80% of All Animals on Earth are Roundworms

Burrowing nematode - Photo by Scot Nelson
Burrowing nematode. Photo by Scot Nelson.

When you look outside your window on a beautiful, warm spring day, what kinds of animals do you see? Birds? Butterflies? Squirrels? While of these animals might seem plentiful, they are not nearly the most populous animals on Earth. The most common animals on Earth are actually quite difficult to see, especially since the majority of them are smaller than your finger. Representing 80% of all individual animals on earth and 90% of all animals on the ocean floor, nematodes (also known as roundworms) are the most common animals on Earth.

Now, nematodes aren’t the prettiest or cutest of the world’s animals. In the grand scheme of nature, they’re actually pretty icky, but because they are, they can thrive in a tremendous variety of environments. They are present in arid deserts, snowy mountains, and deep ocean trenches. They’ve even been found 0.6–2.2 miles (0.9–3.6 km) beneath the surface of the Earth in gold mines in South Africa.

Gravid_adult_female_Nippostrongylus_brasiliensis - Photo by J. Claire Hoving
Gravid adult female Nippostrongylus brasiliensis. Photo by J. Claire Hoving.

Unfortunately, many nematodes are parasitic, and they are present in the bodies of other animals, including humans. Hookworms, pinworms, and whipworms are just a few nematodes that parasitize humans. Heartworms are nematodes that commonly parasitize dogs and cats. And, if you’re SpongeBob Squarepants, nematodes might even eat your pineapple house…