Do any bugs live in the ocean? Short answer: Not really.

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By some estimates, insects make up 80 percent of named animal species. They’re found all over the world and in all manner of places, from rainforests and deserts to (much to our displeasure) human homes and bodies. But surprisingly, Earth’s largest environment, the sea, is almost insect-free.

Many insects live in fresh water, or close to the sea in salt marsh and beach habitats. There are also several species of water striders in the genus Halobates, which live on the ocean’s surface, far from dry land. Halobates are the closest thing we know of to a marine insect, but even they don’t actually live in the sea, just on it. So why isn’t the ocean swarming with insects?

Insects come from crustaceans

The most likely explanation, according to a 2022 article on Halobates, is that “by the time insects evolved, the seas had already been well populated by all major phyla of marine invertebrates.” The ancestors of insects left a crowded sea behind by adapting to life on land. You might say that they were relocating from a bad job market to one with better opportunities.

Modern insects share a common ancestor with crustaceans, like crabs and shrimp. After evolving in the sea about 500 million years ago, crustaceans expanded into a huge range of undersea niches. Even today, they fill many of the same ecological roles in the sea that insects do on land. Crustaceans are nibblers of plants, scavengers of carrion, and parasites of larger animals. Blood-drinking “sea lice” are named for the insects they resemble, but they’re actually copepods, a type of crustacean.

Insects left the sea with plants

When insects evolved around 440 million years ago, it was to take advantage of a relatively untapped niche: land, and more specifically, land-dwelling plants. Related groups of animals, such as the ancestors of today’s millipedes, crawled out of the sea a bit earlier than insects did. But it’s no coincidence that insects appear in the fossil record around the same time as the first land-dwelling vascular plants (a grouping which includes most modern plants).

Ancient millipedes, such as the eight-foot-long Arthropleura, roamed modern-day Europe and North America around 344 to 292 million years ago. Video: When Giant Millipedes Reigned, PBS Eons

Plant and insect evolution are deeply intertwined. Most modern insects rely on plants, sometimes a single species of plant, for food and shelter. Fossil evidence suggests that, as plants expanded from sea to land, the animals that relied on them the most followed. And as plants diversified and spread across the Earth, so did insects, acquiring special adaptations that gave them an edge to survive away from the sea.

Insects are adapted to life on land

Marine environments feature crushing pressure, strong currents, and high levels of salt, among other obstacles to survival. Marine crustaceans, which have been in the sea for hundreds of millions of years, are well-adapted to these challenges. For example, a 2012 study of crustacean gills described them as a “multi-functional organ,” used for breathing oxygen as well as for regulating the intake of salt and other chemicals.

According to BBC Wildlife Magazine, “because insects almost certainly evolved on land, many of their adaptations, from reproduction to the physiology of eating and breathing, are tailored to a terrestrial existence. Even those insects that live much of their lives in fresh water cannot stray too far from dry land.” As the marine ancestors of insects adapted to their new home, they lost features that were no longer necessary and gained new ones.

Unlike crustaceans, insects developed a system of oxygen intake through tiny holes in their bodies. This is more efficient than gills for breathing air. According to one major hypothesis, the gills of early insects, no longer used for breathing, became wings. Flight was another crucial adaptation, as it helped insects travel overland and to high places like treetops with ease. And for maximum stability on land, the many legs of crustaceans reduced down to six in insects.

A macro photograph of a diving bell spider submerged underwater, clinging to the thin, needle-like green leaves of an aquatic plant. The spider is positioned beneath a large, translucent air bubble that it has trapped under a silken web, with a smaller, shimmering bubble of air visible on its abdomen. The water has a murky green tint, and tiny particles are suspended in the surrounding environment. — While not technically an insect, the diving bell spider or water spider lives almost entirely underwater. *Image: Oxford Scientific / Getty Images*

There are numerous other examples of traits that give insects mastery of land environments, but wouldn’t be as useful under the sea. Many insects have specialized mouthparts for sucking plant juices or chewing leaves. Most go through a life cycle called complete metamorphosis, which helps them deal with seasonal food availability. The same insect can munch leaves in summer as a caterpillar, sleep through winter as a pupa, and sip flower nectar in spring as a butterfly.

Losing these traits and evolving ocean adaptations again might present a prohibitive cost of time and energy. This is especially true given that crustaceans have never lost their monopoly over the sea. Insects might be better off staying put where they are than trying to compete with marine crustaceans.

Insects are one of the great success stories of evolution

The ancestors of insects came from the sea. But their long evolution on land may have made them too specialized to easily readapt to it.

For comparison, we can look at the few species of land-dwelling crustaceans: woodlice (also called pillbugs) and some crabs. They’re successful within specific niches on land, but insects are far more versatile and widespread. Terrestrial crustaceans are slow-moving compared to insects. They can’t fly, and require damp environments because they breathe through gills. Some still breed in the sea, so they can’t live too far from it. You won’t find a land crab on a mountain any more than you’d find an insect in a coral reef.

Ocean-dwelling Halobates may be an outlier, but they’re also a remarkable success in their own right. They’re the only animal that lives entirely on the ocean’s surface, and “one of the most widely distributed organisms in the world” to boot. This says a lot about just how incredibly adaptable insects are. With or without the ocean, insects are thriving.

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