Scubaism

Of all the strange worlds that summer brings us close to, few are as strange or as close as the one beneath the bikinied bottoms of beachgoers. There, between the grains, is a microscopic ecosystem populated by sand-lickers, sticky-toed worms and four-legged “water bears.” It’s a world that remains largely unexplored, despite being near enough to touch.

“I always tell people: If they only knew what they had their toes stuck into,” said Linda Schaffner, a professor at the Virginia Institute of Marine Science in Gloucester Point, Va.

The animals living in the sand are often less than a millimetre long and sometimes as small as one-twentieth of a millimetre. They make up for size with numbers: Scientists estimate that a bucket of sand might hold thousands of these tiny creatures; in a few square metres of beach, there might be millions.

The sand is a buffet, as well as a shelter. Scientists say the grains are often covered in bacteria or tiny plants called diatoms. Enough sunlight penetrates the sand that these plants can survive even an inch under the surface.

This food is licked off by worms that crawl over the surface of a grain or is munched on by tiny shrimp-like creatures with waving legs called copepods. An animal called tetranchyroderma looks like a flying carpet with a mouth, propelling itself with a bellyful of hairs and vacuuming up bacteria in a giant maw. Some worms called polychaetes simply eat the sand whole and let their digestive systems clean it off. Out the back end, eventually, comes a trail of clean sand.

“It really is a different kind of existence, the interstitial environment,” said Douglas Miller, a professor at the University of Delaware. Scientists call these creatures “interstitial” because they live in the interstices, or empty spaces, between grains.

Life in this world is short: Most creatures live only a few weeks. That means they need to be ready for reproduction quickly, often a few days after birth. Some creatures have both male and female organs, although they don’t usually fertilize themselves.

“Some animals can actually switch back and forth” between being male and female again and again, said Rick Hochberg, a professor at the University of Massachusetts at Lowell.