Home on the grain; minuscule and precocious, newly found species of aquatic invertebrates offer a delightful solution to a deep sea mystery.

Home on the Grain

They live fast and die young. Theyleave not their footprints, but their skeletons, affixed to single grains of the sea's shifting sand. For the newly found species of bryozoan invertebrates that colonize sand grains, it's a small, small world indeed.

Most of the more than 5,000 describedspecies of bryozoans cluster in colonies resembling lichens that comprise hundreds or thousands of individuals. They encrust boats, dock pilings, old cans and shells and rocks. An entire phylum of invertebrates, the bryozoans are found in fresh and salt water areas throughout the world. "They are like little calcified boxes, each with a little orifice that it sticks its tentacles through. And there's almost nowhere you can go, where you can't find them,' says Judith Winston, associate curator in the Department of Invertebrates at New York's American Museum of Natural History.

Despite the bryozoans' adaptability,marine biologists have puzzled over how the small creatures spread across the sand from seashell to rusty can, from a beached log to rocks in the bottom of the bay--the short-lived motile form in the bryozoan life cycle is not a long-distance swimmer. But now clues to the mystery have been found, wrapped within recent discoveries off the Atlantic coast of Florida by Winston and Eckart H kansson from the University of Copenhagen.

Winston and H kansson describe inthe Dec. 18 NOVITATES their taxonomic names for nine new species of bryozoans they collected between 1983 and 1985, plus 24 earlier-described species--all capable of establishing colonies on single grains of sand. These sand-encrusting species, because they have no room to build "cities' of hundreds or thousands of bony boxes, exhibit an accelerated reproductive cycle and do not expend energy to produce protective soldier-type members like other bryozoans.

Although most bryozoans live manyyears, the new species apparently die within a year. But their explorations on a tiny scale leave a mark on a watery world. "It is a way of getting [bryozoans] distributed across wide sandy areas,' says Winston. "If they just have to make it to the next grain, it explains how you could have the species so widely spread.'

Photo: A live colony ofTrematooecia psammophila perches atop a single grain of quartz about 1 millimeter across. The new species' name means "sand lover,' and, unlike most sand grain species, it colonizes rounded surfaces of grains, rather than crevices.

Photo: The naming of newspecies is based on "skeletons'--such as these of Cribrilaria parva seen through a scanning electron microscope. Parva means "small': individuals of this species range in length from about 0.1 to 0.3 mm.

Photo: A colony ofMembranipora triangularis encrusts a single grain of sand. This new species is capable of "jumping' from grain to grain via tubular connections.

Photo: Although the speciesCribrilaria innominata had been previously described as encrusting pieces of shell, it is one of the few species also capable of colonizing sand-size grains.

Photo: A skeletal colony of T.psammophila on the edge of a grain shows developing ovicells, the brood chambers where embryos change into swimming larvae.

Photo: Double spines, one oneach side of the oral cavity, are evident in the species Bellulopora bellula, previously described on other surfaces. However, only when this species encrusts larger surfaces in larger colonies are the spines part of its defense.