Among these are flat-bodied mayflies that can crouch so close to a stone that torrential currents cannot pull them away, black flies that use silk and a ring of hooks at the end of their abdomen to anchor themselves to a rock and boldly-patterned stoneflies that crawl through the interstices hunting and engulfing smaller insects.

Some 30,000 species of insects inhabit Earth’s aquatic environments—about four percent of the estimated global diversity of insects. Some are found at depths of 1,300 meters [4,290 feet] in Siberia's Lake Baikal, some are only found in pitcher plants and rain-filled tree holes, some inhabit very salty lakes and some live in caves and underground aquifers. From the most ephemeral wetlands to the largest lakes and rivers, insects occupy nearly every available aquatic niche, providing a lifetime's worth of discovery. Given the diversity of insects in freshwater, one might think that the Earth's oceans would support an almost infinite number of insect species. Only 0.0091 percent of the Earth's surface water is contained in lakes and rivers, and 95.96 percent is in the oceans. With nearly 30,000 insects inhabiting freshwater, can you guess how many insects occupy the open ocean?

Five. Yep, five species, all belonging to one genus, are the only insects adapted to live freely in the world's most vast ecosystem. There are actually about 1,400 insect species found in marine environments, but they mostly only dwell in coastal areas. Intertidal zones, estuaries, salt marshes and mangrove swamps support the greatest diversity—from beetles to bristletails, flies to fleas and springtails to snakeflies. Chewing and sucking lice are found in the open ocean, but only on their vertebrate hosts—that hardly counts!

Scientists find it mysterious why insects are virtually absent from the open ocean. Some think the ocean is too salty or too deep for insects, or that ocean water frequently has too little oxygen for larval insects, or that key nutrients are in limited supply. But there are examples of aquatic insects that refute these ideas, such as those that tolerate lakes far saltier than the ocean, or species that thrive in habitats without oxygen. The most commonly accepted explanation for the absence of insects in the ocean is that insects first evolved and diversified on land, while at the same time, crustaceans were diversifying in the oceans. Just as insects rule the continents, crustaceans rule the oceans. Insects were not able to invade the oceans because crustaceans already occupied most available niches; ecologists term this phenomenon “competitive exclusion.”

Sea Skaters

For an entomologist who never cared much for the ocean to begin with, the paucity of insects only strengthened my sentiments. However, I always admired the brave insect species that have made the ocean their home. They are called sea skaters, or ocean striders, in the genus Halobates. While their terrestrial and freshwater kin diversified and flourished on the continents, they swam beyond the tides, watched the continents slip below the horizon, surrendered to the oceans currents and made a life on the high seas. As suggested by their name, sea skaters live only on the ocean’s surface, occupying a niche that crustaceans were never able to fill.

Sea skaters are true bugs (Hemiptera) in the family Gerridae. Although the genus Halobates has over 40 species, only five live exclusively in the open ocean. They are common in the tropical Pacific, but one species, Halobates micans, exists in the Atlantic Ocean and could be found off Cape Cod. Close relatives are commonly seen in local streams and ponds, usually in the genus Gerris or Aquarius. They have unwettable hydrofuge hairs on the tips of their legs that allow them to move about on the surface of the water, without breaking the surface tension. Despite being scarcely more than 6 millimeters (0.24 inches) long, they can skate across the ocean surface at speeds of 50 to 100 centimeters per second (1.6 to 3.3 feet per second) and jump 10 to 12 centimeters (four to five inches). Sea skaters do not have wings to fly, nor can they dive below the water's surface. No other animal on Earth lives in such a vast two-dimensional habitat.

Sea skaters are predators that feed on zooplankton or other small creatures caught in the surface film. They are the only marine invertebrates constrained to traveling, feeding and reproducing only at the surface of the ocean. They are not on equal footing with their predators, which include fish that attack from below or birds that attack from above. It must be truly frightening for a Halobates to see a blue-gray noddy swoop toward it, with no place to hide!

Among the difficulties of living in a two-dimensional world, one of the most amazing is how Halobates find each other to breed and lay eggs. One study estimated that oceanic diffusion alone could transport them 1,250 kilometers (nearly 800 miles) in 60 days, and in addition, they must contend with storms, wind and waves. A plastic milk jug, perhaps cast overboard by a careless sailor in the tropical Pacific Ocean, was thought to have served as a “beacon” for wandering sea skaters—833 sea skaters were collected in one scoop near the jug, and there was an egg mass on the jug comprised of nearly 70,000 eggs. They will lay eggs on anything they find floating, such as debris and carcasses.

Like distant galaxies or the surface of Mars, the open ocean was a place where my mind never wanted to wander. Yet knowing the ecology of its residents, especially insects that are similar to those living in the lakes and streams that I am familiar with, has drawn my thoughts to the ocean. I would like to lace up a pair of hydrofuge skates, step off the rocks at Pemaquid, ride the currents to Cape Cod, cross the Gulf Stream and leave the continent behind. To see the world as Halobates does—trace the full arc of the sun and moon, ride swells and look out upon a fierce landscape of waves and foam. Or spend a starry night on the Sargasso Sea, when the sea surface is like glass, broken only by the faint ripples of a pair of Halobates skating in tandem toward a piece of driftwood that has come from their faraway ancestral continent.

Ethan Nedeau is a science translator for the Gulf of Maine Council. He lives in Amherst, Massachusetts.