Lunar Periodicity in Obelia

Abstract

IN Proc. Roy. Soc., vol. 95, 1923, Mr. H. M. Fox directed attention to a number of cases of “Lunar Periodicity in Reproduction” in marine organisms. To these may be added the hydroid Obelia geniculata. At first sight the periodicity is masked by the irregular breeding of colonies which are wave-worn or much eaten down by nudibranchs, but if attention is confined to healthy and perfect colonies, the lunar periodicity seems quite definite. During 1924 several colonies on the piers were located and watched; the best result was from a colony on Laminaria on Millport old pier, which was giving off medusæ during the ten-day periods beginning with the third week of the moon in July, August and September, and not at other times. Other colonies gave definite results in two consecutive months, but were then attacked by nudibranchs or lost.