Abstract
The daily growth rates of Modiolus modiolus and Cerastoderma edule were measured during continuous illumination, continuous darkness, and during periodic light with 18 hrs of illumination and 6 hrs of darkness. M. modiolus increased its growth rate significantly both during and after continuous darkness, while no such effect was found for C.edule. It is discussed if the pigmentation of the periostracum may be a link between light exposure and shell growth.