The Mysterious Love Lives of Oysters

A Chesapeake Lab hatches nearly two billion oysters each year
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Chesapeake Bay oysters

Chesapeake Bay oysters

Everyone has a happy place. For me, it’s bellying up to a raw bar while an expert shucker loads up a rock salt-covered platter with plump, juicy oysters. Add in a sudsy India pale ale in a frosty mug and that’s perfection.

Despite my great appetite for oysters, it wasn’t until recently that I began to wonder about the creatures inside all those shells. How do they reproduce? How does oyster reef restoration happen? Where is this black magic performed? After a little research and connecting with an oyster hatchery, I discovered that oysters are indeed fascinating creatures—especially when it comes to their romantic encounters. But first, a bit of Chesapeake Bay oyster backstory.

Two hundred years ago, Chesapeake Bay oysters were plentiful. There were so many oysters, in fact, that they could all filter all the Bay’s water in about a week, according to the National Oceanic and Atmospheric Administration (NOAA). Today, NOAA reports that the Chesapeake’s wild oyster population has dwindled to about 1 percent of historic highs. The filtering that used to take a week now takes more than a year.

Despite those depressing figures, local, state and federal agencies have made significant progress restoring the Bay’s oyster populations by planting oysters and restoring oyster reefs in off-limits sanctuaries, bolstering funds for oyster research and hatchery facilities, and cutting bureaucratic red tape for individuals setting up oyster farms. I wanted to find out how they culture the oysters used in these efforts, so I got in touch with the University of Maryland Center for Environmental Science’s Horn Point Laboratory in Cambridge, Maryland, where as many as 1.78 billion oysters are born every year.

I arrived at the 800-acre facility on a beautiful, windy spring day and was quickly ushered from the lobby to the oyster lab by Oyster Hatchery Manager Stephanie Tobash Alexander. The entire operation is organized and clean. The facility is rimmed by lab desks and microscopes, computer monitors, 10,000-gallon fiberglass tanks, oyster- and water-filled holding tables and what looks like many miles of white PVC plumbing.

Alexander with broodstock oyster

Alexander with broodstock oyster

“The rain is giving us a lot of problems this year,” Alexander says. “The salinity of the river water we pump in to the lab is down to around 5 parts per thousand. It’s stressing the oysters, which means it’s been a struggle to get them spawning. We like to see 10 parts per thousand for spawning purposes.”

Alexander explains that oysters are relatively finicky animals, at least when it comes time for love-making. “In nature, oysters’ reproductive lives are largely tuned to water temperatures,” she says. “But if the water isn’t salty enough or there isn’t enough food, the oysters use available resources to stay alive, not produce eggs and sperm for spawning. All of this makes spawning oysters a challenge. “We can salt the water if we have to, or feed them more, but we’re almost always at the mercy of Mother Nature,” she adds.

The life cycle of an oyster is wildly interesting. In late summer and early fall, wild oysters are largely done with spawning and focus on glycogen production to fatten up for winter. Then, in spring, as water temperatures rise, oysters shift from storing energy to producing gametes, which are male and female reproductive cells. Once the water temperature reaches between 74 to 86 degrees, males start releasing sperm in the water. Scientists in the lab can raise water temperatures to mimic summer conditions, which “tricks” oysters into spawning in a controlled environment.

Alexander points to one of the water-filled tables where staffers are staring at oysters laid out neatly in rows like toy soldiers. “They’re sexing those oysters, which are called broodstock. They’re wild oysters we dredge up from various Bay locations and we’ve raised the water temperatures slowly to encourage them to spawn.” Alexander says. “When male oysters spawn, they release a steady stream of sperm from the side of their shell, which shows up pretty clearly on the black table background. Female oysters release eggs by ‘clapping’ their shells together, releasing a cloud of microscopic eggs into the water.”

As we’re all watching the table, I see an oyster sending sperm into the water. “That’s a male,” Alexander says, as an intern removes the oyster and puts it into a water-filled bucket. Next, I notice an oyster clapping its shell. “That’s a female—she’ll go in a separate bucket,” Alexander adds. She explains that the males and females are “allowed to continue their spawning, and when it’s all over they are removed, and the sperm- and egg-filled contents of each bucket are combined.” The spawned-out adult oysters are then returned to their cushy lives in the lab’s carefully controlled holding tables to be used again later.

Tangier watermen tend to floats filled with lab-born oysters. 

Tangier watermen tend to floats filled with lab-born oysters. 

Oyster larvae are then added to 10,000-gallon fiberglass tanks where they drift around in a soup of Choptank River water and a nutritious, lab-grown brew of green and brown algae that the larvae devour as they quickly grow. The feeding in the tanks is highly automated and precise, controlled from a multi-monitor computer workstation nearby.

After about two weeks, the oyster larvae develop a “foot” and begin crawling around looking for a place to cement themselves. “They’re called pedivelegers when they start pulling themselves around with that foot,” Alexander says. “That’s when we have to get them on a substrate, or they will start gluing themselves to the sides of the tank. They’re called ‘spat’ when they’ve glued themselves down. In the wild, they normally settle on another oyster shell. Here we have two substrates for them to use.”

Some larval oysters are given microculture to settle on (normally finely ground oyster shell), while others are put in large tanks filled with whole, clean oyster shells. The oysters cultured on fine shell are often called seed oysters, while the ones that are settled on oyster shell are classified as “spat on shell.”

Spat on shell are often used in oyster restoration projects, while seed oysters, which resemble dark sand when they are small, are often sold to aquaculture (farming) outfits around the Chesapeake Bay. “In a good year, we can produce just under 2 billion oysters,” Alexander says. “Once the oyster is glued down, it’s there for good. It takes about three years for an oyster to fully develop
reproductively, growing about an inch a year, and they are often harvest size around that time, too.”

There’s much more to an oyster’s life than reproducing, and I guarantee you’ll thank me for not delving into the world of diploid, triploid and tetraploid oysters. Trust me. But maybe your next oyster slurp will be accompanied by a newfound appreciation for these crafty little mollusks. Pass the cocktail sauce while you’re at it. 

This article originally appeared in the August 2019 issue.