Coral Spawning Season Has Begun in Zanzibar
From October to March, coral reefs along the East African coast enter the period when many coral species release their reproductive cells, known as gametes, into the water in a synchronized event.
A Year in the Making
Our coral spawning project, supported and funded by Zoo Basel, launched in October 2025 together with Sound Ocean Science. Over this first project year, our team together with the lead Scientist Dr. Ashlee Lillis, has been hard at work establishing the groundwork for coral reproduction research and restoration. We successfully observed spawning events and collected valuable data — data that now allows us to move into the next stage of the project this season.
Why Coral Spawning Is So Hard to Catch
Most reef-building corals are broadcast spawners: instead of reproducing continuously, a single colony typically spawns only once per year, and only for a window of a few minutes, on a specific date, at a specific time of day or night. This timing is driven by environmental cues — primarily water temperature and the lunar cycle, with most spawning events occurring several days after a full moon, after sunset.
Because the window is so narrow, predicting exactly when a colony will spawn is one of the biggest challenges in coral reproduction research. This is precisely why last year’s observations were so valuable: by recording which colonies spawned, on which dates, and at what times, we’ve built a dataset that helps us anticipate and prepare for this year’s events with much greater precision.
From Gametes to Baby Corals: How It Works
Once we’ve identified the expected spawning window for a colony, our team prepares collection tools and gathers the released gametes (eggs and sperm) directly from the water column.
The gametes are then transported in containers of seawater to our on-site laboratory, where eggs and sperm from different colonies are mixed to allow fertilization. This cross-colony mixing is a key part of the process: it increases genetic diversity in the resulting offspring, which is essential for building a coral population that can better withstand future stressors like heat and bleaching.
Within the lab, fertilized eggs develop into free-swimming coral larvae, which our team monitors under microscopes. Once the larvae reach the right developmental stage, they’re moved back out into the ocean into our nurseries, where they’re housed in fine mesh bags. There, they begin searching for a surface to attach to — a process called settlement.
Watching New Reefs Take Shape
Larvae that successfully settle attach themselves to substrates that we’ve custom-designed for this purpose. If settlement is successful and the young coral survives its most vulnerable early weeks, small coral recruits become visible within the following months — the first visible sign of an entirely new generation of reef.
The long-term goal is to grow a population of corals descended from parent colonies that have already survived previous bleaching events — giving this next generation a genetic head start in resilience, and contributing to a healthier, more adaptable reef for the future of Zanzibar’s coastline.