Abstract
The length increase of Mytilus edulis shells was measured once or twice daily by laser diffraction. There was a pronounced individual variation between shells of equal size given the same treatment. The growth rate was significantly higher in animals kept in the dark. Specimens grown under normal conditions of day and night, however, increased significantly more during the day than during the night. This diurnal rhythm was maintained also in continuous darkness. It is suggested that a minimum period of darkness is necessary to give rapid growth. All specimens responded to starvation within 24–36 hrs by a reduced growth rate, regardless of previous feeding. The decline was most rapid in the case of starved and underfed individuals. The response to feeding was slower and more variable (approx. 2–4 days), starved specimens being the first to respond.