Warning: Graphic images

Locals have got back in the water at a Melbourne beach where a teen was left bleeding profusely from both legs, likely to have been caused by thousands of tiny bites from sea fleas.

Sea fleas are about half-a-centimetre to one-centimetre long and tend to clean up the fish carcasses found on the sea floor. You often find them in shallow water.

Sam Kanizay, 16, emerged from the water at Dendy Street Beach in Brighton, in Melbourne's southeast, on Saturday covered in what his family said were tiny marine creatures eating his legs.

Doctors later struggled to stem the bleeding from scores of what looked like tiny pin-prick bites.

Dr Walker-Smith told Fairfax he believed the bleeding wouldn't stop because of the anti-coagulant released by the fleas to stop blood clotting.

Sam's father has since posted a sickening video of the creatures, scooped out of the water with a net and believed to be sea lice - feasting on chunks of raw meat.

Local swimmer Paul Duckett said he'd never seen anything like it, despite daily swims at the same spot for the past 16 years.

"We swim there every day and we're in the water for anything from 15 to 30 minutes, and no one's ever experienced anything like this," he told Fairfax Media.

"This was a first, so that's why I query whether it's sea lice or some other creature that have caused the issue."

Associate Professor at Monash University's School of Biological Sciences Richard Reina, however, was confident Sam's bites were caused by sea fleas.

"I think it's very rare," Mr Reina told news.com.au.

"When it happens you brush it off, or move, or get out of the water and there's no consequence, whereas in this case, Sam was standing in cold water for quite a long time.

"He probably thought the pins and needles he described was the cold and didn't realise there were crustaceans chewing on his feet."

Sam's father Jarrod Kanizay said while his son was recovering in hospital, the family was waiting for answers and doctors had been at a loss to explain what had eaten through Sam's skin.

"When he got out, he described having sand on his legs, so he went back in the water," he said.

"He went back to his shoes and what he found was blood on his legs.

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

"There was a massive pool of blood on the floor (at the hospital).

Associate Professor Reina said sea fleas did not travel in a pack, but once a couple began chewing on his feet, releasing some blood into the water, others would have been attracted.

"They are very good at finding food," he said.

"It looked really bad in the photo, his feet looked like they went through a mincer, but it's a superficial injury and more like a graze than anything else. Because it's a larger area it looks pretty terrible.

"I would expect and hope he will recover pretty quickly."

Associate Professor Reina said sea fleas were common in waters but had a theory about why they may have been hanging around Brighton on the weekend.

"Maybe the strong westerly winds pushed them towards the shore and they congregated on the eastern side of the bay where Sam went," he said.

The night after Sam's attack, Mr Kanizay went back to the beach with a pool net full of meat and captured the creatures he said were responsible.

"What is really clear is these little things really love meat," he said of a video, shared with AAP, showing the bugs in a tray of water devouring chunks of meat.

Sam is not the first Melburnian attacked by sea fleas.

In 2015 a father and son were bitten while taking a dip at Sandringham beach, in Melbourne's southeast.

Nick Murray and his son Will entered the cold water, also in the month of August.

Mr Murray told the Bayside Leader he did not feel pain initially and they only realised their feet were bleeding when they got out of the ocean.

They sustained multiple bites to their lower legs and they saw the creatures still clinging to their feet once they'd washed away the blood.

"I wonder if it was just that night, just that spot, at that particular time," Mr Murray said.

"We stood still for 10 minutes so it may not have happened if we moved about, but I wouldn't want to stand still there for a couple of hours or it may get quite bad."

Associate Professor Reina said sea fleas should not alarm people or deter them from swimming.

"They are just little animals following their natural instinct and eating what they thought was a free meal," he said.

"The situation is really unusual and I don't think it's something people should be concerned about. They are quite small and mainly eat dead and dying animals and because he was standing still while they were biting him gave them the opportunity.

"They wouldn't thought he was dying or dead."