Oysters deprived of a sex life


The million-oyster march is nearing completion at the Virginia Institute of Marine Science.

Sometime next month, 1 million Asian oysters will be ready to be raised by Chesapeake Bay watermen and then sold for consumption - the first large-scale test of the growth and market potential for this non-native species in the bay.

But these foreign oysters will differ from their native cousins in one crucial respect: They won't have the ability to reproduce. And that characteristic is reassuring even the most cautious environmental watchdogs.

Acutely aware of the dangers foreign plants and animals can pose when they're introduced unchecked into new environments, scientists say it's crucial for the Asian oyster, Crassostrea ariakensis, to be tested in such a way that it won't start breeding out of control.

They worry that it could bring a new disease or infection, or perhaps interfere with efforts to resuscitate the bay's native oysters, Crassostrea virginica.

Scientists point to countless examples of seemingly harmless invaders that have wreaked havoc after being let loose, including the mute swan and the gypsy moth. A type of Japanese oyster that was dropped into the bay in the 1960s is blamed for introducing one of two deadly infections that have decimated the Eastern oyster.

"It is impossible to really predict what would happen," said Dennis Hedgecock, a professor at the Bodega Marine Laboratory at the University of California, Davis. "Once you put a reproductive oyster in the bay, you have no control."

Hedgecock is co-chairman of a National Research Council committee that released a report last month calling for the continued controlled study of the Asian oyster in the bay.

Noting that the bay's native oyster harvest has been devastated by disease - from more than 2.5 million bushels per year in the 1980s to 53,000 bushels last season - the panel's report essentially endorsed the approach taken by Virginia for expanded experimentation with sterile Asian oysters.

Committee members said as much as five more years of study should be done before they would be confident that fertile Asian oysters could safely by tested in the bay, calling on Maryland officials to slow their ambitious plans to introduce breeding Asian oysters as soon as next summer.

So attention now turns to Virginia's production of hundreds of thousands of sterile oysters - a process that comes after almost three decades of research.

Tucked away in the hatchery at the VIMS Aquaculture Genetics and Breeding Technology Center in Gloucester Point, researcher Stan Allen and his team are growing Asian oyster batches in which fewer than one in 1,000 are able to reproduce.

"It's biology, not physics, so there are some glitches that happen biologically," Allen said. "We're doing what is humanly possible."

The key to making sterile oysters is creating a condition known as triploidy, or having three sets of chromosomes. Oysters are natural diploids, having two sets of chromosomes. One comes from the mother, the other from the father, and each set has 10 chromosomes. (Humans are also diploids, with two 23-chromosome sets.)

Triploid oysters look essentially the same as "normal" oysters. Allen said. However, the triploid condition means the oysters are sterile during the period they would normally reproduce, as the three sets of chromosomes interfere with their ability to produce gametes. The commercial benefit is that triploid oysters frequently grow faster and bigger, since the energy for reproduction gets diverted toward growth.

In the late 1970s and early 1980s, scientists began creating triploids by manipulating eggs very early in the growth process - generally applying a chemical to keep fertilized eggs from completing development. But that technique worked for only about 90 percent of a batch.

While Allen was at Rutgers University, he and a colleague - Ximing Guo - discovered a method of creating oysters with four sets of chromosomes, known as tetraploids. Unlike triploid oysters, the tetraploids are fertile. When they are mated with two-chromosome diploids, the product should, in theory, be 100 percent triploid.

The threshold for bay experiments is that no more than one in 1,000 of the triploid larvae should be able to reproduce. The first batch of 1 million produced by VIMS this summer was slightly over, with four in 3,000 found to be fertile.

Those were discarded, and testing of the second batch now being grown has shown only two fertile oysters per 3,000 tested.

In the meantime, a stock of tetraploid Asian oysters is kept in a hatchery pond to ensure more triploids can be produced.

When produced in the hatchery, sperm from tetraploids are typically used to fertilize the eggs of diploids.

After being raised for about 10 to 17 days in large tanks, the larvae - each about a third of a millimeter long and known as spat - are set on shell chips that are just about the same size. That ensures there's just one larva per shell chip.

After being allowed to set for a couple of days, the oysters are given another week of nutrients to let them grow to a size of about 1 to 2 millimeters

At that point, the oysters' demand for food exceeds what the hatchery can produce, and they're transferred to devices called upwellers that bring in raw water full of the bay's food and nutrients. The water is treated before being released.

While hatchery-grown oysters are often put in the bay after reaching 2 or 3 millimeters, this million-sample batch of Asian oysters is being grown to 20 millimeters, about three-quarters of an inch.

As the first group of 100,000 or so reach the target size, they'll be divided among 10 Virginia oystermen who have agreed to try growing them - secured in containers to ensure none of the Asian oysters is accidentally released. After several years, some triploid oysters are known to revert to fertile form.

About 5,000 of the oysters also will be headed to Maryland, where they'll be evaluated for suitability in the upper bay - an area that has lower salinity and greater sediment than the Virginia waters, said Kennedy T. Paynter, a researcher at the University of Maryland Center for Environmental Science Chesapeake Biological Laboratory.

Once permit applications are approved, the triploid Asian oysters will be grown side-by-side with triploid native ones in mesh cages in three sites off Maryland shores, as well as one Virginia location as a control.

Over two years, they will be sampled monthly for disease and growth, as well as checked for any signs that they're reverting to a fertile state.

"This is just a first step in trying to acquire a fundamental base of data so that we can make an intelligent decision about fertile introduction," Paynter sad.

"The last thing we want to do is accidentally introduce fertile oysters ourselves. That's why these triploid oysters are so important to our work."

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