The creatures behind an alarming incident in Brighton over the weekend have been identified as amphipods, and they usually don't go around attacking people like piranhas, a Museums Victoria marine biologist says.
When he arrived home, still bleeding profusely from hundreds of little pinhole bites all over his legs, his father Jarrod Kanizay described the scene as "horror movie-ish".
WARNING: This story contains a graphic image showing pooling of blood.
Mr Kanizay returned to the beach with Sam's siblings the next day to see if they could collect some of the culprits.
"I went out to the same distance and same area that Sam was in, and we loaded the net up with some bloodied meat, some bones, and after 15 minutes I came out of the water," he told ABC Radio Melbourne.
"We caught thousands of little mite-type characters, we took them home in the Esky, and we were playing scientists for an hour thereafter, putting them in different dishes and feeding them different things.
"What we found is they were very attracted to red meat, they went straight for it."
Marine biologist Genefor Walker-Smith has also since studied the samples Mr Kanizay collected and identified them as lysianassid amphipods, a scavenging sea creature.
"They are naturally occurring in the environment," she said.
"These animals are scavengers, they live on meat or animal matter, they've got a really keen sense of smell I suppose you'd say.
"They have chemo-receptors, so they can detect chemicals in the water and the chemicals given off by decaying meat or fish, so they're attracted to that smell.
"They occur in swarms, so they'll swarm on a dead fish and eat it in next to no time."
But she said swimmers at Melbourne's beaches were unlikely to suffer the same fate as Sam.
"It was just unlucky. It's possible he disturbed a feeding group but they are generally not out there waiting to attack like piranhas," she said.
"It's possible that he just happened to be standing quite close to a dead fish, or maybe he knocked it or touched it with his foot.
"The crustaceans would have swarmed off that piece of dead fish and unto his leg, he may have already had a cut perhaps and they were able to smell that wound or any chemicals that the wound was giving off."
She said he was probably also an easy target for the creatures because he was standing still in the water.
"His legs were numb, he couldn't feel the biting ... I think as long as you're moving around, they're unlikely to attack you," he said.
Lysianassid amphipods not venomous
Mr Kanizay said his son was still in a bit of pain after the attack, but was doing well.
"Sam's a really positive kid and remains in the great spirits and when he got home, he said 'look Dad, look what happened' and he was sort of anaesthetised by the cold water at that stage," he said.
"But throughout the process, he's been really positive and happy … he's in good health and we expect a full recovery."
Dr Walker-Smith said the creatures were not venomous.
"It's really just that they've punctured the skin and he's bled," she said.
"It's possible that because there was so much bleeding it's indicating that they've possibly got some sort of anti-coagulant chemical that they're releasing, so a bit like when a leech bites you and it bleeds a lot, it just stops the blood clotting."
