A Scleractinian Coral (Plerogyra sinuosa) with 'Photosynthetic Organs'

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Abstract
The scleractinian coral P. sinuosa DANA displays tentacles at night and globular bubbles during the day. Tentacles have few zooxanthellae (760 mm-2 of a transverse section) and a high nematocyst density (1600 mm-2); they are used for capturing planktonic prey. Bubbles have few nematocysts (140 mm-2) and a high density of zooxanthellae (2300 mm-2); they are used to foster the photosynthetic ability of the coral. At low light intensities the bubbles inflate up to 45 mm in diameter, while they contract at high light intensities. P. sinuosa preferentially occupies shaded areas in water of < 10 m depth. A new method for in situ O2 measurements within the bubbles demonstrated a high photosynthetic activity (.apprx. 70 mg O2 m-2 h-1) of the bubbles. These are interpreted as photosynthetic organs, enabling the coral to compete under a wide range of light conditions.