Climate models predict that mass coral bleaching events will become more common in the future, which means that corals may eventually become a thing of the past. However, scientists are doing all they can—including collecting coral sperm—to make sure that this doesn’t happen.
Corals release sperm and egg bundles into the water when they spawn. [Photo by Gary Cranitch, Queensland Museum]
Corals are a beautiful, vital part of our marine ecosystems, but they’re also very sensitive creatures. Even small changes in temperature—small to us, anyway—can have devastating effects on the reefs. Because human activity has caused the world’s oceans to grow warmer, large portions of coral reefs around the world have died. These die-offs are called “bleaching events”, due to the bone-white appearance of corals when they die.
Urgent climate change action is the best and surest way of making sure that we don’t lose the corals in our oceans. This, of course, is easier said than done. As a start, scientists are building the world’s largest coral sperm bank at the Taronga Conservation Society’s Western Plains Zoo in Australia.
Collecting coral sperm isn't the easiest task. [Photo by Gary Cranitch, Queensland Museum]
Over the course of a three-week expedition, scientists were able to collect 171 billion sperm from 31 coral colonies. These sperm belong to eight different hard coral species. The samples were then flash frozen and sent off to the sperm bank for safekeeping. A new study describes the researchers’ efforts to harvest coral sperm for freezing.
The sperm bank already houses trillions of sperm from 16 different species, collected over six spawning seasons. In the long term, the scientists aim to collect sperm samples from all the 400 or more coral species found in the Great Barrier Reef.
Researchers collect sperm and egg bundles from a seawater tank. [Photo by Gary Cranitch, Queensland Museum]
However, scientists need to move fast. Currently, they are in a race against decreasing coral populations and the loss of genetic diversity. The northern part of the Great Barrier Reef has already sustained considerable damage as it is. Thus, if the researchers want to reach their goal, they need to collect their samples before another bleaching event occurs.
Unfortunately, collecting coral sperm samples isn’t easy, nor simple. Great Barrier Reef corals typically produce sperm over a period of just one week per year. This week usually comes in November, occurring after a full moon, at specific times during the night. The collections would also be near-impossible to do in the ocean itself, so the researchers have to bring entire corals into a tank filled with seawater and stationed under the moon and stars. This way, the corals would have the environmental factors necessary for spawning.
One of the researchers checks the temperature of the frozen sperm. [Photo by Gary Cranitch, Queensland Museum]
Corals release egg-sperm bundles when they spawn. “We collect the bundles and separate and clean the eggs and the sperm,” says Mary Hagedorn, a marine biologist at the Smithsonian. “We then freeze the sperm and thaw and test it, to make sure it is capable of fertilizing eggs.”
Theoretically, coral sperm can last for hundreds of years frozen. Hagedorn and her colleagues have actually managed to fertilize a fresh coral egg using coral sperm that was frozen for several years. This reportedly produced healthy coral larvae.
But what about coral eggs? Shouldn’t they be frozen as well? As of now, not yet. Coral eggs are larger, more difficult to thaw, and may sustain damage from the freezing process. However, scientists are working on finding a way to safely freeze coral eggs, as well as coral larvae.
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