Circadian Rhythms in Corals, Particularly Fungiidae

The temporal patterns of expansion and contraction of the tentacles of seven species of Fungia, three other species of the family Fungiidae and two other scleractinian corals have been examined in the laboratory. In six species of Fungia, tentacle behavior showed a circadian rhythm, both under natural light and in continuous darkness, while in two species a circadian rhythm was also demonstrated in continuous light of low intensity. Fungia actiniformis, Polyphyllia talpina and Euphyllia rugosa remained expanded under all conditions. The phase of the tentacle rhythm in Fungia could be changed by initiating continuous darkness during the day. Tentacles of Fungia were shown to contract in strong illumination. Contraction could be induced by a bright flash of 1 msec duration, after a latency of about one minute. The significance of these findings is discussed.