. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 26 A. J. SWEATT AND R. B. FORWARD, JR. sunset. The most rapid decreases in irradiance occurred just after sunset. At sunset, irradiance values over the 7 m water column ranged from 1016 to 1019 photons m 2«s '. Photobehavior experiments In the first series of experiments, upswimming was tested for animals adapted to each of five different light intensities (Fig. 6). Animals were used only once in any experiment. Upswimming was tested at each of the original adaptation intensities (LA), at one log unit below those intensities
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 26 A. J. SWEATT AND R. B. FORWARD, JR. sunset. The most rapid decreases in irradiance occurred just after sunset. At sunset, irradiance values over the 7 m water column ranged from 1016 to 1019 photons m 2«s '. Photobehavior experiments In the first series of experiments, upswimming was tested for animals adapted to each of five different light intensities (Fig. 6). Animals were used only once in any experiment. Upswimming was tested at each of the original adaptation intensities (LA), at one log unit below those intensities (LA~'), and in darkness (D). Statistical comparisons were made between responses measured under the three test conditions at each adaptation intensity (one-way analysis of variance). In one case, tests were conducted only at the original adaptation intensity and in darkness. In the five sets of tests, upswimming responses were weakest when measured at the original adaptation intensity. Leaving animals in darkness invariably led to a stronger response. Reducing the light intensity to one log unit below the original adaptation intensity had variable effects. At the two highest original adaptation intensities, upswimming following such intensity reductions (LA"1) was not significantly different from the response observed with no change in light intensity (LA). However, at two lower adaptation intensities, responses following the one log unit reduction became more like those measured for animals left in darkness (D). This indicates that reducing the intensity by one log unit (LA"1) led to marked and significant increases in upswimming (relative to the 80- 0. c E 60- 401 20- 0 n 9 D* 15 LA' LA — 4l5 •j Original Adaptation Intensity (log photons/m -s) FIGURE 6. Upswimming responses of Sagitta hispida adapted to various light intensities. Means and standard errors are shown on the vertical axis. Sample size for each experimental condition
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