Frontiers of Science:
The Octopus—
Chameleon of the Sea

By Dr. Sherwood B. Idso

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    This shy little animal keeps mostly to itself, hiding under rocks along the shore or along regularly exposed coral reefs. It is nocturnal, venturing out mostly at night to capture small fishes and crabs and mollusks. When frightened, it can emit a cloud of “ink” to cover its escape from danger. But usually the octopus protects itself by merely blending in with the colors of the surrounding environment, becoming almost indistinguishable from the rocks, sand, coral, and seaweeds that comprise its watery habitat.

    How is it that the octopus can change its appearance so dramatically? To answer this question scientists have trained octopuses to perform simple tasks. Generally, these tasks involve attacking or not attacking certain objects. For instance, octopuses have been taught to distinguish between vertical and horizontal rectangular blocks by rewarding them with a small fish when they attack a vertical block and giving them a slight electrical shock when they attack a horizontal block. After several of these experiences, the octopuses soon learn to attack only the vertical blocks that result in their getting a fish to eat.

    In similar experiments, octopuses have been taught to distinguish between light and dark objects. But when the tests have dealt with different colors, the octopuses have always failed, indicating that they are probably color-blind.

    So how is it that octopuses can rapidly change their color to blend in with the environment? The answer lies in the unique composition of their skin. Spread out over the surface layer of the octopus’s skin are about a million small pigment sacs called chromatophores. Muscles contract or expand these sacs, stimulating nerves that act in response to the brightness of light sensed by the octopus’s eyes. When fully expanded, the chromatophores give the octopus a dark brown color. When they are contracted, however, the underlying iridophores and leucophores are exposed to view. Because these cells are highly reflective, they naturally reflect whatever color of light is most abundant in the environment.

    If an octopus is exposed to a greenish hue resulting from creeping through an algae-covered pool, it will take on a greenish appearance, merely because the light reflected from the iridophores and leucophores in the skin will be similar in color to the light that strikes it. Consequently, the octopus’s ability to change its color is nothing more than a side effect of an ability to detect changes in the intensity of light so that it adjusts its skin composition to become more or less reflective.

    But changing colors is not the only unusual feat performed by octopuses. Since they have no bones, they can also dramatically change their shape and squeeze through extremely small openings. The story is told of a naturalist who caught an octopus about a foot long. He put it in a wicker basket and went for a ride on the streetcar with it. About ten minutes later there came a bloodcurdling scream from the other end of the car. Sure enough, the octopus had squeezed through a half-inch crack in the basket and was now sitting on the lap of a hysterical passenger.

    Perhaps an even better description of this chameleon of the sea would be the Houdini of the ocean!

    1 The octopus is generally remembered for its eight arms with their many suction cups.

    2 The great strength of these arms with their suction cups allows the octopus to grasp and tear apart such animals as hard-shelled crabs for food. Here an octopus attacks a stick that has intruded upon its territory.

    3 Here the octopus takes on the blue gray color of a background piece of coral.

    4 This is the ever alert eye of the octopus. Although the octopus is almost certainly color-blind, he can still blend in with the colors of its environment.

    5 The suction cups are largest near its main body and get smaller as they move out to the tips of the arms.

    6 Up against a light brown piece of coral, it is almost impossible to tell where the octopus and coral meet. The creature at the top of the coral is a sea anemone that we will meet in a future story.