Abstract

The optical density of the shell-material and that of the underlying pigmented epithelium has been determined for a number of species — littoral, sublittoral, operculate, and pedunculate. In all the species except Lepas fascicularis Ellis & Solander there is a marked change in the attenuation coefficient at a thickness of ≈ 0.20 mm; above and below this thickness log (100 II0) is linearly related to thickness i.e., the attenuation coefficient is constant. In Balanus crenatus Brug. the optical density appears to reach a constant value independent of thickness.

The shell material transmits more at longer wavelengths but the pigmented epithelium acts virtually as a neutral density filter above ≈ 400 nm. Some properties of the pigment are noted and its development relative to the degree of insolation was investigated.

The bearing of the results on the light-limitation of endolithic algae and the response of the photoreceptors to light are discussed.