New research suggests adult blue crabs are surprisingly tolerant of low-oxygen (hypoxic) conditions, contradicting earlier studies on the crustacean's ability to function in oxygen-poor waters.
A team of scientists used high-tech computer-controlled respirometers to help explain what they describe as an "ecological mystery," the William & Mary's Virginia Institute of Marine Science reported.
"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 lead author Rich Brill, a fishery biologist with NOAA's National Marine Fisheries Service and adjunct faculty at VIMS.
Concerns have been raised about decreasing levels of oxygen in coastal waters across the globe, and how this phenomenon might affect vulnerable marine wildlife including blue crabs. These dead zones are linked to excess inputs of nitrogen from fertilizers and other human activities. The added nitrogen causes algal blooms to act as a plentiful food source for bacteria, which in turn deplete the water's oxygen levels through decomposition.
"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.
The new studies improved on past methods of monitoring the crabs' ability to thrive in hypoxic waters by allowing the scientists to automate the entire data-recording system so they could observe the crabs around the clock with minimal human intervention. The findings showed that contrary to previous studies, the crabs were able to maintain a constant rate of aerobic metabolism until a critical oxygen level was reached. Past findings had suggested blue crabs are "oxygen conformers," and their metabolic rate fell when environmental oxygen levels were reduced.
The new findings reveal blue crabs can survive in oxygen levels as low as 1.3 milligrams per liter at moderate temperatures, which is only about 15 percent of the oxygen levels seen in regular saturated water. The problem is the crabs may not be as viable in low-oxygen waters if temperatures continue to rise. Water temperatures in the Chesapeake Bay have risen by between 1.5 and 2.5 degrees Fahrenheit since 1960, and are predicted to rise by as much as 9 degrees 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 warned.
The findings were published in a recent edition of the Journal of Experimental Marine Biology and Ecology.
