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Recently, the attention has been drawn to the vertical migration of planktonic crustaceans in the fact that the phenomenon is an interesting subject not only as itself but also as a problem concerned in the transport of living organic substances from surface to deep layers in the ocean. In the present paper, the moving behaviour of planktonic crustaceans to light and the mechanism of the vertical migration were analysed under the conditions of various surface light intensities and extinction coefficients of water. The media of various extinction coefficients were prepared by regulating the volume of blue-black ink to be dropped into seawater or freshwater filled in glass tube-tank (5 cm diameter × 100 cm height). The intensity of underwater illumination of the tube was measured by photometer with Selenium photo-cell. It was observed that the planktonic crustaceans (some species of copepods and daphnids) gave rise to the vertical migration in the tube when the surface light intensity was changed gradually. In the case of adult, their moving behavior was recognized to be a photokinetic one showing that their swimming speed towards the light source was regulated by the intensity of underwater illumination at the depth where the animals were present, while in immature, it was likely to phototaxis. When the surface light intensity of artificial light source was changed, the depth of distribution-centre (Z) of the species living in surface layer or the intensity of illumination at the depth (I_z) could be given by the following equation with two parameters, surface light intensity (I_0) and extinction coefficient of water (μ); I_z = aI_0^b, a = f(μ) where ‘a’ and ‘b’ are constants having each a different value by species. In the case of Daphnia pulex, for example, the equation was given as follows; logI_z = (1.021-0.035μ) + 0.603logI_0 Z = 2.3/μ(0.397logI_0 + 0.035μ － 1.021) and, for Acartia erythrea, ‘a’ in the equation is 0.467－0.085 μ, and ‘b’ is 0.676. It was confirmed that the depth of the distribution-centre of these species calculated from the above equation corresponded with the actual one from the field observations. Thus, it was approved that the general equation could be sufficiently applied to estimate the depth where these species distributed in the daytime in the sea or lake. In the species living in surface layer, it was recognized that the value of ‘b’ in the above equation was positive, and under the condition of a constant coefficient (μ), the higher the surface light intensity (I_0), the deeper the depth of the distribution-centre (Z). The intensity of illumination at the depth (I_z) does not show such a constant value as had been described by many authors. According to the present reconsideration about the data from the observations of DSL or SSL, it was revealed that the higher the surface light intensity, the lower the intensity of illumination at the depth, that is to say, the intensity of illumination at the depth of DSL was given by the following equation; I_z = aI_0^－b It was seen, in detail, that ‘b’ value in both I_z － I_0 relationships changed gradually to a smaller value as the surface light intensity became higher. In the species living in surface layer, however, ‘b’ value was regarded to be constant within the limits of empirical condition. Then, it is well known that, as the light is transmitted to the deeper layer, the degree of the angular distribution becomes more uniform. It was observed in A. erythrea and other species that when the angular distribution became extremely uniform (almost completely diffused light condition), the planktonic organisms could not maintain their position and regardless of the intensity of illumination they sank passively down as they were in the dark condition. Therefore, it is apparent that these species are directly susceptible and react to a certain directional component in the angular distribution of the transmitted light at the depth. It is considered that the difference of the I_z － I_0 relationships between the species living in surface and deep layers and the change of ‘b’ value with surface light intensity are caused by the difference of the functional character of their light receptor and the change of the light spectra with depths. The wavelengths of the transmitted light in such a depth as DSL appears are dominant at the range between 470－480 mμ, for which the photosensitive pigment in light receptor of organisms in DSL has been reported to show the largest absorbance. Whereas, the species’ living in surface layer are susceptible and react to the transmitted light of the wide range of wavelengths. Therefore, the different signs of ‘b’ in the above mentioned equations given for the species living in surface and deep layers can be understood as the results of measuring the light intensity by the Selenium photometer which is susceptible to a certain range of wavelength of light. Thus, it can be concluded that the animals migrate tracing photokinetically the depth of equal intensity of underwater illumination of vertically directional light which is susceptible to the photo－receptor. In addition, it is recognized that, according to the character of light in water, the product of the rate of illumination change per unit of depth in metey (dI_z/dZ) and the reciprocal of the intensity of illumination at certain depth (1/I_z) indicates a constant value, When the species living in surface layer undergo the vertical migrations, they show upward or downward migration with restless up-and-down small movements. On this basis, it can be said that the maintenance of the positions of species living in surface layer (which adapt to relatively high light intensity) is determined by the change of the magnitude of dI_z/dZ, while that of species living in deep layer (which adapt to very weak light intensity) may be determined mainly by that of the magnitude of I_z itself.
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