Grad student Sarah Roney offers an oyster shell in each palm for comparison. Baby oysters the size of red lentils speckle the smooth inner surface of one shell. Babies the same age but nearly twice as big crowd the other. 

The secret to growing those bigger babies? 

Crab pee.

The spat, or baby oysters, that have settled on the oyster shell on the left have been treated with the organic compounds homarine and trigonelline. The spat on the right have not been treated with the organic compounds. The research trainee and Ph.D. student is examining the effect of these “fear chemicals” on the growth of oysters. Credit: Emily Kenworthy/UGA Marine Extension and Georgia Sea Grant.

More specifically, it’s two compounds found in the urine of blue crabs — homarine and trigonelline.  They’re “fear molecules,” Roney said, chemicals that alert oysters that predators are nearby.  

They could help researchers grow stronger oyster beds to prevent erosion as sea levels rise. They could also be a boon to commercial oyster farmers wanting a stronger, more predator-resistant product.

Roney, a 23-year-old Georgia Tech Ph.D. student from Statesboro and a 2019 graduate of Georgia Southern University, is working out of the Shellfish Lab at the UGA Marine Extension Service and Georgia Sea Grant on Skidaway, which is helping to fund her research. It’s a hot, sunny day in late September as her oysters face their first field test. 

She fills plastic mesh bags with the shells on which “induced” oysters have settled. These are the bigger ones, exposed to the fear chemicals. She stuffs another four bags with controls, shells on which baby oysters have grown free of any hint of nearby predators. Four more bags get bleached oyster shells from a pile behind the building. Roney will use these to keep track of how many wild baby oysters, called spat, are recruited from the estuary.    

Sarah Roney bags oysters at the Shellfish Lab before placing them in the river. The research trainee and Ph.D. student is examining the effect of “fear chemicals” on the growth of oysters.

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

“And so one of the things that they noticed, way back a decade ago, is when you grow oysters near blue crabs —  just in the same water — the oysters are stronger. And so over time, through the process of elimination, they figured out that something is coming from these blue crabs to cause this response.”

That something was in the urine, which brings us to a wacky fact about crabs. They pee from their face. 

“So I don’t know if you’ve ever caught crab (while) fishing,” Roney said. “If you pull them up out of the water, it looks like they’re spitting. They get this foamy stuff. That’s actually not spit, it’s urine, because they pee out of their faces. And it’s a social signal in blue crabs. That’s how they assert dominance when they’re living in the water. Out of the water, it just foams up.”

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

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

Now that researchers  know which molecules are doing the work, they order those fear compounds from an aquaculture supply company.

 “And it’s relatively inexpensive,” she said. “So it’s a very practical way to create oysters that are more likely to survive when you put them out in the field. Because that’s kind of the end goal here ecologically. That’s why these oysters have this defense. When you have a thicker shell, you are less likely to be eaten.” 

Whether that defense lasts remains to be seen.

Watching the babies grow

That’s part of the point of putting the oysters out 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 expects to repeat the process in the spring.

She’s also sent some young oysters off to Alabama to be tested for strength with a penetrometer. 

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

Her research will also look at whether other oyster predators such as mud crabs and oyster drills and some fish emit the same fear chemicals as blue crabs. And she’ll be delving more deeply into whether the growth triggered by fear molecules is based on adding calcium carbonate to grow bigger or if it’s adding organic molecules and protein to their shells, making them thicker and stronger. 

With her oyster bags in a wheeled cooler, Roney pulls on her mud boots and heads to the nearby banks of the Skidaway River. 

Sarah Roney places her bagged oysters in the Skidaway River. The research on the effect of “fear chemicals” on the growth of oysters could lead better opportunities to create living shorelines in Georgia. Credit: Emily Kenworthy/UGA Marine Extension and Georgia Sea Grant.

Tom Bliss, director of the shellfish research lab at the Marine Extension and Georgia Sea Grant  is eager to see Roney’s results. 

In Georgia, oyster reef restoration typically relies on natural recruitment. The process involves putting shell or other material into tidal water and waiting for tiny oysters to settle there. That works pretty well here, much better than in Gulf where natural recruitment is much iffier. But it does limit when these restorations can happen, because the oyster larvae are only in the  water from about March through June. 

Results could extend oyster harvest season, strengthen shoreline

Bliss is getting ready to start a grant looking at the use of spat on shell — shells that already have oyster larvae growing on them —  for reef restoration. 

“If she sees an increase in survival using this method, is that something that will translate into us into being able to get shell with spat on it in areas that are spat-limited,” he said. 

Instead of putting the shell out from March through June, the season could be extended through September or October, he said. 

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

“Hopefully, we can create living shorelines that are more successful and increase the amount of living shorelines here in Georgia that are being used as opposed to sea walls, or groins or other way kind of wave breaking mechanisms and coastal engineering options,” Roney said. 

As Roney staked her oyster bags into the mud just below the marsh grass, two big blue crabs scuttled across the bank near the dock, as if they were hurrying to a meal. 

Roney will be back periodically to see where these crabs and other predators have been dining most frequently.