Sailing along in calm seas a few hundred or even thousands of miles from any continent, you idly glance over the rail at the passing sheen of water. You notice several mysterious tiny silver dots zipping this way and that across the water’s surface, like minuscule UFOs. Eventually, one of the dots slows down enough that you can make out its form.
You see it’s a stout little bug with a body no more than a half-inch in size, four lengthy rear legs, and two shorter ones near its head. A wingless insect, alive and well, way out here in the open ocean. Believe it or not, this individual is just one of millions like it that spend their entire lives at sea.
Of the millions of insect species in the world, just five species successfully populate the open ocean, which covers 70 percent of the planet. These hardy insects, collectively known as ocean skaters, ocean striders, or sea skaters, are related to the long-legged water striders or “boatmen” commonly seen on freshwater lakes and streams. Discovered some 175 years ago but little known among entomologists or marine scientists until the past two decades, these creatures are remarkable for their ability to thrive in an unforgiving habitat that has thwarted all other attempts at colonization by insects. Indeed, ocean skaters are the only organisms to live exclusively atop the ocean surface, rather than in the water itself or in the air, like seabirds. Because these surprisingly charismatic insects are small, fast-moving, fully oceanic, and difficult to keep alive in captivity, the humans most likely to ever have the chance to observe them (apart from marine biologists) are offshore sailors.
Ocean skaters are generally confined to tropical waters between 40 degrees north and 40 degrees south latitude, where water temperatures consistently hover above 18 to 20 degrees Celsius. All five species of ocean skaters live in the Pacific, two of them also inhabit the Indian Ocean, and only one ranges into the Atlantic. The species are all classified in the genus Halobates and have only scientific names, not common names.
Of the five, Halobates micans is the only species that occurs in all three of the major oceans. Within the Pacific, H. micans is most common between 20 degrees north and 20 degrees south latitude. The nearly identical-looking H. sericeus takes over to the north and south of that, while the three remaining species have more limited distributions in the eastern equatorial Pacific or near Southeast Asia.
Along with these fully oceanic species, approximately 35 other species of Halobates occur in coastal waters of the mainland and islands of the Pacific and Indian Oceans, frequently along mangrove shorelines. No coastal species of Halobates have been found in the Atlantic Ocean.
“We may discover even more species of Halobates,” said Lanna Cheng, a scientist at Scripps Institution of Oceanography in La Jolla, California. “There are areas in the world, especially remote islands, where we have never been to look for them.”