Feb. 14, 2021
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Coral: passing down genetic life lessons
In conversation with Iliana Baums, a molecular ecologist at Penn State University.
The first thing to know about coral is that they’re relatives of jellyfish.
While jellyfish expand and contract their bell-like bodies to float around and drag their tentacles with them, “corals lay down on the bell side,” says Baums. Slowly, over decades, they painstakingly construct calcium carbonate skeletons to protect their fragile polyps, and dangle their tentacles outside to munch on drifting plankton.
Coral and their skeletons provide the backbone for entire reef ecosystems. Their skeletons are homes to algae, with whom coral swap carbon dioxide for sugar. Coral polyps and tentacles secrete a thick, nutrient-rich mucus—which happen to be the favorite food of bacteria. Bacteria, in turn, happen to be the favorite food of plankton, which happen to be the favorite food of the corals themselves, as well as the tiny fish and crustaceans who seek refuge in the nooks and crannies of corals’ skeletons. These small fish are the favorite food of larger fish, who are the favorite food larger fish, and so on and so on. It’s a beautiful, snot-based economy, says Baums.
A coral reef with a school of fish swimming nearby. Image credit: NOAA.
The second thing to know about coral is that they’re uniquely able to pass down survival tips through their genes to their offspring. Scientists call this having “an un-sequestered germ line.” What this means is that mutations coral pick up over the course of their lifetimes get passed down to their offspring. Though it’s common in plants, it’s rare in the animal kingdom, and it helps make coral masters of immortality.
Let’s dive into that more:
Like kelp, coral are stuck where they settled as larvae; they’re at the mercy of their environment, says Baums. This means that if coral hope to survive, they better find ways to constantly adapt. One way to do so is to collect beneficial mutations over time.
For us, mutations are generally a bad thing. They’re an unfortunate byproduct of staying alive: The longer we live, the more copying mistakes our cells make over time. Some of these mistakes are random, and some are the result of external exposures, like UV rays or chemicals.
The more copying mistakes, the greater the likelihood that one of them will lead to cancer other diseases. We have some internal anti-cancer protection: The DNA in our cells are equipped with telomeres, which shorten with every division. wWhen telomeres get to a critical length, they forbid the cell from dividing anymore because the risk of developing a cancerous mutation is too high. As a result, the cells senesce and eventually self-destruct—that’s a big part of aging for us.
We keep our gametes (eggs and sperm) separate from the rest of our cells that pick up these mutations over time. That’s why a person who develops an injury, like a burn, or an illness, like cancer,* won’t pass it down to their kids. For example, I have numerous scars on my knees from falling and scraping the skin; if I ever had children, they wouldn’t be born with scar tissue on their knees. They’d have smooth skin, like mine was before I picked up trail running.
(*Cancer can be the result of an environmental exposures, like smoking, drinking alcohol, or certain chemicals. People who develop cancers related to one of these exposures may not pass it on to their children. But sometimes, cancer is the result of a combination of genes and environment; it’s an incredibly complex disease. Some of these genes could get passed onto children, even if they didn’t guarantee that child would develop cancer.)
But coral are different. Coral reproduce sexually and asexually. The giant colonies of corals, like the one pictured above, are the result of hundreds of thousands of rounds of asexual reproduction.
Asexual reproduction is essentially cloning. It doesn’t always go perfectly; sometimes, a clone may have a genetic mistake that leads the offspring polyp to die. But, because there are hundreds of thousands of successful clones, the colony as a whole survives.
When a colony’s environment changes, like if the water gets warmer or more more acidic, coral have to adapt. While much of it may die off, some of its polyp clones will have mutations help them survive in this new environment. Those polyps have got a positive mutation on its hands (tentacles)! Polyps with this beneficial mutation can clone itself again and again, and the colony survives.
But then it’s coral’s turn to ~sexually~ reproduce. This mixing of genes across gametes is critical for the survival of the species, and not just the immediate colony. It happens just once a year. For Caribbean corals, it’s like clockwork “three nights after the August full moon between 10pm and 11pm,” according to Baums. Corals release their egg and sperm into the water column and hope they meet up to form a zygote that can float and finds a suitable surface to latch onto to build a colony of its own.
Coral gametes, however, collect the mutations of their parents. Theoretically, this means they have all the genetic life lessons of their parents: How to survive in warmer water, how to survive in more acidic water, and maybe even how to survive with a little less food available. And while that inheritance could be dangerous for us humans, for coral, it could be the key to survival.
It also brings up an unusual question about a coral’s age, which Baums and her team are studying: Because scientists can count the rate mutations in coral over time, like rings on a tree, they can figure out how old its parent coral was when it sexually reproduced to start a new colony, at least roughly. It’s usually give or take a few hundred years, Baum says. And then, based on the size of the colony itself, you can figure how long ago that was.
Theoretically, if all coral keep adapting over time, which allows them to grow their individual colonies until it’s time for sexual reproduction, coral would be live forever. We know that coral dies, though, when its environment changes too quickly for it to adapt. Being immortal doesn’t mean evading death entirely; it just means you don’t age into death. And coral, it seems, may have cracked the code for a form of immortality—at least for a creature remains forever in its place.
That’s all for now—stay curious, friend ❤️
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Top image by Rachel Couch; headshot by Matt Anzur.