We live in the Northern Territory, where we have wonderful swimming weather, but alas cannot swim at the beach because of sharks, crocodiles and jellyfish. It may sound silly, but for half the year beaches are closed by tiny little jellies you can barely see.
The deadly jellyfish are not standard ones, but box jellyfish. These are a class of jellyfish with many different species that all have a cuboid or box-like umbrella. And they’re definitely not floating blobs at the mercy of the currents! They are actually complex predators, in both anatomy and behaviour.
Box jellyfish have 24 eyes in four different types. They come in clusters of six on four little lobes. Four of the eyes, the pit eyes and slit eyes, are simple eyes that only detect light and shadows. But the other two eyes are complex eyes with lenses, irises and corneas that can actually detect shapes.
In at least some species the lobe containing the eyes has a heavy crystal at the bottom which always pulls it down. This means that whichever way the jelly is swimming, one eye always points up and the other down. Researchers have done behavioral experiments on one species that suggest for them the upper eye is designed to look up out of the water at landmarks while the other eye can watch out for collisions.
The dangerous part of the jellyfish is the long tentacles with the stinging cells. These are specialised cells like tiny harpoons that shoot into anything they contact and pump in poison. They are designed to catch fish for the jellies to eat but will react to anything, including humans.
No-one is sure why they have such a strong poison, but it is thought it is to provide quick kills. As jellyfish they are fragile and can’t chase injured prey, killing the fish instantly means they can catch it or stop an enemy.
The tentacles are extremely long, for larger species they can be many metres. The tiny irukandji jellies are only about the size of your fingernail but can have tentacles a metre long.
If people are stung the intense pain can stop them being able to swim to shore. Vinegar will deactivate any stinging cells that have not been shot and stop further stings but unfortunately it won’t do anything about the venom already inside.
Box jellyfish don’t just drift but actively swim to find prey. Larger species have been seen swimming at up to 6m per minute.
One species has been tracked patrolling back and forth along a beach in the zone where fish swim. Another that hunts in mangroves will quickly head back towards the nearest trees if it is moved into the open.
Most of the deaths in Australia have probably been caused by the larger species of box jellyfish. This can be stopped by nets and some beaches are protected. However there is another deadly type of jellyfish here, the two tiny species of Irukandji.
These cause Irukandji syndrome, which was actually described and named a decade before the jellyfish themselves were found. It was named after the Aboriginal people in an area where it is common.
Irukandji have a slightly different type of stinging cell that means in humans the effects are delayed for minutes or hours. However once it begins there is severe pain, cramps, high blood pressure, nausea and psychological symptoms.
It has been described as:
“It’s like when you’re in labor, having a baby, and you’ve reached the peak of a contraction—that absolute peak—and you feel like you just can’t do it anymore. That’s the minimum that [Irukandji] pain is at, and it just builds from there.”
The severe pain usually lasts from 4-30 hours but can take weeks to resolve completely. Definitely enough to keep me out of the ocean!
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