If you're terrified of the ocean because of sharks, you should reconsider—the real thing you need to be afraid of is invisible.

As reported by Australian news outlet The Age, 16-year-old Sam Kanizay decided to cool off after a difficult football practice on Saturday night by taking a dip in the water at the Dendy Street Beach in southeastern Australia. When he came out, his legs were dripping with blood.

Sam Kanizay's father, Jarrod Kanizay, told Australian news outlet news.com.au that his son didn't know he was bleeding until he stepped out of the water.

"He went back to his shoes and what he found was blood on his legs. As soon as we wiped them down, they kept bleeding," he said. "There was a massive pool of blood on the floor [at the hospital]."

But the culprit wasn't sharks, stingrays, or any of the usual suspects in marine attacks on humans.

There seems to be consensus among local biologists, including Genefor Walker-Smith, the collection manager of marine invertebrates at the Museum Victoria, and Richard Reina, a professor of biology at Monash University, who told The Age and news.com.au that they believe sea fleas are the culprit. They were tipped off by Kanizay's tiny, pin-like bite marks.

Image: Wikimedia Commons

Sea fleas encompass a number of flesh-eating species that are native to the Port Phillip Bay, where Dendy Street Beach is located. One of the most common sea flea species in the Port Phillip Bay is Natatolana woodjonesi, which prefers dead and dying fishes and animals. Since sea fleas generally prefer shallow water, Kanizay was a vulnerable target.

Walker-Smith told The Age that there's no reason to panic or shut down the beaches, since this incident was highly unusual. Kanizay just happened to be standing still in the wrong place.

"[Sea fleas are] there all the time," she said. "You could put a piece of meat in the water, anywhere in the bay, and you could find them."

Reina told news.com.au that the sea fleas could have been pushed to shore by strong winds, and they typically don't travel in swarms. Kanizay's bleeding most likely just attracted a lot of other fleas.