Moving Shoals

For many reasons, the sea floor remains an unfathomed, and largely unfathomable, mystery. Adventurers such as Jacques Piccard, dropping six miles down in bathyscaphes, touched only a dark valley or two in the deeps. For the rest, seafarers rely on sounding devices, which can mistake even a dense layer of cold water for an uncharted--and wholly imaginary--shoal. Last week, in Scientific American, Dr. Robert Dietz of the Navy Electronics Laboratory in San Diego, Calif., reported on yet another obstacle to sounding the depths: congregations of shrimplike crustaceans and lantern fish.

In a series of sonar explorations, Dr. Dietz found that the tiny troublemakers, ranging in length from 1 in. to 3 in., swam in layers. In some places, Dietz was able to detect five separate strata, superimposed like plywood laminations. Some of the layers remained placidly "at the same depth day and night," while others rose from as low as 2,400 ft. to the surface in the dark.

Such shifts, suggests Dietz, may mean that the sea creatures are more influenced by light than by temperature. The layers, bouncing back a sonar echo, may also be responsible for many of the hundreds of "E.D." (existence doubtful) markings that identify shoals on nautical charts. And, according to recent research, the organisms provide the food that barking fur seals chase to their lonely winter habitat far out in the Pacific. Somehow sensing a rich dinner below, the fur seal waits patiently for night, when the immense underwater horde comes up to feast on plankton riding the ocean swells.

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