Sea lice, stingrays and jellyfish larvae are all suspected of attacking Melbourne teenager Sam Kanizay who emerged from a beach swim with both legs bleeding profusely.
Trying to solve the gruesome mystery, Sam's father Jarrod Kanizay took meat in a net into the water and recorded footage showing dozens of critters feeding on the chunks.
Jeff Weir, executive director of the Dolphin Research Institute, who has also suffered a similar injury while diving, says the critters in the video are likely sea lice.
Scientifically referred to as amphipods, the tiny crustaceans scavenge on decomposing plant and animal scraps, like slaters do in the garden.
"In the deep ocean, these little guys can grow bigger than a rat. But in our bay they are about one centimetre," Mr Weir told AAP.
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But parasite expert Dr Thomas Cribb from the University of Queensland said it was "really unusual" for amphipods to cause such extensive bleeding or injury.
"It's not a parasite I've ever come across," he told AAP.
Doina Canta, a member of the Brighton Icebergers swimming group, says a sting ray could be to blame.
A man in her swim group was stung about six months ago on one leg and it bled extensively, she told Fairfax.
Marine expert Michael Brown was puzzled by the small bugs in the video and suggested they could be jellyfish larvae.
"I've been doing this for (nearly) 20 years and I've never seen anything like this," he told Channel Seven's Sunrise program.
"I don't think it's a stingray, it can't cause this type of laceration... it's a mystery."
