Studies of Protective Color Change

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Abstract
Gambusia which had been kept for over 10 weeks in black and in white tanks, respectively, and the resulting darkness or pallor of which had become to some extent fixed, were placed together in some numbers in a single tank, and exposed to the attacks of a largely predaceous species, Apomotis cyanellus. In the majority of experiments, equal numbers of fishes of "black" and "white" history were used. When this experimental tank was black, the Gambusia of "white" history were eaten preponderantly (more than 75% of those eaten) ; when the tank was pale gray, Gambusia of "black" history were those most largely eaten (66%). In some other experiments, in the pale tank, twice as many of the pale Gambusia were offered as of the dark ones. Nevertheless, the proportion of "blacks" and "whites" among those eaten was about the same (67% and 33%), although the "whites" were twice as available as the "blacks." Decrease in the degree of difference between the 2 contrasting lots of fishes, resulting from being kept on a common background, led to a marked decrease in selectivity on the part of the predators. Ten minutes of such sojourn on a white background was sufficient to annul the differential survival rate; but some hours on black proved to be necessary. Rendering the food-fishes immobile by suitable anesthetics led to their being much more promptly devoured by the predators, despite the contention of some writers that quiescence is the best protection of a preyed-upon species, whether or not it is concealingly colored. Therefore the mechanism for color change in fishes is one of high utility, and its main function is concealment.

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