National Geographic : 1953 Nov
613 Like a Circus Performer, Pterotrachea Swims Upside Down Through a Wedding Ring An almost invisible foot, or flapper, drives this strange mollusk through the water. Viscera and vital organs are compacted in the white patch (left); head and elephantlike trunk hang down at right (page 610). this was a true adult member of the genus Myctophum, I should certainly have thought it to be a common minnow-that is, until I turned the fish over and examined its belly. Suddenly I had the Milky Way in my hand, and the planets, and a dozen suns. Some 40 shining light organs were implanted there on a background of shimmering silvery scales. They glowed an intense white, partly with self generated light, partly with light reflected from the lamp. I was a little excited, al though this was by no means my first experi ence with luminescent organs, and I quickly dropped the creature into one of the glass jars of sea water. We found many more myctophids that night, as well as Argyropclecus, Chauliodus, and Trachypterus. None of these did we examine closely: that was a job for next morning in the laboratory. It was into the jar with the catch, and then quickly back to the lamp in the hope that new or rarer species would show up. Occasionally, while watching and waiting, I would play at focusing my eyes into the waters far below, down where deep-sea fishes call it home, down where pressures are in credible and the blackness is broken only by glowing galaxies of living things. I wondered how far down into this world the sunlight of next day would penetrate. I recalled that when William Beebe descended in his bathysphere near Bermuda some years ago, he reported that at about 1,900 feet he could still perceive faintly the blue remnants of daylight, but that at 2,000 feet all turned into absolute blackness.* Actually, experiments with photographic plates lowered into the sea show that there is still enough daylight at 3,000 feet to affect the emulsion. The intensity at this depth, however, as Beebe noted, is so slight as to be imperceptible to the human eye. The depth to which daylight penetrates the sea depends on such factors as the angle of the sun to the surface, and the amount of light-absorbing plankton and other suspended material in the water. Thus, in northern seas, where plankton is relatively thick,t the black ness of the abyss begins at a much higher level than in tropical seas where waters are clearer. Owls of the Deep-sea Twilight Also of interest is the fact that daylight penetrating the sea is absorbed differentially: that is, the red waves of the spectrum dis appear first, then the greens and yellows, and lastly the blues and violets. Hence, in the twilight zone just above the abyss, the world is one of blue-violet. Many of the creatures that inhabit this zone have big eyes, efficient light gatherers comparable, perhaps, to the eyes of such terrestrial twilight dwellers as owls and tarsiers. * See, in the NATIONAL GEOGRAPHIC MAGAZINE, "Half Mile Down," December, 1934; "Depths of the Sea," January, 1932; and "Round Trip to Iavy Jones's Locker," June, 1931, all by William Beebe. SSee"Strange Babies of the Sea," by Hilary B. Moore, NATIONAL GEOGRAPHIC MAGAZINE, July, 1952.