Diel cycles of expansion and contraction in coral reef anthozoans

Diel patterns of expansion and contraction are widespread in coral reef anthozoans, yet no theory adequately explains this behavior. We have observed a wide variety of behavior patterns in 14 sea anemone species at 9 sites along the Caribbean coast. The distribution of zooxanthellae in anemone tissues was quantified by sectioning preserved specimens and calculating zooxanthella density in the endoderm. We show that polyp structures containing dense populations of zooxanthellae respond positively to light (expansion, positive orientation) and those with few or no zooxanthellae respond negatively (contraction, negative orientation). Structures capable of prey capture, feeding tentacles, are expanded at night when prey is available. Structures adapted for photosynthesis, auxiliary structures of the column and tentacles with dense zooxanthellae, are expanded during the day. Such independent reactions of structures acting as functional units for photosynthesis and/or prey capture combine to give the observed variety of behavior patterns. We hypothesize that the need to conserve limiting nutrients and energy could be the ultimate cause of expansion and contraction rhythms in coral reef anthozoams.