Blue crabs tolerate hypoxic conditions

"The notion that blue crabs are relatively intolerant of oxygen-poor waters was counterintuitive"

Adult blue crabs are “much more tolerant” of low-oxygen, hypoxic conditions that previously thought, but global warming could change things, according to new research by the Virginia Institute of Marine Science.

The findings - based on experiments using high-tech, computer-controlled respirometers - contradict earlier studies of the crustacean’s ability to function in oxygen-poor waters. The researchers undertook the study in light of concerns about decreasing levels of oxygen in coastal waters worldwide, and how the growing prevalence of low-oxygen “dead zones” might affect populations and management of blue crabs and other coastal marine life.

“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.

According to the findings, increase in dead zones is down to excess inputs of nitrogen from fertilizers, sewage, and other human sources.

“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,” added Mr Brill.

The results reveal blue crabs are “oxygen regulators” that can maintain a constant rate of aerobic metabolism until they reach a critical oxygen level. The critical oxygen level varied with the metabolic state of the tested crabs – at 62°F and 72°F, the critical oxygen level was less than 2 milligrams of oxygen per litre of water; this increased to between 2 and 3 milligrams per litre at 82°F, and when testing recently fed and infected crabs.

The results show that blue crabs can - at moderate water temperatures - survive at oxygen levels as low as 1.3 milligrams per litre, just 15% of the oxygen available in fully saturated seawater.

Researchers are now concerned what happens to the crabs’ critical oxygen level as water temperatures continue to rise with global warming.

“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,” Mr Brill concluded.

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