The lights that save

Marine biologists have wondered for years why fish and other creatures that live at middling ocean depths carry rows of little searchlights on their bellies. The searchlights (photophores) are cup-shaped organs that are lined with highly reflective tissue and contain luminous cells whose light is concentrated into a downward-pointing beam. Biologists reason that since photophores evolved independently in fish (vertebrates), shrimps (crustaceans), and squids (mollusks), they must have important survival value. But what was it? The bright beams of the photophores shining downward would seem to be a disadvantage, serving only to draw the attention of predators.

In Britain's Nature, William D. Clarke of General Motors Defense Research Laboratories, Santa Barbara, explains a likely purpose of the photophores. The creatures that carry the belly searchlights, he says, live at ocean depths (less than 3,000 ft.) where sunlight barely penetrates. These waters are the hunting ground of fish with eyes that point permanently upward. What they normally see is the last faint trace of sunlight, which looks like a dim blue ceiling. When they see a dark and edible-looking object silhouetted vaguely against the ceiling above, they dart up and grab it.

Dr. Clarke believes that photophores actually protect their bearers by confusing the enemy. The fish cruising below are rather nearsighted, so they do not see the little searchlights as points of brightness. Instead, the lights blend together as in a badly focused photograph, making the silhouette look dim and fuzzy against the lighted ceiling. So the hungry fish with the upturned eyes look elsewhere for dinner.

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