horns. Its diet, sexuality and regeneration methods are also part of its unusual features.

The charming, but foolish, turtles

Some of the creatures living here are far more famous - but no less outstanding. The loggerhead is the most common sea turtle in the Mediterranean - but tourism and getting caught in fishing nets mean it's endangered. Their large heads, which support powerful jaws and enable them to feed on hard-shelled prey, justify their name. But in some languages they are known as the 'foolish turtles'.

'Turbot' is watching you!

Even with newly purchased professional goggles, it would be hard to notice this fish. But it is somewhere there, just under our feet. The turbot is a flat fish with both eyes on the left side of its - surely flat - head. It likes blending in with the sand and mud on the sea bed, but despite its camouflage, it is sought by fishermen around the world, as it is considered a highly-prized species.

Much more than a scary presence

There are not many natural dangers in the coastal areas of the Mediterranean Sea, and that might be the reason why jellyfish create such a panic. But despite the bad rap these annoying bathing companions receive , they are actually remarkable animals. As an example, the compass jellyfish has 24 impressive tentacles and, upon maturity, it changes sexual functions from male to female.

Here comes the star

The spiny starfish can reach an impressive diameter of 70 centimeters, even though the central disk is relatively small. Each of its five arms has three longitudinal rows of purple spines, each surrounded by a white cushion. Despite being native to the eastern Atlantic Ocean, its habitat range extends from Iceland to the Mediterranean Sea and the Azores.

Aquatic scorpions

Not everything is peace and love under the Mediterranean Sea. The small red scorpion fish is a very venemous species due to its sharp spines coated with mucus. It is mainly found in rocky littoral habitats, even though sometimes it can be found up to 700 meters under the sea. It is carnivorous and feeds on small fish and crustaceans - of course, one would expect no less from a scorpion fish!

Related content

Environment | 07.03.2018

'Blue economy'

Environment | 11.08.2017

Whales need a vacation from tourists

Environment | 19.12.2016

Hunting dynamite fishers in Tanzania