Fear of blue crabs makes oysters stronger

Mary Landers, the current

Graduate student Sarah Roney offers an oyster shell in each palm for comparison. Baby oysters the size of red lentils stain the smooth interior surface of a shell. Babies of the same age but almost twice as many as each other.

The secret to making these bigger babies grow?

Crab pee.

Specifically, these are two compounds found in the urine of blue crabs: homemary and trigonelline. These are “fear molecules,” Roney said, chemicals that alert oysters that predators are nearby.

They could help researchers develop stronger oyster beds to prevent erosion as sea levels rise. They could also be a boon for commercial oyster farmers who want a stronger product that is more resistant to predators.

Roney, a 23-year-old Georgia Tech PhD student. A Statesboro student and a 2019 graduate of Georgia Southern University, works at the Shellfish Lab at UGA Marine Extension Service and Georgia Sea Grant on Skidaway, which helps fund her research. It’s a hot and sunny day at the end of September as his oysters face their first field test.

She fills plastic mesh bags with the shells on which the “induced” oysters are deposited. They are the biggest, exposed to the chemicals of fear. She fills four other bags with checks, shells that the baby oysters have grown on without any hint of predators nearby. Four other bags receive bleached oyster shells from a pile behind the building. Roney will use them to track the number of baby wild oysters, called spat, recruited from the estuary.

Roney works with Marc Weissburg at Georgia Tech, whose lab examines chemical ecology, or how animals interact through the use of chemical signals. Think of pheromones for a more familiar example.

“And so one of the things they noticed about ten years ago is when you grow oysters near blue crabs – just in the same water – the oysters are stronger. over time, through the process of elimination, they figured out that something was coming from those blue crabs to cause this response. ”

That something was in the urine, which brings us to a wacky fact about crabs. They piss on their faces.

“So I don’t know if you’ve ever caught crab (while) fishing,” Roney said. “If you take them out of the water, they look like they spit.” They get this sparkling stuff. It’s actually not saliva, it’s urine, because they pee on their faces. And that’s a social signal among blue crabs. This is how they assert their dominance when living in water. Out of the water, it just foams.

Roney catheterized crabs to capture their urine for examination, a few milliliters at a time.

“We actually come in with a catheter and put that catheter into their nephropore, which is right under their eyes, on their face,” she said. “And we extract their urine.”

Now that the researchers know which molecules do the job, they are ordering these fear compounds from an aquaculture supply company.

“And it’s relatively inexpensive,” she said. “So it’s a very convenient way to create oysters that are more likely to survive when you put them in the field. Because that’s kind of the end goal here from an ecological point of view. This is why these oysters have this defense. When you have a thicker shell, you are less likely to get eaten.

Whether this defense will last remains to be seen.

Watch babies grow up

Part of the point of putting the oysters in the Skidaway River. Roney will be checking the bags periodically over the next six months to see how the baby oysters are doing. She hopes to repeat the process in the spring.

She also sent young oysters to Alabama to be tested for strength with a penetrometer.

“It’s a very small instrument that we use to crush the shell,” she said. “And he measures the force it takes to crush the oyster shell. So when we use it, in short, we can compare the oysters that have been grown in these compounds versus not. And we can see that when we use these compounds to grow the oysters, the shells are like 30% stronger. “

His research will also examine whether other oyster predators such as mud crabs and oyster drills and some fish emit the same fear chemicals as blue crabs. And she will look in more detail at whether the growth triggered by fear molecules is based on adding calcium carbonate to make things bigger, or whether it adds organic molecules and proteins to their shells, making them thicker and stronger.

With his bags of oysters in a roller cooler, Roney dons his mud boots and heads for the nearby banks of the Skidaway River.

Tom Bliss, director of the Seashell Research Lab at the Marine Extension and Georgia Sea Grant, is eager to see Roney’s results.

In Georgia, restoration of oyster reefs generally relies on natural recruitment. The process involves putting shells or other material in the tidal water and waiting for tiny oysters to settle there. It works pretty well here, much better than in the Gulf where natural recruitment is much more reliable. But that limits when these restorations can occur, as oyster larvae are only in the water from about March to June.

The results could extend the oyster harvesting season and strengthen the coastline

Bliss is about to launch a grant to use shell spat – seashells on which oyster larvae already grow – for reef restoration.

“If she sees an increase in survival using this method, that’s something that will translate into the possibility for us to get shellfish with spat on them in areas where spat is limited,” he said. he declares.

Instead of popping out the shell from March to June, the season could be extended until September or October, he said.

Stronger shells would also be a boon for living shorelines that fight erosion.

“I hope we can create more successful living shorelines and increase the number of living shorelines here in Georgia that are used as opposed to dikes, groynes, or other types of wave-breaking mechanisms and dams. ‘coastal engineering options,’ said Roney. .

As Roney planted his oyster bags in the mud just below the swamp grass, two large blue crabs scurried to the shore near the dock, as if rushing for a meal.

Roney will return periodically to see where these crabs and other predators have eaten most often.

This story comes to GPB through a reporting partnership with The stream, providing in-depth journalism for Coastal Georgia.

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