Duration of bundle release by Acropora aff. tenuis corals in the field

Simultaneous mass spawning is an essential reproductive strategy for maintaining zooxanthellate coral populations, but many of the details remain unclear. Most corals that participate in simultaneous mass spawning are hermaphrodites, and they release capsules (called bundles) that contain sperm and eggs. The advantage of simultaneous mass spawning is that it increases the fertilization success between colonies for sessile organisms such as corals, but while there have been many reports on the timing of spawning in previous research, there is almost no information on exactly how long these bundles are released for. In this study, we measured the duration of bundle release in the field for one species of the genus Acropora , which is a representative coral group that participates in mass spawning. The measurements were made by direct observation and estimation from continuous underwater camera footage. The results of direct observation were 9 ± 4 min (mean ± SD) in 2021 and 15 ± 6 min in 2022. The results of estimation from the underwater camera were 6 ± 2 min. These were considerably shorter than the bundle release periods obtained from observations in aquariums in the past, and it was found that bundle release is completed in a short time in the field. The results obtained in this study are considered to be useful for accurately predicting the fertilization rate in the field.