Gametogenesis Cycle, Spawning Time, and Gametes Microstructure of Two Acroporid Species (Acropora digitifera and Acropora gemmifera) from the Red Sea, Egypt

Synchronization and gonadal development of broadcasting acroporid species (Acropora digitifera and Acropora gemmifera) were studied in northern Hurghada, the Red Sea. Histological investigations of the gonads were examined every month for approximately two successive spawning seasons. Forty tagged colonies were collected from the two selected coral reef sites in Hurghada. Coral branches were collected from March 2019 to March 2020 for dissection and histological examination. Small living colonies of both studied species were collected and put in plastic aquaria to observe the synchronization of the studied species between the colonies in the field and the aquaria. The current study revealed that A. digitifera  and A. gemmifera colonies were broadcast species with one annual gametogenic cycle (10 months), which started in July 2019 and ended in March 2020 for Oogenesis, while spermatogenesis developed in 7 months. Both species exhibited prominent synchronization between their colonies in both aquaria and the field. The spawning timing of A. digitifera  and A. gemmifera was affected mostly by surface sea temperature, the lunar cycle, and photoperiod (day length). They released their egg-sperm bundles around the April 2019 full moon. Both studied species spawned two days before the April 2019 full moon (17 April 2019). This study represents the first integrated work employing multiple techniques to accurately detail the gonad development and spawning timing of acroporoid corals in the Red Sea. It is also the first to describe the structure of their egg-sperm bundles and the process of spermatogenesis in this region.