Early development of two Pocilloporid corals
Michio Hidaka
Dept. of chemistry, Biology and Marine Science, Univ. of the Ryukyus
Nishihara, Okinawa, 903-0213 Japan
I would like to present two topics on early development of Pocilloporid corals: (1) Origin of planulae in the coral Pocillopora damicornis, and (2) Early development of zooxanthella-containing eggs of P. verrucosa.
The coral P. damicornis develops gonads and releases planulae on alunar cycle throughout most of its range. Planulae of P. damicornis in some populations display the same allozyme pattern as their mother colonies suggesting that planulae might be produced asexually. To investigate the origin of planulae in this coral, histological studies on maturation of oocytes and early development of planulae were performed on P. damicornis in Okinawa. Blastula- and gastrula-like stages were observed for the first time in P. damicornis. Blastula-like embryos consisted of a poorly defined cellular cover over an indistinct yolky mass, and were approximately the same size as mature oocytes, c.a. 100 μm in diameter. A continuous series of developmental stages from blastulae to planulae was documented. The appearance of early embryos and small planulae occurred about the time that large oocytes disappeared. These observations, together with lack of any sign of production of planulae by budding, strongly suggest that planulae of P. damicornis in this population are derived from oocytes that undergo typical embryogenesis.
Some hermatypic corals spawn eggs that contain zooxanthellae. We followed development of zooxanthella-containing eggs of P. verrucosa. We also documented changes in the distribution pattern of zooxanthellae during development. Oocytes took up zooxanthellae 3-4 days before spawning. At first, zooxanthellae were evenly distributed in oocytes, but later moved to the hemisphere that contained the germinal vesicle. After fertilization, the first cleavage furrow started at the hemisphere that contained zooxanthellae, dividing the zooxanthella complement of the zygote approximately equally into the two blastomeres. The 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 blastocoel. The blastocoel disappeared at about 5 hours after the first cleavage and the central region of the embryo was filled with cells containing either zooxanthellae or lipid droplets, forming a stereogastrula. Our results suggest that only blastomeres that had been determined to develop into gastrodermal cells receive zooxanthellae during cleavage. This determination appears to take place, at the latest, by the second cell division at the four cell stage.
This work was done in collaboration with Mrs. Diah Permata, Mrs. Mamiko Hirose, and Dr. R. A. Kinzie III.
