Seasonal and daily planulation patterns of two pocilloporid corals at a high-latitude habitat in Southwest Kochi, Japan

In order to clarify the characteristics of planulation and reproductive strategies of brooding scleractinian corals at high-latitude coral community, seasonal and daily planula release patterns of Pocillopora damicornis and Stylophora pistillata were studied at Nishidomari, Kochi, Japan in 2006 and 2007. Distribution of recruits and adult colonies were also examined. Colonies were collected at the study site and kept in tanks, and the number of daily released planulae was counted. Branch fragments were sampled from tagged colonies in the field, and oocytes and planulae in polyps were observed under a binocular microscope after decalcification. Observation of captive colonies and branch fragments showed that planulation occurred twice each year, either in June and July (P. damicornis) or in July and August (S. pistillata). Brooding of planulae in polyps was observed both years during the same periods. The duration of planulation was longer in S. pistillata (approximately 25 days) than in P. damicornis (approximately 15 days). The planulation of P. damicornis peaked around the new moon, but that of S. pistillata did not show a clear peak synchronized with lunar phase. In both species, planula release occurred all day long during the planulation periods although the number of planulae released was slightly higher at night. Compared to the studies from low latitude coral-reefs areas, both species had less frequent planulation periods per year but longer duration of planulation periods. In the distribution study, recruits of P. damicornis and S. pistillata were significantly higher around their parental colonies. These planulation and distribution patterns are supposed to reflect the reproductive strategies of these two species in high latitudes; increasing the opportunity of planulation and recruitment around parental colonies to maintain their local population.