photo of coral spawning
In May, free-diving volunteers and staff for the U.S. Fish and Wildlife Service documented sexual reproduction of cauliflower coral off Shark Island in French Frigate Shoals, part of Hawaiian Islands National Wildlife Refuge and Papahanaumokuakea Marine National Monument. (Lindsey Kramer)

Each event lasted about 10 minutes. As early morning sunrays pierced the water, coral reef fish went about their business as usual. But for volunteers and staff for the U.S. Fish and Wildlife Service—it was a rare moment of scientific discovery.

On consecutive mornings in May, we formally documented sexual reproduction of cauliflower coral—more precisely, Pocillopora meandrina—for the first time in the Northwestern Hawaiian Islands. We were able to capture on video coral spawning off Shark Island in French Frigate Shoals, part of Hawaiian Islands National Wildlife Refuge and Papahanaumokuakea Marine National Monument.

Shark Island is a small sand island about 500 miles from Oahu. Its lagoon is ideal for observing spawning because of its protected shallow waters, its abundant healthy cauliflower coral and its proximity to a Service field station on Tern Island two miles away.

Corals are colonial invertebrates. They reproduce in two ways: asexual fragmentation or sexual reproduction and larval settlement to suitable habitat.

Asexual reproduction occurs when part of a colony breaks off during a storm or other high-energy event and the broken fragment reattaches to the bottom and continues to grow. Often, fragments are not able to lodge into the substrate and are washed away or buried in the sand. Even when successful, asexual reproduction results in genetically identical colonies, which can render a population susceptible to disease and unusual environmental changes.

Sexual reproduction occurs either by fertilization of eggs within the coral tissue (brooders) or by hermaphroditic egg-and-sperm release into the water column with external fertilization (broadcast spawners). New colonies created via sexual reproduction are genetically unique, which increases the likelihood of a population’s survival during a stressful event, such as a viral outbreak or elevated sea temperatures.

Cauliflower corals are unusual in that they are known to spawn just after sunrise in the main Hawaiian Islands (many species spawn in the middle of the night). Observations near the Big Island (Hawaii) over the past decade by the volunteer organization ReefWatchers—led by Sara Peck of the University of Hawaii Sea Grant Program—have helped shape a predictive model of coral broadcast spawning based on seasonality, lunar cycles, water temperature and ocean chemistry. Our team adjusted the Big Island spawning predictions to fit the later sunrise and moonset at Shark Island.

Our first observation at Shark Island, on April 19 from 7:30 to 8:45 a.m., was uneventful. The water felt fairly frigid that morning, which may have precluded spawning. On May 19, two days after the full moon, we monitored 50 coral colonies, and approximately 30 percent of the colonies spawned around 7:30 a.m. On May 20, our team of eight monitored 70 colonies, and 95 percent spawned between 7:40 and 7:53 a.m.

The events were spectacular. The outgoing tide seemed to increase just before spawning began, perhaps cueing the corals. Typically, one or two colonies began to spawn; then neighboring colonies followed. Smoke-like puffs of reproductive materials were ejected from each coral in waves until the water became a hazy gray. The outgoing tide washed away the spawning materials within minutes.

Although significant, these observations represent just a small piece of the puzzle. More observations are needed to fully characterize and understand the reproductive timing of cauliflower coral.

We do know that successful sexual reproduction helps ensure the resilience of a coral population by improving the odds that it will be able to weather new diseases and difficult environmental conditions, which may become critical in a changing global climate. Knowing where and when reproduction takes place also enables coral reef managers to minimize human interference with the process and help facilitate the creation and replenishment of new corals to the reef.

Lindsey Kramer, a marine science field technician based in Kailua-Kona, HI, is a U.S. Fish and Wildlife Service volunteer. The Shark Island coral spawning video can be viewed at:—com/photos/usfwspacific/tags/coralspawning/