17 Awesome Facts About Giant Isopods

A giant isopod in the Gulf of Mexico in 2017.
A giant isopod in the Gulf of Mexico in 2017. / NOAA Office of Ocean Exploration and Research, Flickr // CC by SA 2.0

There are 20 known species in the genus Bathynomus; B. giganteus, or the giant isopod, is the biggest. Yes, they're kind of creepy looking. But they're also kind of cute! Here are a few things we know about these internet-beloved creatures.

1. Giant isopods aren’t bugs.

They’re crustaceans, and are closely related to marine crustaceans like shrimp and crabs, and terrestrial crustaceans like the pillbug (Armadillidium vulgare).

2. They’re bottom dwellers.

NOAA Okeanos Explorer Program, Gulf of Mexico 2012 Expedition, via NOAA Photo Library, Flickr // CC by 2.0

Giant isopods live between 550 to 7020 feet deep (and potentially deeper), and prefer a mud or clay floor, which they burrow into for shelter. “Bathynomus giganteus is more of a coldwater species,” Dee Ann Auten, an Aquarist II at the Aquarium of the Pacific—which has some giant isopods in its Wonders of the Deep gallery—told Mental Floss in 2014. “They live in the Pacific Ocean, off Japan and in the South China Sea. That’s the kind of area where you would find them.”

3. As their name suggests, giant isopods get big. Really big.

Typically, giant isopods are between 7.5 and 14.2 inches in length, but they can get much bigger: One specimen pulled up with an ROV in 2010 was 2.5 feet long. Scientists aren't quite sure why these isopods get so enormous, but believe that their huge size might be an adaptation that helps them survive the extreme pressure of the deep ocean.

4. They come in different colors.

The giant isopod shell—which is comprised of overlapping segments—comes in two varieties: brown and pale lilac.

5. Giant isopods are carnivores.

Though they’re generally believed to be scavengers, feasting on dead animals that fall from above, some evidence suggests that they might also eat slow-moving live animals like sponges. Giant isopods also attack trawl catches.

6. They can go a very long time without eating.

One giant isopod in Japan went for five years without eating a single bite before dying in 2014. That same year, Auten told Mental Floss that she attempted to feed her giant isopods every day, a ritual that required a lot of patience. “The trick is what to feed them and how to eat them,” she said. “Here at the Aquarium of the Pacific, the hit is mackerel. It's usually what I feed them. I’ll butterfly a dead mackerel so that the insides are coming out, and then I will present it in front the isopod. I try offering food once a day and that's just because one day they might not be active as much, and one day they could be really hungry and I might miss that opportunity. ... It's fascinating and it's rewarding when you put so much effort into taking care of them and a lot of patience and you finally figure out this what they like to eat.”

7. But when they do eat, they gorge themselves.

Giant isopods have four sets of jaws—which are adapted to cut and tear at prey—and they get a workout when the animals are hungry. “When they're hungry and they're eating, definitely have a lot of food around them, because they'll keep eating,” Auten said. “They'll eat a lot at one time and then they can go for a long time without eating. There’s a comic of one giant isopod eating a dead whale, and it eats the whole thing except for the bones. It's sitting on its back like ‘ughh I'm full now.’ That's totally true! If they eat, they eat a lot.” In fact, they eat so much that they compromise their ability to move.

Still, they’re not aggressive feeders. “I've never had [an occasion] where they all ate at once,” Auten said. “I will make sure to bring enough food for all of them but whenever I feed them, it's one will eat at that one setting. They definitely would not fight.” And apparently, they’re not picky: Three giant isopods collected in the southern Gulf of Mexico had ingested large quantities of plastic.

8. They live in a constant state of semi-hibernation.

Since meals in the deep sea can be few and really far between, giant isopods limit their energy expenditure. “They have a slower metabolism,” Auten said. “Their bodies mechanics are like that because they're deep-sea creatures and don’t find food all the time. So they're able to slow down their metabolism and energy level so they can survive. Watching ours, you can see they do that. They don't move around that much. They only ever move around when I'm trying to feed them. Normally they're just sitting there.”

9. Giant isopods have something in common with cats.

A giant isopod at Loveland Living Planet Aquarium in Draper, Utah.
A giant isopod at Loveland Living Planet Aquarium in Draper, Utah. / cifraser1, Flickr // CC by SA 2.0

And it’s all in the eyes. Giant isopods have widely spaced, fixed compound eyes with more than 4000 individual facets. Cats and the crustaceans (and many other animals) have a reflective layer at the back of the eye called the tapeum, which reflects light back through the retina and increases the ability to see at night. It’s also what makes cats' and isopods' eyes appear to glow.

10. They probably don’t see all that well...

It’s pretty dark where isopods live, so, according to Auten, vision isn’t really a factor for them, or many other deep-sea animals. “They use other senses to maneuver, to communicate, to find food, to find a mate,” she said. “I've done experiments with my flashlight to see if the isopods sensed a difference in light or anything like that. They don't move, they're not doing anything. Therefore, I'm thinking that maybe they might not see well.”

11. ... So they use their antennae.

Giant isopods have two sets of antennae that they use to experience the environment around them. “The small antennas are used more for chemical sensing,” Auten said, “and they have large antennas that are used for physical sensing. When you put the food in front of them, you're letting them sense it, physically and chemically.” They might have a sensory receptor that responds to distortion in the water around them.

12. Want to tell males from females? Look for the peenies.

Courtesy Dee Ann Auten

Female isopods have a brood pouch, or marsupium, located on their undersides, where they hold 20 to 30 eggs (top right). Males have two specialized organs: Small white appendages, called peenies (top left), that carry sperm (fun fact: smaller isopods usually have bigger peenies, according to Auten), and appendices masculinae (bottom), which they use to transfer sperm to the female. “They'll inject that sperm transfer organ anywhere within the female after she molts—because she's softer—and she's able to take on that sperm,” Auten said.

13. They have the largest eggs of all marine invertebrates.

They measure .51 inches in diameter. Females don’t eat when they’re brooding; instead, they bury themselves in sediment to reduce energy use and to protect the eggs.

14. babies come out looking just like big isopods.

Juvenile giant isopods, or mancae, don’t have a larval stage; they’re approximately 3.4 inches long when they emerge and look exactly like adults. All they’re missing is the last pair of pereopods, or legs; when fully grown, they will have seven pairs of legs total.

15. To Grow, They Shed their Exoskeletons.

Younger isopods molt often to gain size, but “when they get older, they don't molt as much,” Auten said. “They are capable of molting, but they’ve already reached their size, so they’re not going to molt as much—or they’re not going to molt at all, because molting is only for growth.”

16. Isopods bite!

“I wear gloves when I work with ours,” Auten said. “They're scavengers—they're definitely going to bite on anything. But it's a small bite, it's nothing big. They don't have big mouths.”

17. They curl up when threatened.

Auten said that isopods can potentially be eaten by anything that’s bigger than them, and when they’re threatened, they curl up into a little ball—just like their land-locked relatives, pillbugs. “If it's eating something and a fish is trying to come over and take the food from them or bite their appendages, they'll roll over to keep their food or to keep their soft organs underneath protected,” Auten said. ”They would cover themselves so that nothing will attach to them. Or they'll hide in a crevasse somewhere so that nothing can find them.”