Tests show that the foreign oyster, Crassostrea ariakensis dramatically outperforms the native Crassostrea virginica in its own habitat. When bags of each oyster are placed separately in the Bay, ariakensis grows up to twice the size of virginica in the same amount of time.
But that doesn’t necessarily mean virginica would easily be brushed aside if ariakensis got loose in the Bay. When it comes to head-to-head competition, there is a preliminary indication that it is virginica that has the upper hand.
In tests conducted at a quarantined laboratory, Virginia Institute of Marine Science researchers placed tiny larvae from the two oysters on a series of 4-by-4 inch ceramic tiles, then watched them grow for 12 weeks. At the end of the experiment — to the shock of researchers — the virginica oysters were larger, and their survival rate greater, than ariakensis.
“Very much to my surprise, it was the opposite of what I thought,” said Mark Luckenbach, the scientist conducting the study. He cautioned that the conclusions were “very preliminary,” and that the results so unexpected that he plans to repeat the experiment this fall.
But Luckenbach said his tentative interpretation of the results is that virginica is a better reef-building oyster than ariakensis. In situations where the two oysters physically encountered each other, virginica responded by growing up, whereas ariakensis remained mostly flat.
“Where we were able to observe actual head-to-head, one-on-one competition between individual oysters of different species, neither won most of the time because of the short duration of the experiment,” Luckenbach said. “But when one did win, it was almost always virginica.”
Little is known about the natural history of ariakensis.
Stan Allen, the VIMS scientist who has been leading much of the work on the species, has traveled to Japan and China to collect ariakensis specimens. On those trips, Allen said, he never found evidence of ariakensis building reefs — but much of their habitat was either inaccessible, or had been heavily impacted by humans. “When they’ve been collected, they’ve been scattered about,” he said.
Historically, the ability of virginica to build reefs gave it many advantages for living in the Chesapeake. By forming high reefs which sometimes broke the surface of the water, the oysters were able to outpace the natural rate of sedimentation covering river bottoms.
Also, by getting off the bottom — where oxygen levels were often low even before European settlement — the oysters were able to take advantage of better water quality. The reefs also provided three-dimensional habitat that was used by a host of fish and other Bay species.
So, while ariakensis may grow well in aquaculture, it might make a poor replacement for the native oyster in the wild.
“Those that would hope ariakensis has some value in restoring the wild fishery would be pretty disappointed if that were the case,” Luckenbach said.
On the other had, he said it may also mean there would be less concern about its impact on virginica if it were to escape from aquaculture operations.