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Reactions to Light

placed, buccoo and tank


The remarkable habitat of Tropiometra in Buccoo Bay naturally suggested that in its reactions to light it would be positively photo tactic. A tank 6 feet long, a foot wide, and 8 inches deep, having the bottom covered with broken coral and stones, was used for the experi ments. It was placed near high-water mark on the beach, in the sun, and two boards, 5 or 6 inches wide, were laid across it, giving shaded areas; one end was made quite dark by covering with a broad piece of board and burying it in the sand. Numerous individuals of Tropio metra were tested in this tank on many different occasions and under diverse conditions of temperature. The results were surprisingly unsatisfactory. No individual showed any decided reaction to light and none sought the darkness. In most cases, when the specimen was fresh from Buccoo Bay, if placed in a shaded area it remained there; if placed in the sun close to a shaded area, it moved into the shade, but if the board were then removed, it made no effort to move out of the sun; if placed in the sun some inches from shade, it remained quietly in the light. Generally the individuals were placed upside

down in the tank, so that the process of righting might serve to initiate movement. But although they righted themselves promptly, they rarely wandered more than a few inches thereafter. If placed in the tank right side up, they were usually content to remain where put, regardless of whether it was in sun or shade.

Experiments were made particularly to see whether the brightly col ored specimens from under rocks reacted differently from the dark individuals from Buccoo Bay, but no constant differences could be detected. Brightly colored examples of Tropiometra seemed to be a little less hardy and became sluggish and inert in the tank more quickly than the dark ones, but the difference was not striking. The final conclusion reached, as a result of all the experiments and observations, is that Tropiometra at Tobago is slightly negatively phototactic, but that other factors in the environment in Buccoo Bay neutralize this tendency almost or quite completely, while on Buccoo Reef it is for some reason strengthened.