If you’ve spent the day at the beach and left with little bites on your feet, you might have been the victim of tiny little nuggets called sand piranhas. Thankfully, they’re not going to hurt you (although you might feel a little pinprick if they decide to take a chomp out of your foot) and they’re not dangerous to humans. In fact, swarms of these creatures help to keep our shorelines clean.
What is a sand piranha?
Sand piranhas (Excirolana chiltoni) are a type of ocean-dwelling isopod that live in intertidal zones along Pacific Ocean coastlines where they feed on dead and decaying matter. This might sound gross but it’s an important part of keeping beaches clean and breaking down carcasses so their nutrients can be returned to the habitat for other animals to benefit from.
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Some beachgoers may have come across sand piranhas and, although they’re not dangerous, it might have been a slightly unpleasant experience. If you spend too long loitering in the wet sand, these curious critters might come along to see if you’re a delicious snack – ouch!
What do they look like?
Sand piranhas are very small – no larger than a grain of rice – so they’re rather hard to spot when they’re nestled under the seabed but you might see little lines in the sand indicating where they’ve been.
If you do see one (or several) of these marine critters, you might notice that they look a little like tiny woodlice – in fact, they are both types of crustaceans called isopods.
Sand piranhas have segmented bodies, lots of legs and special feeding parts that help them scrape away at carcasses while they’re eating.
Where do sand piranhas live?
These animals are found in warm waters on coastlines around the Pacific Ocean from the U.S.A to Canada and along the shorelines of Japan. They spend much of their time buried underneath wet sand of the swash zone – where the waves break on the beach – hiding from predators such as seabirds and emerge at high tide to feed.
What do they eat?
These teeny scavengers help keep shorelines clean by devouring dead and decaying matter such as carcasses, seaweed, kelp or other organic debris rotting in the sand. This gives the little scroungers a great source of energy and protein and helps to recycle nutrients, putting them back into the ecosystem.
They prefer dead matter to living prey but that’s not to say they won’t try to get a little taste of your tootsie if they get the chance. If you stand still for long enough when sand piranhas are nearby, they might try to take a nibble out of your feet – so watch out. It’s not agonising but you might feel a sharp sting like a papercut when they nip you and they can draw blood.
How do they hunt?
Hunting probably isn’t the best term for it because their preferred prey is already dead but when these tiny isopods detect chemical cues from something tasty, they swarm towards it in their hundreds.
Individuals have tiny hooks on their legs to help them get a good grip on their prey so they can start feasting. Once they’ve clamped on, they use their mouthparts to start chomping on their dinner with their jaws, using their front legs to direct food to their mouth.
How do they reproduce?
These small creatures are ovoviviparous, meaning the mother incubates eggs inside her body then later gives birth to live young. A study in Japan suggested their breeding season runs from early April to late September.
The newborn – known as a manca – look like smaller versions of the adults except that they are missing their last walking legs. These grow later as they mature.
Are they dangerous to humans?
No. Although no one wants a bite taken out of them at the beach, getting nipped by these isopods isn’t going to kill you. Their bites feel like a pinprick and may sting a little.
Thankfully, there’s an easy way to keep them at bay: stay on the move and keep walking. These little nibblers only bite you when you stand still long enough for them to swarm.
Are they under threat?
Not that we know of. Sand piranhas haven’t yet been categorised by the IUCN Red List of Threatened Species.
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Top image: University of California San Diego, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons
