Observations of the Effects of Changes in Hydrostatic Pressure and Illumination On the Behaviour of Some Planktonic Crustaceans

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Abstract
1. Daphnia magna responds vigorously to a large increase in hydrostatic pressure when in total darkness, but this response is very much depressed in the presence of light. 2. The weak response of Daphnia in light is apparent over a wide range of intensity. 3. The effect of pressure on Daphnia appears to be largely kinetic and does not involve a specific orientation of the swimming activity. 4. The overt threshold for Daphnia and Calanus under laboratory conditions are in the region of 3.5 bar (50 p.s.i.) and 0.7 bar (so p.s.i.) respectively. 5. The barosensitivity is not abolished by repetitive compression and decompression. 6. Observations made of the behaviour of Calanus indicate that the pressure response is not inhibited by light as it is in Daphnia. 7. Calanus which have been maintained at high ambient pressure for several hours remain sensitive to small increments of pressure. The response appears similar to that obtained at the level of atmospheric pressure.