For 500 million years, jellyfish have been part of the maritime ecosystem, but now they’re poised to take over the earth.

They have no brain, no eyes, no spine, not even blood, but they have a remarkable capacity to reproduce and can pack an impressive sting, both literally and figuratively.

Most recently, vast numbers of bluebottle jellyfish were pushed ashore by unusually strong winds and spells of hot weather in Queensland, Australia, stinging thousands of people and forcing the closure of popular swimming spots. About 13,000 stings were recorded in the past week.

In June last year, over the course of just one week, over 1,000 people were stung in Volusia County, Florida, following a period of exceptionally prolific jellyfish blooms. The explosion in their numbers has been attributed to warming seas and even increased pollution; unlike many other marine creatures, jellyfish can cope with reduced oxygen levels.

Small but deadly – at least some of the time

Typically, jellyfish range in size from 1cm to 40cm. But they can be significantly larger – the Lion’s Mane Jellyfish, for example, can reach 1.8 metres wide, with tentacles over 15 metres long.

Lion’s Mane Jellyfish: not to be tangled with Image: Tony Hisgett/CC2.0 via Wikimedia Commons

For the most part, the sting of a jellyfish is more unpleasant than it is harmful. The pain comes from venom delivered via millions of microscopic barbs in the creatures’ tentacles. Most jellyfish stings will only have a localized effect on the victim – redness, swelling, and discomfort where the barbs make contact with the skin.

Some, however, will prompt a systemic, whole body, reaction. These may take several hours to emerge and can include symptoms such as headaches, nausea and drowsiness.

In rare cases, the sting can be fatal. This is true of the box jellyfish, which is spreading into waters that had previously been too cool to support it; its venom causes a severe reaction that can cause death within minutes.

A force of destruction

But these booming jellyfish populations are doing far more harm than ruining people’s trips to the beach. In fact, the scope of their disruption has extended far beyond the water’s edge.

In 2011, both reactors at the Torness nuclear power plant in Scotland were shut down after an invasion of jellyfish started blocking the cooling filters. Two years later, the jellyfish struck again – this time in Sweden. They forced the closure of the Oskarshamn nuclear power plant, which contains the world’s largest boiling-water reactor.

The island of Luzon, home of the Phillippines’ capital Manilla, suffered a blackout in 1999 due to jellyfish, and in 2006 the USS Ronald Reagan, a nuclear-powered aircraft carrier, was brought to a standstill by thousands of the little creatures. And while these events may stand out as exceptions, they are increasing in both scale and frequency. From sea-bed diamond mining in Namibia to salmon farming in Ireland, even jeopardising the sustainability of beluga caviar farming in the Caspian Sea, jellyfish are as destructive as they are abundant. And that abundance is being caused by a variety of factors, many of which are related to human activity.

Some like it hot

Over the last hundred or so years, the average surface temperature of the world’s seas has risen by about 0.9°C. As the oceans get warmer, marine animals are able to spread into areas that had historically been too cold. Oxygen levels in the sea have fallen by around 2% over the last 50 years, due to rising temperatures and pollution.

Jellyfish can thrive in areas with lower oxygen levels, where other animals suffer. But there are other factors at work, too. Fishing has depleted the global stocks of some of the jellyfish’s natural predators – such as tuna and swordfish – and some they compete with for food – such as anchovies. With more food and fewer predators, some jellyfish populations can grow unchecked.

The strangely ethereal, oddly named warty comb jellyfish Image: Vidar A, Gozo/CC2.0

In the Black Sea, unchecked population growth is precisely what’s happened. Anchovy fishing in the region had caused harm to the Black Sea’s ecosystem by the time stowaway jellyfish made the journey there from the eastern seaboard of the USA. Most likely transported in the ballast water of ships that made the crossing, 1982 saw the arrival of the warty comb jelly. By 1990, there were 900 million tons of them in the Black Sea.