Documenting coral spawning in East Africa: New in situ observations from Zanzibar for three reef-building species

Abstract

Broadcast coral spawning is a vital reproductive event for many reef-building species and is essential to the resilience of coral reef ecosystems. Understanding spawning dynamics is key to assessing the reproductive health and resilience of these ecosystems. While extensively documented in the Indo-Pacific and Caribbean, coral spawning remains understudied in the Western Indian Ocean (WIO), a region facing rapid environmental change and increasing coral decline. This study presents the first in situ coral spawning records from Zanzibar for three Scleractinian species: Galaxea astreata, Favites pentagona, and Platygyra daedalea. Observations were made during monthly nighttime monitoring dives from November 2024 to February 2025, timed with lunar and solar cycles linked to peak spawning in other regions. Results showed species-specific spawning timing: G. astreata spawned on the fourth night after the full moon in November and December; F. pentagona on the sixth night in November; and P. daedalea on the sixth night in December and January. Spawning behaviour, including gamete setting, release windows, and split spawning, also varied by species. These findings offer the first detailed insights into reproductive patterns for these species in the WIO and underscore the importance of local spawning studies to support regional coral conservation and restoration.