Picture a world where tiny minibeasts are anything but small; where you can admire their incredible mechanics and intricate features without needing a magnifying glass.

You might assume that a woodlouse the size of a cat belongs in the realm of science fiction, akin to Honey, I Shrunk the Kids. However, such a creature does exist—not languishing beneath a garden flowerpot, but in the chilly depths of the ocean.

Meet the giant isopod (Bathynomus giganteus), which isn’t exactly the same as a common woodlouse. Nonetheless, these two creatures share numerous traits: both boast a tough exoskeleton, possess 14 walking legs, and utilize gills, leading to a lifestyle that is predominantly tranquil and straightforward.

As the largest member of approximately 20 species within the Bathynomus genus, the giant isopod thrives around 2 kilometers deep in the ocean, where it encounters a dark and frigid environment. Here, it meanders across the expansive ‘abyssal plain’—broad stretches of soft silt—alongside various other deep-sea specialists, earning its living much like its more familiar land-dwelling relatives: through scavenging.

Diet of the Giant Isopod

Studies that involve examining the stomach contents of giant isopods pulled from their depths, combined with footage captured by remotely operated underwater vehicles (ROVs), reveal that these creatures primarily act as ‘detritivores’.

They mainly consume decomposing animal matter that drifts down from above, scavenging on everything from whales to cephalopods. Interestingly, they likely also hunt down smaller prey like worms and mollusks. There’s even astonishing footage showcasing them clinging to the heads of small sharks!

Food Detection in Giant Isopods

Giant isopods locate their food in total darkness with the help of their highly sensitive antennae. This is similar to how terrestrial woodlice search for decaying materials beneath logs or stones. They have two sets of antennae: one long pair that feels the substrate constantly, and a shorter pair that remains elevated, detecting chemical ‘tastes’ in the surrounding water.

Size of the Giant Isopod

Given its name, it’s no surprise that the giant isopod can reach impressive dimensions. Records indicate they can grow up to 50 centimeters long, with some unverified reports suggesting even larger specimens.

Understanding Giant Isopod Size

Giant isopods illustrate a phenomenon known as abyssal or deep-sea gigantism—the tendency for deep-dwelling creatures to be significantly larger than those in shallower waters, including sea spiders and squid. While the reasons behind this are not fully understood due to the challenges of studying such depths, it’s believed that being larger offers several benefits.

One hypothesis posits that increased dissolved oxygen levels at greater depths may contribute to this gigantism, as terrestrial invertebrate size is thought to be limited by lower atmospheric oxygen levels and their inability to absorb oxygen through lungs. This explains why we don’t find beetles the size of basketballs.

Another theory suggests that because abyssal plains are vast, animals living in these areas may have to endure long periods without food (one giant isopod in captivity went without eating for four years). A larger body size allows them to capitalize on scarce feeding opportunities and store nutrients for future use. When they do feast, they often consume so much they can hardly move, explaining a potential advantage of size: being less vulnerable to predation.

Main image: Getty

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