JELLYFISH SUCH AS THIS CEPHEA CEPHEA, ALSO KNOWN AS THE CAULIFLOWER JELLY. ARE BRAINLESS, BLOODLESS AND MOSTLY AIMLESS.
An animal that is 98% water is not the obvious suspect to threaten an aircraft carrier. But it happened. Brainless wanderers don't seem likely to capsize a fishing trawler. But they do. A soft bag of jelly with no teeth doesn't spring to mind as the most dangerous beast in the ocean. But so it is. These are the extreme aspects of jellyfish, known to most of us as the transparent blobs that wash up on the beach after strong onshore winds. Obviously, there's a lot more to them that just jelly.
Jellyfish have been around, according to the fossil record, for more than 500 million years. That's well over 100 million years longer than fish. They are creatures of the sea, wave-borne and current swept, and depending on the species, their soft bodies are made up of between 95 and 98% water. The big ones get heavy though -- a jellyfish known from the Sea of Japan can weigh 200kg, That's about the same weight as a sumo wrestler and unsettlingly, they are made into a vanilla icecream. A Japanese trawler set its nets around a swarm of these jellies, and discovered that the jellies were mightier than their trawl gear when their ship capsized. Jellyfish icecream is perhaps a pleasure to be taken in moderation.
The biggest jellyfish of all, the lion's mane jellyfish, is found in the all the northern oceans, can weigh 1000kg and reach a bell diameter of 2.3m with tentacles of 37m; longer than the biggest blue whale. That's a stack of slightly solidified water.
Their basic body form is the medusa, a beautiful usually transparent bell shape with trailing tentacles. Muscles, digestive organs and gonads are all crammed together inside the bell. Sensory detection organs and tentacles are usually on the bell margins.
Jellyfish can shrink in times of famine, reducing the amount of food they need, and then grow bigger again once they are around food. Depending on the jellyfish, that food could be fish or invertebrate larvae found floating in the plankton, small fishes, shrimps or other jellyfish. They store oxygen in their flesh and can cope in low oxygen environments. They manage well in highly saline waters, partly because the increased iodine content helps them grow. All of which make them superbly capable of managing most marine environments, even polluted ones.
They use a form of jet propulsion to get around, expanding and contracting their belled bodies to push themselves through water. When they form blooms, the term for an aggregation of many jellyfish of the same species, they are even capable of swimming against currents to reach these blooms. Not bad for an animal which is statistically all water.
When it comes to reproduction, even more so than in human life, it's complicated. Jellyfish can reproduce both sexually and by cloning. Given enough food, jellyfish spawn daily. Most species are split into males and females, though some species are hermaphrodites. Spawning is usually determined by light cues, and eggs and sperm are released into the water at either dusk or dawn. Apart from this timing, fertilization is left to chance, eggs and sperm meeting in midwater. The fertilised egg will develop into a larval form that settles onto a firm surface and starts to grow. As it grows, it asexually buds free-swimming larvae which then swim away and develop into small jellyfish. That's a lot of jellyfish emerging from just one egg.
They hunt passively by using their tentacles as drift nets, and subdue their prey with stinging cells which are usually grouped all over their tentacles. These are formidable weapons.
The stinging cells are known as nematocysts or cnidocysts, and like the nettles they are named for, they sting. Microscopic investigation has revealed that they consist of a special sac with a coiled tube with a barb at the end inside it. There are sensors on the outside of the sac which detect pressure and chemicals. This combination of sensor types ensures that the cell only fires when a prey animal or predators are about. As anyone who has been stung by a bluebottle knows, these stings can be ferocious – nowhere more ferocious than off the east cost of Australia, where a species of box jellyfish lives. These animals have tentacles that can be 3m long and that are covered with millions of stinging cells. If they sting a large enough area of a human and the sting is left untreated, the person will die within two to five minutes.
That's one good reason to avoid swimming off the Australian east coast in summer. Another is the thumbnail-sized jellies called Irukandjis, which have recently been identified as the cause of several mysterious deaths: though the initial sting isn't noticeable, terrible pain develops, and patients become so convinced of their imminent death that they beg their doctors to kill them to end their suffering sooner. Hard to decide between those two as the most dangerous.
Despite stories about treating the stings with urine, it seems the best treatment involves washing the affected area with vinegar and using a blade to scrape off any remaining tentacles, along with rinsing with sea water. Fresh water seems to cause remaining stinging cells to fire. Helpers need to wear gloves to avoid being stung themselves. And like vampires, jellies can still sting after they are dead. There's even a species which seems to be immortal. A few days after an individual dies, cells emerge from the corpse, find one another and re-form as a tiny polyp which then grows and repeats the cycle.
For all their ethereal beauty, jellyfish are in fact formidable animals. They are also on the increase. They cope in a wide range of temperatures, so ocean warming isn't a problem for them, and they grow well in polluted seas, using pollutants as food. Not having a shell, increased ocean acidification doesn't trouble them and our habit of chucking solid plastics into the ocean generates many solid surfaces for their budding larvae. On top of that, the fish and turtles which usually eat them are under pressure. All of which explains why jellyfish blooms are becoming more frequent.
The USS Ronald Reagan, a nuclear-powered aircraft carrier, was visiting Brisbane in 2006 and had to shut down part of its capabilities when it sucked a swarm of jellies into its water intake. The navy lost the battle of the jellies and the ship had to leave port to find uncontaminated water for its cooling systems. Much to the locals' relief: nuclear power and overheating are a dangerous combination.
Jellyfish blooms can shut down power stations and can't be deterred by electric shocks, chemical repellents or acoustic shocks. They clog filters and are expensive to remove. Fragile and brainless though they are, these beautiful animals may well become the dominant life form in our oceans. Perhaps we will have to get used to eating jellyfish in more than icecream.
text: Georgina Jones
photo: Jenny Strömvoll
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