The Response of Protodrilus symbioticus (Giard) (Archiannelida) to Light

Abstract

By means of a photonegative klinokinesis the animal was shown to accumulate on the shaded side of a light/shade boundary. Although strong light inhibited activity, within a light gradient an optimum region of light intensity between 30.13 lux and 0.40 lux was selected. Inlight intensities above 30.13 lux the animal moved to the darkest areas available. This preferendum, in the natural habitat, will keep P symbioticus below 2.8 mm. and above 5.1 mm. depth of sand, in bright sunlight. An equation was derived for calculating light penetration into beach sand.