What is an isopod?

Isopods are an order of marine invertebrates (animals without backbones) that belong to the greater crustacean group of animals, which includes crabs and shrimp.

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During the first dive of the Gulf of Mexico 2017 expedition on NOAA Ship Okeanos Explorer, we saw this giant deep-sea isopod, Bathynomus giganteus. While we often see these giant underwater 'pill bugs' resting on the seafloor, we don't always get to see them swim, so seeing this isopod, which measured nearly 30 centimeters (almost one foot) long, come in for a landing was exciting! Video courtesy of the NOAA Office of Ocean Exploration and Research, Gulf of Mexico 2017. Download larger version (mp4, 24.0 MB).

Scientists estimate that there are around 10,000 species of isopods (all belonging to the order “Isopoda”). They are one of the most morphologically diverse of all the crustacean groups, coming in many different shapes and sizes and ranging from micrometers to a half meter in length. They also live in many different types of habitat, from mountains and deserts to the deep sea, and they are distributed worldwide.

Isopods often do not look alike, but they do have common features. For example, all isopods have two pairs of antennae, compound eyes, and four sets of jaws. The body, or pereon, of all isopods consists of seven segments, each with its own pair of walking legs. Isopods have a short abdominal section composed of six segments, called “pleons,” and one or more of these segments is fused into a tail section. Each pleon has a set of biramous (branching in two) limbs called “pleopods” that are used for swimming and respiration.

About half of the known species of isopods live in the ocean. Some are large and spiny and live in the deep sea, while others are very small and live as parasites on fish. Others live in coastal and shelf waters, moving around on the seafloor or living in plants.

Those species of isopods that live in the oceans are able to do so by using their pleopods to ‘breathe’ by gas exchange. Those that live on land have developed their pleopods to include air sacs called “pseudotrachea” that function as gas exchange organs that don’t need to be fully submerged in water.

The most familiar isopod is probably the terrestrial pill bug (sow bug or wood louse), which can be found scurrying around any backyard in moist, dark conditions. The largest isopod species are those from the genus Bathynomus. These animals live in the deep sea, and (like many animals that live in the deep) they are much larger than their shallow-water relatives.