Page:Popular Science Monthly Volume 88.djvu/205



CIENTISTS rarely go a-fishing in troubled waters; Professor S. O. Mast, however, of the zoological department of Johns Hopkins, is an exception. The Johns Hopkins professor discovered that such fish as minnows are often found in the little temporary pools left in the sand by the tide, but rarely, if ever, after the water in such a tide is so low that the outlet is closed.

When the tide is falling, these fish—fundulus majolisis, the scientific name for them—swim out, somehow knowing when the tide is about to get so low that they might be trapped in the little pools in the sand. As the tide falls, they swim in and out of such tide-pools at short intervals. Thus, these fish avoid being trapped in the pools and killed when the little collections of water dry during low tide.

Professor Mast has observed that the outlets of such tiny pools may be closed while the tide rises, but if they should close while the tide is falling, the fish swim about rapidly in various directions to discover water. If they find none, they leave the water and actually flop over land to the sea. Professor Mast has seen scores and scores of these fish leave large pools and travel across sandbars more than twelve feet wide and half a foot high. The fish nearly always leave the pools on the side towards the sea. They evidently remember the direction of the outlet and the direction from which they entered.

Curiously enough, they never make any mistakes in "walking" on dry land, either. Professor Mast never found one to take a wrong direction for any great distance. Although he admits that it is not yet definitely known how fish are guided in the right direction, it is certain that light reflected from the water is not a factor in this sense of direction.

Perhaps one of the most interesting discoveries made by the Johns Hopkins zoologist shows how fish can make their way on dry land..

Of course, locomotion on land by fish can be brought about only by successive leaps and jumps produced by rapid bending and wriggling of the body or sideswiping by the tail.

When trapped in a pool which rapidly dries up or evaporates, they swim about for a few minutes, then come closely to the edge of the water and swim up and down the side of the pool nearest to the sea. Finally a dense aggregation of fish forms in the outlet near the dam. and three minutes by the watch after they are shut in, they manage to climb out on the sand. They leave in groups of twelve and "march" like General Sherman to the sea. These fish are superior to some men in finding their way home.