December brought the hope that we were longing for in Laamu with an incredible surprise: coral spawning! Despite the challenges posed by the April 2024 bleaching event, multiple coral species defied the odds with a late but remarkable mass spawning event—a stunning testament to the resilience of our reefs.

In this article we’ll explore how this extraordinary event is fueling MUI’s coral larval restoration project, a vital effort to regenerate degraded sections of the reefs around Laamu. This project involves collecting coral sperm and eggs,(gametes), fertilizing them, and reseeding the resulting larvae onto damaged reef areas.

It All Begins: The Gravid Coral Check

As the full moon approached, we conducted our gravid coral check—a process to determine if corals are ready to spawn. This month, MUI intern Thuva and research coordinator Dr. Nisha carefully "cracked" corals, breaking small branches of Acropora corals to check for mature eggs. After last month’s disappointment of no mature eggs, our hopes were low. But to our delight, two coral species revealed clusters of pink, mature eggs. The excitement was palpable—this meant spawning was highly likely following the next full moon!

Coral Researcher Coco preparing the Coral Larvae Pool. Photo by Owen Bornhorst.

The Preparation

With spawning on the horizon, preparations kicked into high gear. Coco, MUI’s coral researcher, meticulously organized equipment for gamete collection and larval cultivation. Dr. Nisha readied the data sheets, and the entire MUI team came together, diving into the moonlit waters to survey the reef. The goal of these surveys were to detect signs of “setting,” when egg bundles are held in the mouth of the coral, signaling that spawning is very near.

The Wait

Day 1 after the full moon: No activity. After Day 2: Still nothing. Days 3 and 4 passed and the silence continued. On Day 5, just as the sun set, four Acropora tenuis colonies finally spawned— however, this was just a teaser of what was to come. The real action unfolded on Day 6. At sunset, Acropora tenuis colonies began spawning. By 8 PM, over 20 Acropora secale colonies had joined in. It was an all-hands-on-deck moment.

Coral Spawning. Photo by Owen Bornhorst.

Growing the Next Generation

The dive team deployed collection devices onto the spawning colonies, gathering eggs and sperm, which snorkelers transported to the surface. Here the gametes were mixed in tubs to allow them to fertilize. In the lab, Coco worked tirelessly through the night to assess the results. Success! An impressive 88% of coral eggs were fertilized.

The following morning, fertilized eggs were transferred to a floating pontoon equipped with a net, allowing them to grow and develop into free-swimming larvae over the course of six days. Coco finally got some well-deserved rest as the larvae matured.

Waiting for fertilization. Photo by Owen Bornhorst.

Releasing 1.2 Million Coral Larvae

Six days later, the moment arrived, the corals were ready for release. The MUI team released 1.2 million coral larvae onto the reef! To monitor the success of this effort, bare reef tiles were placed at the release site. If larvae settled on the tiles, it would indicate successful settlement across the reef.

The Initial Result

Coco is currently at the microscope counting coral recruits. Early results are promising—one 10 cm² tile already hosts nine coral babies. New year, new coral babies on the reef!

SPAWNING BRINGS NEW HOPE FOR OUR REEFS