How do corals have sex? New secrets of spawning revealed

Corals play a crucial role in ocean ecosystems, but climate change and human activities threaten their existence. To better protect corals, understanding their reproductive life cycle is essential. Now, researchers led by the University of Tokyo have created a model for coral spawning, based on environmental factors, using data from an often overlooked source: an aquarium.

Coral spawning after a full moon

Corals, often mistaken for underwater plants, are actually colonies of small anemones that reproduce with eggs and sperm. 

Coral mass spawning is a synchronized event where corals release their eggs and sperm in bundles over several days following a full moon. 

Despite decades of research, the environmental drivers behind this synchronous spawning remained unclear due to the sparse nature of coral spawning data.

Spectacular reproduction event

“Coral mass spawning, where corals release their eggs and sperm in synchronized bundles over several days following a full moon, is one of the most spectacular reproduction events in the world,” said senior author Shinichiro Maruyama, an associate professor of biosciences at the University of Tokyo. 

“However, despite decades of studies, environmental drivers of the synchronous spawning remain unclear. Coral spawning data are very sparse; it’s usually only an annual event. Putting such sparse data into a model that can explain the overall pattern of spawning has been impossible, but we’ve finally found a way that works.”

Coral spawning in their natural habitat

Studying coral spawning in their natural habitat is theoretically possible but practically challenging due to the need for robust environmental sensors and daily diving for observations. However, Maruyama and his team found an alternative in the Okinawa Churaumi Aquarium.

“The Okinawa Churaumi Aquarium had kept 15 years of coral spawning records for Acropora corals, a reef-building coral commonly found in Japan’s southernmost prefecture Okinawa, but this data had not been used for this kind of research before,” Maruyama explained. 

“By collecting, interpreting, and interrogating this data, we found that corals use multiple environmental inputs, like rainfall and wind speed, to adjust their spawning timing and synchronize it to achieve a peak time for spawning. Water temperature seems to be the primary trigger to determine the annual window of opportunity.”

Understanding coral reproduction 

The study has potential applications such as more accurate coral spawning predictions and assessments of environmental changes affecting coral reproduction. 

Understanding coral reproduction activities is crucial for maintaining coral reef ecosystems and protecting related marine life. 

Despite the valuable data available from aquariums, they have been overlooked as research resources due to perceptions that they don’t accurately reflect natural conditions and lack organization.

Overlooked possibilities

“Aquariums are treasure troves of research resources with a wealth of untouched and valuable data. On the other hand, some scientists tend to think that they don’t truly reflect nature, and also that they are not as well organized as a laboratory,” noted Maruyama.

“We were fascinated by that gap in recognition and the possibilities that had been hidden, overlooked, and sometimes neglected.”

“Here, we analyzed past data to make a model that fits that data. Next, we plan to produce a mathematical model to predict future spawning events in nature.”

More about coral spawning 

Coral spawning is a fascinating natural phenomenon where many coral species release eggs and sperm into the ocean simultaneously for the purpose of reproduction. This event typically occurs just once a year and is highly synchronized. 

Timing of coral spawning 

The timing of coral spawning is influenced by several factors including water temperature, lunar cycles, and the length of the day, ensuring that a majority of the corals in a region release their reproductive cells at the same time.

Colorful fertilization 

During coral spawning, the corals eject colorful bundles of egg and sperm that float to the surface. These bundles often create a vibrant, snowstorm-like effect on the water’s surface before they break apart to allow fertilization to occur. 

Survival strategy 

This synchronized spawning increases the likelihood of successful fertilization given the vastness of the ocean. Once fertilized, the eggs develop into larvae called planulae, which eventually settle on the ocean floor and begin to grow, forming new coral colonies. 

This event not only contributes to the genetic diversity of coral populations but is also crucial for the recovery and sustainability of coral reefs, which are vital to marine ecosystems but are currently threatened by factors such as climate change and pollution.

The study is published in the journal Royal Society Open Science.

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