Under the light of the moon and stars, scientists in Australia recently scooped up some of the most valuable treasures in the sea: packets of coral sperm from the Great Barrier Reef.
On a three-week expedition to Heron Island in mid-November, teams collected 171 billion sperm from 31 coral colonies, representing eight species of hard corals. These samples were then flash frozen and added to the vaults of the world’s largest coral sperm bank, held at the Taronga Conservation Society’s Western Plains Zoo in New South Wales.
Ultimately, the scientists would like to bank all 400 or so coral species that make up the famous 1,400-mile-long reef system, says Mary Hagedorn, a Smithsonian marine biologist who pioneered the technique of freezing coral sperm.
But the ongoing project is a race against time to collect samples from as many important reef-building corals as possible before populations shrink and genetic diversity is lost. Much of the north section of the Great Barrier Reef already sits in ruin after warm waters caused unprecedented coral die-offs in the summers of 2016 and 2017.
“The problem for us,” says Hagedorn, who is based in Hawaii, “is that we can’t train enough people or move fast enough.”
Romance by Moonlight
Reef-building corals are colonies of tiny, tentacled marine animals that protect themselves by growing hard skeletons on the outsides of their bodies. These calcium-based structures are what we see as coral reefs. Like rain forests of the sea, coral reefs are biodiversity hot spots, with a wide array of ocean animals depending on them for food and shelter.
Corals are very sensitive to temperature changes, and as the ocean warms due to human activity, large swaths of the world’s reefs have died, leaving behind only lifeless white coral skeletons. If these so-called mass bleaching events become more frequent and intense, as climate models predict, then coral reefs as we know them could become a thing of the past.
That’s why scientists from Taronga and the Smithsonian Conservation Biology Institute have been building up a bank of frozen coral sperm. Over six spawning seasons, they have collected trillions of sperm from many individual coral colonies, covering a large portion of the genetic diversity of 16 species.
Collecting and freezing coral sperm is no easy task. Corals on the Great Barrier Reef spawn en masse at specific times of night and typically over a period of just one week each year, usually in November following a full moon.
“We’d booked our flights months in advance, because we knew this was going to happen in a specific timeframe,” says Rebecca Hobbs, a Taronga reproductive biologist and one of the expedition leaders.
Collecting coral spawn in the open ocean is near impossible, so the scientists collect whole corals and bring them into tanks flushed with seawater that are open to the moon and stars, so they can respond to these environmental cues as they would on the reef. When it’s time for a specific species to spawn, they release vast numbers of egg-sperm bundles.
“We collect the bundles and separate and clean the eggs and the sperm,” Hagedorn says. “We then freeze the sperm and thaw and test it, to make sure it is capable of fertilizing eggs.”
Currently, the researchers are only freezing sperm—coral eggs are much larger and are difficult to freeze and thaw without damage. But Hagedorn’s team is working on technologies to freeze eggs and, potentially, coral larvae. And as they recently revealed in Scientific Reports, they have successfully fertilized fresh eggs with sperm that had been frozen for several years, resulting in healthy coral larvae.
Investing in the Future
Theoretically, once sperm is stored in liquid nitrogen at -320°F, it is in a state of suspended animation and could last for hundreds of years.
“The potential significance of the cryopreservation project is that it will enable us to freeze sperm from a range of healthy corals now, just in case in future we’ve lost significant amounts of the genetic diversity,” says Peter Harrison, director of the Marine Ecology Research Centre at Australia’s Southern Cross University, who helped the team collect this year at Heron Island.
Hobbs says that the coral in the southern part of the reef is still relatively healthy and intact, which is why it was important to expand their collecting efforts there this year. In previous years, they collected from the central section of the reef, near Townsville, with the Australian Institute of Marine Science. But bleaching events have created serious declines in those coral populations.
“A lot of the genetic diversity has been lost, and what we’re left with is a remnant of the populations,” says Harris. Reefs around the world are losing corals faster than they can naturally replenish, he adds, which is why bio-banking is so important.
“If we have some of that sperm in our bank, then we can then go back and help re-diversify those populations,” says Hagedorn. “Even if we only have three or four coral individuals, we will have tons of diversity in that sperm. Frozen sperm, even though they are just sperm right now, will have a huge impact on reinvigorating those populations.”