Blue Crabs Tolerate Low Oxygen, Still at Risk to Warming Temperatures

In 2015, the Gulf of Mexico dead zone—an area of water so low or absent in oxygen that it threatens the area’s marine life—sprawled outwards along the coasts of Louisiana and Texas, measuring 6,474 square miles. According to the National Oceanic and Atmospheric Administration (NOAA), the Gulf of Mexico boasts the second largest human-caused dead zone in the world.

With more than 550 dead zones occurring annually worldwide, scientists are attempting to discover the long-term effects dead zones will have on local species.

Bucking previous research, scientists from the Virginia Institute of Marine Science found adult blue crabs are more resilient to low-oxygen water levels 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,” said Rich Brill, a fishery biologist with NOAA’s National Marine Fisheries Service  

Excessive nutrients, such as nitrogen and phosphorous, and waterbody stratification prompt algal growth in dead zones. “As dead algae decompose, oxygen is consumed in the process, resulting in low levels of oxygen in the water,” according to the Environmental Protection Agency (EPA). Dead zone increases are attributed to nitrogen from fertilizers and sewage, among other human resources.

“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,” said Brill.


The study, which appeared in the Journal of Experimental Marine Biology and Ecology, measured an individual crab’s oxygen usage every second, achieved by an array of fiber optic and fluorescent sensors. Called “automated intermittent-flow respirometry,” the technique collected data around the clock for several days. Time was allotted to ensure crabs, which came from tidal creeks near the laboratory facility in Wachapreague, Va., recovered from transfer and handling effects before low-oxygen experiments started.

The researchers found blue crabs are “oxygen regulators,” capable of maintaining a steady rate of aerobic metabolism until reaching critical oxygen level. At 62 F and 72 F, critical oxygen levels were less than 2 mg per liter of water. At 82 F, the levels increased to between 2 and 3 mg/L. Researchers were capable of altering the metabolic state of the crabs by increasing water temperature, feeding the crab and using crabs infected with a parasite known to increase metabolism. Fed and infected individuals had metabolic rates on par with the unfed crabs at 82 F.

The results “support the idea that blue crabs are well adapted to the hypoxic conditions occurring in the estuarine environments they occupy,” Brill said.

The crabs are capable of surviving oxygen levels as low as 1.3 mg/L at moderate temperatures, according to the researchers.

While the blue crabs may be tougher than previously thought, increasing water temperatures are still of concern. The Chesapeake Bay Program Science and Technical Advisory Committee reports that by 2070, water temperatures are expected to rise between 3.5 and 9 F.

“Metabolic rates of blue crabs increase with increasing temperature, and this in turn increase the lowest oxygen levels they can survive,” said Brill. Warming “will exacerbate the effects of hypoxia on blue crabs, as it will with almost all other organisms.”