Reference: Biol. Bull., 151 : 236-246. (August, 1976) ORCADIAN RHYTHMS IN CORALS, PARTICULARLY FUNGIIDAE BEATRICE M. SWEENEY Department of Biological Sciences, University of California, Santa Barbara, California 93106 During the day, most of the many species of corals which comprise a tropical reef are contracted, and the tentacles are withdrawn within the horney or calcareous skeleton (Abe, 1939; Yonge, 1940). Feeding takes place at night when the tentacles are expanded. However, there are soft and hard corals in which the tentacles are expanded during the day, and in a few species, the tentacles are always expanded unless they are touched or during reproduction (Wells, 1966). To this author's knowledge, the behavior of corals has not been observed previously under either continuous light or constant darkness in the laboratory. Without such observations, it is impossible to determine whether polyp expansion is directly deter-mined by environmental light or whether it is wholly or partly under the control of a circadian rhythm. The opportunity for making such observations was provided at the shore station which was established at the Banda Islands, Indonesia by the ALPHA HELIX expedi-tion to Indonesia in the spring of 1975. Species of the scleractinian family Fungiidae are particularly common in the Banda region (Wells, 1966). They are large single polyps not attached to the substrate when mature, and hence they can easily be collected without damage. Fungiids were thus particularly singled out for study. The following is a report of the evidence that tentacle expansion and contraction are controlled both by environmental light and darkness and by a circadian rhythm. MATERIALS AND METHODS Coral specimens were collected from the reefs around the Banda Islands of Neira, Gunnan Api and Banda Bessar while snorkeling. The species studied are listed in Table I. Specimens were placed in a pail under water and then brought to the laboratory without exposure to air. Fungiids were picked up from the bottom, to which they are not attached when mature. Specimens of Tubipora and Euphyllia were dislodged from a colony with a section of pipe or an abalone iron. On returning to the shore station, specimens were immediately transferred to the experimental tank without exposing them to the air. All specimens were assigned a number on collection. All fungiid specimens used in this study have been de-posited in the National Museum, Washington, D. C. Two experimental tanks of 77 1 capacity were filled with unfiltered sea water from the inner harbor at Banda Neira. The water was aerated continuously by a stream of bubbles from an air pump. The temperature of the tanks was 28 0.2 C. The sea water was replaced every four to five days, or sooner if it became cloudy. Zooplankton was occasionally added to the tanks as food, but not according to a regular schedule. One of the experimental aquaria was exposed to natural light from a north-facing window just behind it. Since Banda lies close to the equator at 236
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