Degradation of zooxanthellae in the coral Acropora nasuta during bleaching

To study morphological changes in zooxanthellae in bleached corals, we observed zooxanthellae in colonies of Acropora nasuta that suffered various degrees of bleaching. Colonies were collected from a shallow reef at Bise, Okinawa, from July to September in 2001. Zooxanthellae within the tissue of A. nasuta were classified into 3 types based on their coloration and shape: healthy-looking zooxanthellae (Hz.), pale zooxanthellae (Pz.) and transparent zooxanthellae (Tz.). The Pz. were pale yellow with more or less disorganized cellular content, while Tz. were swelled and vacuolated with no pigmentation. Tz. were the dominant zooxanthellae in colonies that suffered severe bleaching, while non-bleached colonies and those recovered from bleaching had mostly Hz. Some Tz, were ruptured with vacuoles spreading out of the cell. The density of zooxanthellae in severely bleached colonies was about 105 cells cm-2. The present observation suggests that, during natural bleaching due to chronic stress, zooxanthellae remained in host tissue lose pigmentation and become swelled and vacuolated suggesting necrotic death of the zooxanthellae.