Early development of zooxanthella-containing eggs of the corals Pocillopora verrucosa and P-eydouxi with special reference to the distribution of zooxanthellae

Some hermatypic corals spawn eggs that contain zooxanthellae. We followed d evelopment of zooxanthella-containing eggs of two such species, Pocillopora verrucosa and P. evdouxi. We also documented changes in the distribution p attern of zooxanthellae during development. Oocytes of both species took up zooxanthellae 3 to 3 days before spawning. At first, zooxanthellae were ev enly distributed in oocytes, but they later moved to the hemisphere that co ntained the germinal vesicle. After fertilization, early cleavage events we re holoblastic, progressing by furrow formation. The first cleavage furrow started at the hemisphere that contained zooxanthellae, dividing the zooxan thellate complement of the zygote about equally into the two blastomeres. T he second division divided each blastomere into one zooxanthellae-rich cell and one with few zooxanthellae. With continued cell division, blastomeres containing zooxanthellae moved into the blastacoel. The blastocoel disappea red at about 5 h after the first cleavage, and the central region of the em bryo was filled with cells containing either zooxanthellae or lipid droplet s, forming a stereogastrula. Our results suggest that only blastomeres that had been determined to develop into gastrodermal cells receive zooxanthell ae during cleavage. This determination appears to take place, at the latest , by the second cell division at the four-cell stage.