Adult blue crabs have adapted to waters with lower oxygen concentrations, according to researchers from William and Mary's Virginia Institute of Marine Science (VIMS). Such conditions, also known as hypoxic waters, are associated with increasing nitrogen levels, but researchers have found that blue crabs may be more resilient than previously thought. 

"The notion that blue crabs are relatively intolerant of oxygen-poor waters was counterintuitive because this species often occupies estuarine environments that can become hypoxic even in the absence of human activities," Rich Brill, lead author and an adjunct faculty at VIMS, explained in a news release.

The researchers sought to explain how low-oxygen "dead-zones" in coastal waters worldwide were affecting marine species. Low-oxygen areas result from increased nitrogen concentrations contaminating ocean waters. Essentially, oxygen is absorbed by bacteria feeding off nitrogen-fueled algal blooms that have died and sunk to the bottom.

"Because coastal hypoxia can significantly impact the movements, distribution, growth, and reproduction of inshore fish and invertebrate species, understanding their ability to tolerate hypoxia is becoming crucial; especially in species of ecological and commercial importance," Brill said.

Using a respirometer, the researchers measured the rate at which blue crabs were consuming oxygen in a closed container. From their study, the researchers concluded that blue crabs are oxygen regulators, which means they consume oxygen at a constant rate, until the moment a critical oxygen level is reached.

"Our results are consistent with the oxygen levels shown to influence blue crab behaviors in both field and laboratory settings," Brill explained in a statement, and "support the idea that blue crabs are well adapted to the hypoxic conditions occurring in the estuarine environments they occupy."

Overall, researchers found that at moderate temperatures, blue crabs can survive in waters that have oxygen concentrations as low as 1.3 milligrams per liter, which is equal to roughly 15 percent of the oxygen available in fully saturated seawater.

Concerns remain about how blue crabs will respond to water temperatures that continue to rise in the Chesapeake Bay. Since 1960, water temperatures have risen 1.5 to 2.5 degrees Fahrenheit and they are expected to rise even more by 2070.

"Our data show that the metabolic rates of blue crabs increase with increasing temperature, and this in turn increases the lowest oxygen levels they can survive. So warming of the Bay will exacerbate the effects of hypoxia on blue crabs, as it will with almost all other organisms," Brill added. 

The study's findings were recently published in the Journal of Experimental Marine Biology and Ecology

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