The Deep Ocean Giant Isopod Looks Like a Huge Roly-poly Bug

Chances are, you've never encountered a giant isopod unless you've spent time trawling the deep ocean and accidentally snagged one in the tangles of your fishing net.

Perhaps you may have seen one in captivity at one of the few aquariums that house them. Regardless, you may be fascinated to learn that there are tons of these humongous bug-like creatures living in the darkest depths of the sea. And yet, they remain somewhat a mystery to us humans.

"There's a lot we really don't know about these guys," says Ruth Carmichael, senior marine scientist at the Dauphin Island Sea Lab and professor of marine and environmental sciences at the University of South Alabama. But what scientists do know is that "they have a really important job [debris recycling] that we need animals to be doing in the deep ocean." More on that in a minute.

Isopod Versus Giant Isopod

Giant isopods are, essentially, giant versions of isopods. What's an isopod, you ask? Isopods are a type of arthropod, Carmichael explains. Arthropods are invertebrate animals like lobsters and shrimp, but also insects and spiders. The common denominator is that they all have tough, flexible exoskeletons.

Isopods are further narrowed down taxonomically into an order of crustaceans called Isopoda, which consists of various segmented, flattened animals. There are more than 10,000 species of isopods worldwide that include marine, freshwater and terrestrial species. Perhaps the most well-known isopod is the roly-poly, also known as the pill bug or potato bug.

Not all isopods look alike, but they do have some features in common, such as:

two pairs of antennae
compound eyes (eyes with multiple lenses)
four sets of jaws
a body consisting of seven segments each with its own pair of legs

For the most part, marine isopods are small, "about the size of a fingernail or up to 2 inches [5 centimeters] long," and are more common in marshes and estuaries, Carmichael says. Giant isopods are marine animals that live in the very depths of the sea — 500 to 7,000 feet (152 to 2,133 meters) deep — primarily in the western Atlantic, western Pacific and Indian oceans.

They are also much, much bigger than all other isopods.

How Giant Is a Giant Isopod?

There are actually two size classes of giant isopods, Carmichael explains — giant isopods and super-giant isopods. Regular giant isopods are about 3 to 5 inches (8 to 13 centimeters) long. "Think of an index card," she says. Super-giant isopods are even bigger, up to 20 inches (51 centimeters) long. "So, if you think about a professional basketball player's shoe — like Michael Jordan's size 13 — that's sort of the small end of a super-giant isopod. They can get about one and a half times that size," she says.

What makes giant isopods and super-giant isopods so gigantic is the same sort of phenomenon seen in other sea creatures living in the deepest depths of the sea, like the giant squid. In zoology, the term is called "deep-sea gigantism." It's the tendency of deep-sea dwelling animals to grow much larger than their shallow-water dwelling relatives.

Why this occurs baffles scientists, but one theory is that gigantism may help these creatures withstand the extreme pressure of their underwater environment. Another thought is that gigantism is a result of adaptations to scarce food resources in the deep sea. Decreasing temperature are also believed to result in increased cell size and increased life span, which can lead to increased maximum growth and larger size.

What Do Giant Isopods Look Like?

Giant isopods look surprisingly similar to tiny, land-dwelling roly-polies, except their exoskeleton is brown or pale lilac in color. And like their roly-poly cousins, giant isopods can roll up into a ball and use their tough outer layer as a shield.

Similar to roly-polies, the body of the giant isopod is composed of seven overlapping segments. On giant isopods, these segments are called pleonites. The first is fused to the head and protrudes over the animal's large, far-spaced eyes like a helmet. The eyes have more than 4,000 individual facets which reflect light, making the eyes appear to glow.

The remaining six segments of the giant isopod form the abdomen and each isopod has seven pairs of pereopods, or legs. The first pair of pereopods is designed to help push food into the animal's four sets of jaws, which then cut and tear at the prey. The other legs, called natatory legs, are used for swimming.

Opposite the head is a fanlike tail, called a uropod, and fluttering swimmerets, called pleopods. These work together to propel the giant isopod through the water as well as help with respiration.

Giant isopods also have two sets of antennae sprouting out of their heads — a short pair and a long pair that extends about half the length of the body. These help guide the animals around the dark, murky depths of the ocean.

What Do Giant Isopods Eat?

"As far as we know, they are primarily scavengers," Carmichael says, hoovering up debris that's fallen to the deep ocean floor, including live and dead fish, crabs, shrimp, squid, sponges and whale carcasses. "They play an important role in nutrient and element recycling."

Relying on food sources at the bottom of the deep sea can be a feast-or-famine situation, but that's no problem for giant isopods. When food supplies are plentiful, these creatures don't mind gorging themselves, though doing so tends to stimy their locomotion. And at times when supplies are scarce, giant isopods can adapt, as well. In protected environments such as aquariums, they've been known to survive up to four years without food.