Do Oceanic Plankton Animals Lose Themselves?

Abstract

RECENT researches into the behaviour of plankton animals in the sea as to their vertical movements from day to day indicate that light intensity is an important factor. The animals appear in Nature to be brought around an optimum intensity by some tropistic mechanism and, assuming this, there seems likely to be a lower limiting or threshold intensity below which no stimulation takes place. Such, indeed, appears to be the case at night with animals living in our shallower offshore waters, when, with the release of the light stimulus in darkness, they are free to roam anywhere and become evenly distributed throughout the water layers, within the limits of other controlling factors such as temperature and salinity. This lower threshold intensity idea receives support from a paper published in 1926 by two Japanese workers, M. Tauti and H. Hayasi (Jour. Imper. Fisher. Instit., Tokyo, vol. 21, No. 4, p. 42), who found that if a light be projected at night vertically downwards into the water, fish swimming in numbers deeper in the water are only attracted individually to the light when by random movements they swim upwards into a certain threshold intensity.