Degradation of zooxanthellae and regulation of their density in hermatypic corals

Abstract

This study investigated the process of zooxanthellae degradation in hermatypic corals. The number of degraded zooxanthellae in corals taken from different light conditions amounted to 1 to 6% a day, which was similar to the number of dividing zooxanthellae. Zooxanthellae degradation takes place only at night in the connecting sheet and tentacles but both at night and during the day in the gastroderm of the mesenteries. Zooxanthellae degradation continues for about 6 h. DNA staining with DAPI (4'6-diamidino-2- phenylindole) and light, UV and electron microscopic examinations showed that zooxanthellae under degradation lost DNA, protein of pyrenoids and lipid drops. The degraded zooxanthellae particles contained 'accumulation bodies', unpacked thylakoids, starch grains and a pyrenoid starch envelope. Under starvation experiments the number of degraded zooxanthellae in Stylophora pistillata increased in the tissue, as did their release. It is concluded that hermatypic corals are capable of regulating their zooxanthellae population by digestion and extrusion of zooxanthellae remnants.