A Melbourne teenager has gone for a dip at the beach and come out covered in blood and looking like a horror movie extra.

Key points: Boy taken to hospital when bites wouldn't stop bleeding

Boy taken to hospital when bites wouldn't stop bleeding 16-year-old had been standing in cold ocean

16-year-old had been standing in cold ocean Expert says tiny amphipod creatures were the culprit

When Sam Kanizay, 16, felt sore after football on Saturday, he decided to soak his legs at Dendy Street Beach in Brighton just before 6:30pm.

Half an hour later, he walked out covered in what his family said were tiny marine creatures eating his legs.

Museums Victoria has confirmed the tiny animals were identified as lysianassid amphipods, a type of scavenging crustacean.

WARNING: This story contains graphic images.

"I walked out and saw what I thought was sand covering my calf and shook it off, and by the time I'd walked across the sand about 20 metres to put my thongs on, I looked down and noticed I had blood all over my ankles," he said.

What are amphipods? Amphipods are tiny invertebrates, often described as prawn-like

Amphipods are tiny invertebrates, often described as prawn-like There are thousands of species

There are thousands of species Mostly found in marine environments, some found in damp land areas

Mostly found in marine environments, some found in damp land areas An important food source for many creatures

An important food source for many creatures They feed on decaying organisms

Sam said he wasn't in any pain at first, so he walked home from the beach and had a shower.

"[I couldn't feel anything] because the cold water basically numbed my legs, I felt what I thought was pins and needles," he told 3AW.

"It sort of looked like hundreds of little pinholes or pin-sized bites distributed all over my ankle and the top of my foot," he said.

"We've had a few people just guessing that it was sea lice … but there was no real evidence to say what it was."

Hospital staff were at a loss to explain what happened. ( AAP: Supplied )

Sam's mother, Jane Kanizay, said he was very calm when he got home.

"We just thought we'd clean off some of this blood, and when it wasn't washing away [we] made the call to take him to hospital immediately," she said.

She said it started to become painful for Sam once the numbness and cold on his legs wore off.

Sam's father, Jarrod Kanizay, said staff at the hospital were at a loss to explain what had happened but he expected to have answers in the next day or so.

"They've called a number of people, whether it's toxicity experts or marine experts and other medics around Melbourne at least … [and] yep, no one [knows]," he told AAP.

He said Sam was in a bit of pain, but the blood had clotted and they expected he would have a full recovery and hopefully no scars.

The next night, 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 he shot, showing the bugs in a tray of water devouring chunks of meat.

Sorry, this video has expired Sea bugs that attacked Melbourne teen seen swarming meat

But Jeff Weir, the executive director of the Dolphin Research Institute, said he suffered a similar experience while on a night dive under the Portsea Pier taking photographs.

The marine biologist said the culprit was likely to be a type of amphipod — an order of shrimp-like crustaceans.

"It was very cold, probably, about this time of year, and when I got out of the water I realised my forehead was bleeding and parts of my cheeks were bleeding," he said.

"At night what happens is the little amphipods, these little tiny crustaceans, they all swarm into your light.

"They'd crawled in under my wetsuit hood and into my beard and chewed away, very similar to what happened to this young lad."

'Not life-threatening but a great story'

Jeff Weir was able to find some amphipods near Hastings, on the Mornington Peninsula. ( ABC News: Gloria Kalache )

Mr Weir said Sam's case was one of the most extreme he'd ever heard about, but it was not uncommon for divers who remained stationary for long periods to be targeted.

"Most people, if they go in, they're moving around reasonably quickly, either swimming or walking around. You have to be pretty still for quite a while [to be bitten]," he said.

"These are very important little critters that live in the water — just like garden slaters in the garden that clean up the breaking-down debris, these things are a really important part of the ecosystem.

An expert believes amphipods were drawn to Sam because he was standing so still. ( AAP: Supplied )

"It's a bit annoying for the young lad, he must have been there for quite a while and not realising he's getting nipped away. It's not life-threatening, but it's a great tale to tell."

Mr Kanizay couldn't stop his son's legs bleeding when he came home and they went to hospital.

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

"They ate through Sam's skin and made it bleed profusely.

"As soon as we wiped them [his legs] down, they kept bleeding.

"There was a massive pool of blood on the floor [at the hospital]."

Mrs Kanizay said the family would still go for a dip in the bay, but they would make sure they remained active in the water from now on.

"I wouldn't stay still for 30 minutes at that time of the evening," she laughed.

"I'll still swim in summer and I'd probably still ice after my long runs as well.

"It was a normal thing for us to do to, the footy team is down there all the time so that's why [Sam going to the beach] was of no concern to us."

ABC/AAP