It was a not-very-dark and semi-cloudy night. An Australian teenager named Sam went wading in the sea in a Melbourne suburb to cool off after a soccer match. When he stepped back onto the beach, he was covered shin-to-toe in blood, which was streaming from tiny holes. It took a trip to the ER to stop the bleeding.
We already have a prime suspect. Sam’s crafty dad, Jarrod, returned to the scene of the attack, tossing raw steak in the water and catching the ensuing feeding frenzy on film.
Jarrod also caught a few of the bloodthirsty critters and took them to the local museum for identification. The culprit? Amphipod crustaceans, a.k.a. sea fleas.
Case closed? Not necessarily. These sea fleas certainly had opportunity. But being totally down for free beef isn’t the same as boring into live teenager leg.
When Niel Bruce—a curator at the Museum of Tropical Queensland and one of the world’s foremost experts on mini marine crustaceans—watched the raw-steak video, something else caught his eye. Most of the scavenging sea-bugs in the video filmed by Sam’s dad are indeed sea fleas.
But there’s something else in the dish too: cirolanid isopods.
Sometimes known as sea lice, these guys are voracious feeders, known to reduce a seal carcass to nothing but skin and bone in mere hours. Isopods are also good swimmers, which also makes them keen on swarming.
This bone-cleaning efficiency is crucial to keeping dead matter from warping marine ecosystems. But for non-dead creatures, their voraciousness and swimming skills can sometimes be a nasty combination. In 1978, a species of cirolanid suddenly began converging on sharks in the waters off Florida, killing huge predators by boring into their hearts (pdf, p.379). Their cousins Down Under did something similar to sharks in south Australia (pdf, p.5).
And, yep—they go for humans too.
“My knowledge of these scavengers is that cirolanid isopods are actually well-known biters in surf beaches and on coral reefs and have been recorded as biting swimmers and divers on a number of occasions,” says Bruce. “I know of one [incident], in Sydney, where divers were cleaning jetty pylons and were attacked by isopods that chewed the exposed skin between their face masks and wetsuit hoods.”
Even at 6-10 millimeters (about 0.2-0.4 inches) in length, the isopods Bruce spotted in the dish “have wide strong mandibles that do not have any difficulty in biting through human skin,” he says. Amphipods have smaller mouthparts, and reports of their attacks are relatively few.
Since these two types of scavengers tend to hang out in the same places, the amphipods may have been aiding and abetting.
“It is possible that both groups of small crustaceans attacked the boy, possibly with isopods opening the wound and then both isopods and amphipods feeding at the injury,” says Bruce.
So why did the amphipods fall for the steak sting in large numbers, if isopods were the more likely leg-biters?
It might have been, says Bruce, that “while isopods were involved they had moved away or burrowed into the sand when the sample was later collected.”
A classic case of frame the amphipods? Could be. Or maybe they were just already full of leg.