17 Incredible Facts About Giant Isopods

The genus Bathynomus includes 20 recognized species, with B. giganteus, the giant isopod, being the largest. While their appearance might seem a bit unsettling, they also have an endearing quality! Here’s a closer look at what makes these fascinating creatures so popular online.

1. Giant isopods are not insects.

Belonging to the crustacean family, they share close ties with marine species such as shrimp and crabs, as well as land-dwelling relatives like the pillbug (Armadillidium vulgare).

2. They inhabit the ocean floor.

Giant isopods thrive at depths ranging from 550 to 7020 feet, and possibly even deeper. They favor muddy or clay seabeds, where they dig burrows for protection. “Bathynomus giganteus thrives in colder waters,” Dee Ann Auten, an Aquarist II at the Aquarium of the Pacific—home to giant isopods in its Wonders of the Deep exhibit—explained to Mytour in 2014. “They are commonly found in the Pacific Ocean, near Japan and the South China Sea. These regions are their primary habitats.”

3. True to their name, giant isopods grow to impressive sizes.

Giant isopods usually measure between 7.5 and 14.2 inches long, but some can grow much larger. For instance, a specimen captured by an ROV in 2010 was 2.5 feet long. Researchers are still unsure why these creatures grow so large, but they hypothesize that their size may be an evolutionary adaptation to withstand the intense pressures of the deep sea.

4. They exhibit a range of colors.

The exoskeleton of giant isopods, made up of overlapping segments, appears in two primary shades: brown and pale lilac.

5. Giant isopods are meat-eaters.

While they are primarily known as scavengers, feeding on dead creatures that sink to the ocean floor, there is some evidence that they may also prey on slow-moving live animals, such as sponges. Additionally, giant isopods are known to disrupt trawl catches.

6. They can survive for extended periods without food.

A giant isopod in Japan survived for five years without eating before passing away in 2014. That same year, Auten shared with Mytour her daily feeding routine for giant isopods, which required immense patience. “The key is knowing what to feed them and how to present it,” she explained. “At the Aquarium of the Pacific, mackerel is their favorite. I prepare the mackerel by butterflying it so the insides are exposed, then place it in front of the isopod. I offer food daily because their activity levels vary—some days they’re inactive, while on others, they might be ravenous. It’s incredibly rewarding when, after much effort and patience, you finally discover what they enjoy eating.”

7. However, when they do eat, they consume enormous amounts.

Giant isopods possess four sets of jaws designed for cutting and tearing prey, which they put to good use during feeding frenzies. “When they’re hungry, they eat voraciously,” Auten noted. “They’ll consume large quantities in one sitting and then go without food for a long time. There’s even a comic depicting a giant isopod devouring an entire whale, leaving only the bones, and then lying on its back, stuffed. That’s accurate! They eat so much that it affects their mobility.”

Despite their appetite, they aren’t aggressive feeders. “I’ve never seen them all eat at the same time,” Auten said. “I always bring enough food for everyone, but only one tends to eat during each feeding. They don’t compete or fight over food.” Interestingly, they aren’t selective eaters—three giant isopods from the southern Gulf of Mexico were found to have consumed significant amounts of plastic.

8. They exist in a near-constant state of semi-hibernation.

Due to the scarcity of food in the deep ocean, giant isopods conserve energy by slowing their metabolism. “Their bodies are adapted to deep-sea life, where meals are infrequent,” Auten explained. “They reduce their energy usage to survive. Observing ours, you’ll notice they’re mostly inactive, only moving when food is presented. Otherwise, they remain stationary.”

9. Giant isopods share an unexpected trait with cats.

The similarity lies in their eyes. Giant isopods possess widely spaced, fixed compound eyes with over 4000 individual facets. Like cats and other animals, they have a reflective layer called the tapetum at the back of their eyes, which enhances night vision by reflecting light through the retina. This layer also causes their eyes to glow in the dark.

10. Their vision is likely limited...

Given the darkness of their deep-sea habitat, vision isn’t a primary sense for isopods or many other deep-sea creatures. “They rely on other senses to navigate, communicate, locate food, and find mates,” Auten explained. “I’ve tested their reaction to light with a flashlight, but they showed no response. This makes me think their vision might be quite poor.”

11. ... So they rely on their antennae instead.

Giant isopods use two sets of antennae to interact with their surroundings. “The smaller antennae are primarily for chemical sensing,” Auten noted, “while the larger ones are for physical sensing. When food is presented, they use both types of antennae to detect it. They may also have sensory receptors that respond to changes in water pressure or movement.”

12. To distinguish males from females, look for the peenies.

Female isopods feature a brood pouch, or marsupium, on their undersides, which holds 20 to 30 eggs (top right). Males possess two specialized organs: small white appendages called peenies (top left) for carrying sperm (an interesting note: smaller isopods tend to have larger peenies, according to Auten) and appendices masculinae (bottom), used to transfer sperm to the female. “After the female molts and her body softens, the male injects the sperm transfer organ into her, allowing her to receive the sperm,” Auten explained.

13. They produce the largest eggs among marine invertebrates.

Their eggs measure .51 inches in diameter. During brooding, females stop eating and bury themselves in sediment to conserve energy and safeguard their eggs.

14. The hatchlings resemble miniature versions of adult isopods.

Juvenile giant isopods, known as mancae, skip the larval stage and are born looking like tiny adults, measuring around 3.4 inches in length. The only difference is they lack the final pair of pereopods, or legs; adults have a total of seven pairs.

15. They grow by shedding their exoskeletons.

Young isopods molt frequently to grow, but “as they age, molting becomes less common,” Auten explained. “They can still molt, but since they’ve reached their full size, they don’t need to do it as often—or at all, since molting is primarily for growth.”

16. Isopods can bite!

“I always wear gloves when handling them,” Auten said. “As scavengers, they’ll bite almost anything. However, their bites are small and not particularly harmful—they don’t have large mouths.”

17. They roll into a ball when threatened.

According to Auten, isopods are vulnerable to predators larger than themselves. When threatened, they curl up into a tight ball, much like their terrestrial cousins, pillbugs. “If a fish tries to steal their food or attack their limbs, they’ll roll over to shield their meal or protect their vulnerable underside,” Auten explained. “They’ll also tuck themselves into crevices to avoid detection and stay hidden from potential threats.”