Explainer: What are sea fleas anyway?

Teenager Sam Kanizay emerged from the water at Melbourne's Dendy Street Beach in Brighton covered in blood. Until he looked down at his feet, he did not even know he was bleeding. He only felt a slight tingling sensation and numbness on his legs, while he was standing in the chilly water.

The culprit? It's believed hungry, flesh-eating sea fleas are the cause.

What are sea fleas?

Sea fleas are tiny carnivorous crustaceans. There are hundreds of recognised species, and they feed mostly on dead marine life such as fish, crabs, sea birds and even whales, but will nibble live human flesh if it's convenient. Many are so small a microscope is required to recognise them.

You've probably come across them before - they are little creatures that hang about in the shallows and leave you feeling itchy. They cause particular problems for children; the fleas get stuck in their bathers and bite them repeatedly.

How big are they?

Sea fleas can range in size from about half a centimetre to 1cm.

What do they eat (apart from human flesh)?

Dead and dying marine creatures.They are happy nibbling on tissue that is lying on the ocean floor.

Where do they live?

They are common in shallow water, but have been seen as low as 4500 metres deep.

How many are there?

"Hundreds of millions," says Museums Victoria marine scientist Dr Genefor Walker-Smith.

Are they dangerous?

No. "They're not venomous, you normally would feel their bite and move out the environment where you have been bitten. It's unusual for this person [at Brighton] to be bitten so much. Maybe if you were a haemophiliac it would have been dangerous, but for most people they're just an annoyance."

Could they be responsible for this attack?

Yes, says Dr Walker-Smith.

A swarm of sea fleas can be attracted to a cut in human skin, or humans can step near their food, such as a fish carcass, and be attacked.

Dr Walker-Smith said they live in both colder and warmer water, but are more active at night and dusk, to avoid fish predators.

"It probably did make it worse that he was standing still; they may not have been able to cling on too tightly if he had been moving through the water. They're used to eating dead things still on the bottom."

Should we be closing the beach?

Dr Walker-Smith doesn't think so.

"I think this is quite a rare thing. I really just think he was in the wrong place at the wrong time."

She suggested swimmers should "be careful where you put your feet".

The story Explainer: What are sea fleas anyway? first appeared on The Age.

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