Life is short if you’re a millimetre long

Press, 30 September 1983, Page 17

Life is short if you’re a millimetre long

By

BORIS WEINTRAUB,

National

Geographic News Service

It’s a hot, summer day, and you are overjoyed at the chance to go to the seashore. You spread your beach towel on the sand, lie down, and close your eyes, marvelling at the peace and quiet, the absence of any sort of activity for miles around.

If you only knew what was going on just an inch or two beneath your prone, sun-drenched body.

Down there, barely beneath the top layer of sand, a flurry of everchanging, ever-moving animal life is living, breathing, eating, multiplying, and dying at a fearsome rate, perhaps burrowing deeper as the tide comes in, perhaps climbing nearer the surface as it recedes, fulfilling some as-yet-unknown role in the great cosmos.

All that animal life is contained in and among the grains of sand in what are known as interstitial spaces. Scientists agree that in any given volume of sand about 80 per cent is taken up by the sand itself. The other 20 per cent consists of space that is filled by water and minute animal life. What lives where on the beach depends on upon what part of the beach you are talking about. Different tiny creatures are found in the intertidal area — the part under water at high tide, dry at low tide — than are found above the high water mark. But one group dominates. “The biggest group by far are the nematodes,” says W. Duane Hope, a marine nematologist in the Smithsonian Institution’s Division of Worms in the National Museum of Natural History. Nematodes are among the most numerous of Earth’s animal groups, and away from the oceanfront, many of these tiny roundworms are parasites. Even so, the nematodes of the interstitial world are harmless, tiny — no more than a millimetre or two

long — and numerous. Dr Hope estimates that as many as 1.5 million live in a square metre of sand; and there are a lot of different kinds.

John H. Tietjen, chairman of the biology department of the City College of New York, estimates that he could turn up anywhere from 50 to 150 species in that square metre of sand. Dr Hope says there are so many different nematodes that he could find new species just by scooping up a handful of sand on any beach.

Dr Tietjen concedes that it is hard for most humans to understand how complex life is for these tiny animals. “I tell people to pretend they are shrinking down to an eighth of an inch or so tall,” he says. “There you are in these sand grains, which may be 10 times as tall as you are, some of them of boulder size, with different sizes, different shapes. You’ve got to slide around in between these grains, feeding on bacteria or algae, or feeding on other animals, and in turn being fed upon by slightly larger animals.”

The tiny animals require oxygen for life, so they “breathe” it from the water down in the sand. Most are small enough that they absorb the oxygen directly through their body walls and into their tissues, Dr Hope says. Larger ones have developed circulatory systems. To help them get around, and to survive in a difficult environment in which water is always washing up, down, in, and around the sand grains, they have developed a number of defence mechanisms. “Some have elaborate adhesive

organs which enable them to hang on to the sand when water comes in or out,” says Donald Zinn, professor emeritus of zoology at the University of Rhode Island. “On the other hand, there is no light down there, so many of these things don’t have eyes.” These “things” — including minute clams, crustaceans, and a variety of flat and segmented worms — don’t live long. Most live only from 30 to 60 days, but they reproduce rapidly. More are found during the summer than in the winter, perhaps because more food is present, though scientists are not sure.

In fact, scientists are not sure about a lot of things concerning these animals. They speculate that the tiny creatures are part of a giant food chain, feeding on the detritus of decaying organic material and in turn being eaten by slightly larger creatures, like the mole crab or larger worms. They speculate that the little animals help bacteria break down organic material more quickly. But this, too, is speculation.

Dr Zinn recalls that scientists used to think that sandy beaches were biological deserts. Now that opinion has changed. “Nature abhors a vacuum and, believe me, this is no vacuum,” he says. With all this activity below you on the beach, it should come as some comfort to know that mankind cannot hurt the interstitial life much, except through pollution. They cannot hurt us, either. “People have more to worry about from too much sun, or sharks, or jellyfish; or getting stung by sand flies, and the like, than from these little things,” Dr Tietjen adds.