Tiny sea crustaceans are to blame for a teenager's bloody Melbourne beach visit, a marine biologist says.

Shocking images of Sam Kanizay's lower legs and feet have been beamed around the world after the 16-year-old went for an innocent dip at Brighton's Dendy Street Beach on Saturday to cool his aching muscles following a tough game of footy.

It sparked widespread debate among experts about what could have turned the youth's feet into what looked like a scene from a horror film.

Sam's dad Jarrod Kanizay dunked some meat into the water, attracting a swarm of critters which have undergone testing.

Museums Victoria marine biologist Genefor Walker-Smith examined a sample and said they were a type of scavenging crustacean, technically know as lysianassidae amphipods.

Sam's dad said doctors were left confused and curious about what caused the injuries. (Fairfax)

"It was just unlucky. It's possible he disturbed a feeding group but they are generally not out there waiting to attack like piranhas," Dr Walker-Smith said in a statement on Monday.

It was possible the bugs contained an anti-coagulant similar to that produced by leeches, which explained the inability to stem the flow of blood, she said.

Sam Kanizay's feet started bleed after he walked out of the beach. (Facebook/Fairfax)

Sam is unable to walk and remains in Dandenong Hospital.

"As soon as we wiped them (his legs) down, they kept bleeding," Mr Kanizay said.

"There was a massive pool of blood on the floor."

Hospital staff were initially at a loss to explain what had happened as some other community members came forward to report they had experienced similar symptoms in the past.

Jeff Weir, executive director of the Dolphin Research Institute, once suffered a similar experience while on a cold night dive.

"I didn't realise it at the time because the water was cold and my face was numb, but my forehead and cheeks were bleeding," Mr Weir told AAP.

The marine biologist agreed the culprit was likely crustaceans called amphipods, which scavenge on decomposing plant and animal scraps, like slaters do in the garden.

"Most people won't feel it because they move around quickly in the water... the lad must have been standing very still for quite a while (to be bitten)," he said.

St Kilda Football Club players often use the beach for recovery but were warned to stay out of the water following the latest report.

Player Koby Stevens said he's heard of a similar experience.

It's not yet clear what attacked the 16-year-old's feet. (Fairfax)

"A couple of weeks ago, one of my mates was down at the water and he came out and had blood on his feet as well," Stevens told afl.com

"You really need your feet for this game so we were advised not to go down there."