Magazine article International Wildlife

Colors of the Sea

Magazine article International Wildlife

Colors of the Sea

Article excerpt

You can never find a shark when you need one, so photographer Greg Brown and a friend were cutting up bait underwater to attract big predators for the camera. Suddenly the knife slipped and sliced into the friend's hand. What Brown recalls most vividly is that the resulting plume of blood was not red at all but a distinct army green.

Brown tells the story (in which both divers escape safely) to illustrate his longtime fascination with color in the ocean. Water plays tricks with color. Water creatures practice multicolored deceits of their own. And a photographer afloat in this aquatic kaleidoscope gets a lifetime of surprises.

The green blood comes from a benthic quirk: The ocean is a dreadful conductor of light, and as depth increases, water gobbles up one color at a time. Reds, which have the longest wavelengths on the color spectrum, disappear first. As divers drop deeper, the other colors gradually vanish in rainbow order. At the dimmest depths, the world becomes intensely blue, finally fading to bowling-ball black. In the case of the stabbed diver, the mass of water above had absorbed all the reds. Only the shorter wavelengths, like greens, remained.

This drop-off of colors may explain a rule of thumb about marine vision. In dim light, freshwater fish species tend to see reds better than do saltwater fish. The reason: freshwater animals often swim in the shallows, where more red penetrates. Penguins don't see red at all. Their eyes are adapted for hunting in deep water, where greens and blues dominate.

Exactly how underwater creatures see color--and each other--is not clear. The last decade of research has been upsetting long-held axioms. Scientists now realize that sharks have the right physiology to see their prey in color instead of just in black-and-white, as was long thought. And despite years of belief to the contrary, a number of fish, including salmon, detect ultraviolet light. Some even see polarized UV light, which bounces off fish scales in a pattern characteristic of the species.

Whatever the ocular physiology, color apparently plays a role in animal behavior underwater. Sluglike animals called nudibranchs (pages 46-47), sport hues as outrageous as the warning colors of poison-dart frogs on land. Nudibranchs' blobby bodies carry no protective shell and ooze over reefs essentially naked in full sight of predators. Like the colorful frogs, nudibranchs tend to be poisonous, and those see-me shades may be shouting, "Look but don't touch." In more than 3,000 hours of diving, Brown only once has seen a fish gulp down a nudibranch, and that gulp did not stay down for very long.

Other reef spectacles include his-and-hers fish coloration. Brown occasionally swims by a golden yellow cloud of female purple-blotch basslets. …

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