Jun 1, 2024 - Science

The secrets of corals' synchronous sex could help save them

Illustration of coral shaped like a heart.

Illustration: Annelise Capossela/Axios

Wind, rainfall and ocean temperatures all influence the synchronized spawning of some coral in an elusive but critical process scientists are beginning to unravel.

Why it matters: Coral reefs are keystone species supporting ecosystems that underpin coastal economies around the world. But they are under pressure from warming oceans, marine pollution and other threats.

  • A majority of the globe's coral reefs are in the midst of a global coral bleaching event — a consequence of global warming's effects on the ocean, plus an El Niño event in the Pacific Ocean.

The big picture: Understanding the environmental cues that drive coral reproduction could help to protect them and inform efforts to restore reefs.

  • Knowing when coral spawning will occur allows researchers to monitor and collect spawn, fertilize eggs and develop larvae that can ultimately be "returned to the reef through restoration efforts, such as coral seeding," Carly Randall, a research scientist at the Australian Institute of Marine Science, tells Axios in an email.

But coral spawning is difficult to study. Mass spawning events were only first observed and described by scientists four decades ago.

  • "Let's be generous and say we have a decent idea of 100 coral species out of the thousands of them out there," says Rob Toonen, a professor and coral reef biologist at University of Hawai'i at Mānoa.
  • "So we actually don't know very much about how corals reproduce, the vast majority of them remain unknown."

How it works: Many coral generally spawn once each year, when every individual of different species releases (at least) millions of sperm and eggs into the sea in a synchronized sexual event.

  • The season, the day and the hour of the gamete release are all highly regulated so that coral maximize their chances of fertilization, which, if successful, increases the genetic diversity of a reef.
  • Spawning typically happens in summer as the ocean water warms, which may be involved in the development of the gametes in the months before spawning.
  • And it usually happens after the sun sets, with the hour depending on the species.

But predicting details about the day of spawning is trickier, in large part because of the logistics and cost of collecting data on a reef each night in summer months year after year.

  • In many coral, it is precisely linked to the lunar cycle: A few days after a full moon, coral begins to spawn.
  • But for others it is not as simple, says Shinichiro Maruyama, a professor at the University of Tokyo who studies coral symbiosis.

What they found: In a study published this week, Maruyama and his colleagues looked at 15 years of coral spawning data collected at the Okinawa Churaumi Aquarium.

  • They confirmed water temperature influences the annual summer spawning window for Acropora coral species.
  • But they found other environmental factors — like rainfall and wind speed — also affect when peak spawning occurs within that timeframe, the researchers report this week in the journal Royal Society Open Science. When there was more rain, peak spawning happened earlier, for example.
  • The findings suggest these cues allow the coral to "fine-tune" their spawning to avoid releasing sperm and eggs during a storm or other unfavorable conditions that might reduce their reproductive success, but that hasn't been fully determined through experiments, Maruyama says.

Between the lines: Using aquarium data is a unique approach with both benefits and limitations, he says.

  • The aquarium allows for detailed and more controlled data collection and the team found spawning peaked around the same time for the aquarium reef, which uses seawater, and a nearby wild reef.
  • But there are key differences in the amount of water in the aquarium versus in the wild reef and in the number of coral species.
  • Maruyama says the next step is to try to develop a model to predict spawning events and to understand the causal relationship between the environmental cues and spawning.

What to watch: A big question is how the environmental cues that govern synchronized spawning may be disrupted in a changing climate, Randall says.

  • One possibility: Ocean warming could lead to "asynchronous spawning behavior," Randall says. "Some early evidence suggests that could already be happening — but I think this is largely an open question that needs further study."
  • Previous research also found bleaching has a long-term impact on the ability of coral that survive starvation events to reproduce in later years.

The bottom line: "We want to be able to look at when reproduction is impacted and the organisms can't replace themselves, then we can do something about it before they're gone," Toonen says.

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