Histological studies on the origin of planulae of the coral Pocillopora damicornis

The coral Pocillopora damicornis develops gonads and releases planulae on a lunar 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, Japan. Small branches 3 to 5 cm long were sampled for histological observation from 7 colonies every day from August 24 to 30 and September 26 to 30, 1997. 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, ca 100 mu m in diameter. Oocytes occurred in 2 size classes, small (<50 mu m) and large (>50 mu m). A continuous series of planula 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 a 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.