Blue Crabs Can Withstand Low Oxygen Levels, Contrary To Previous Studies

New findings published in the Journal of Experimental Marine Biology and Ecology contradict previous findings showing that adult blue crabs are more tolerant of low oxygen, otherwise known as "hypoxic" conditions.

The study results are based off of high-tech computer-controlled respirometers that examine the crustacean's ability to function in oxygen-poor waters, helping to explain what had been somewhat of an ecological mystery.

"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 study author Rich Brill, a fishery biologist with NOAA's National Marine Fisheries Service and adjunct faculty at VIMS, in a news release.

Researchers took on the study in light of concerns regarding decreasing levels of oxygen in coastal waters worldwide and how these might affect the growing prevalence of low-oxygen "dead zones" in populations and the management of blue crabs, as well as other coastal marine life.
Scientists blame excess inputs of nitrogen from fertilizers, sewage and other human sources for the increase in dead zones. When the algae in nitrogen-fueled algal blooms die and sink, they provide a rich food source for bacteria, which in the act of decomposition take up dissolved oxygen from nearby waters.

"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 Brill. "The older studies took place prior to the easy availability of personal computers and were thus of limited duration. The instruments were set up and the data recorded with pencil and paper during 'business hours'."

In this recent study, researchers used automated intermittent-flow respirometry, placing the crabs in a second container so that their oxygen usage was measured every second via fiberoptics and fluorescent sensors.

Study results revealed that, contrary to previous studies, blue crabs are "oxygen regulartors" that can maintain a constant rage of aerobic metabolism until they reach a critical oxygen level. Previous studies have also suggested that blue crabs were "oxygen conformers" whose metabolic rate fell in concert with reductions to oxygen concentration of surrounding waters.

The critical oxygen level varied based on metabolic state of the tested crabs, which researchers raised by increasing the water temperature and feeding a crab a meal of menhaden or using crabs infected with a prasite that's known to boost metabolism. At 62°F and 72°F, the critical oxygen level was less than 2 milligrams of oxygen per liter of water; this increased to between 2 and 3 milligrams per liter at 82°F, and when testing recently fed and infected crabs.

"Our results are consistent with the oxygen levels shown to influence blue crab behaviors in both field and laboratory settings," says Brill, and "support the idea that blue crabs are well adapted to the hypoxic conditions occurring in the estuarine environments they occupy." The results show that blue crabs can--at moderate water temperatures--survive at oxygen levels as low as 1.3 milligrams per liter, just 15% of the oxygen available in fully saturated seawater.

Study results revealed that the metabolic rate of tested blue crabs doubled when going from 62°F to 72°F, and increased almost 6-fold with another 10°F rise in water temperature from 72°F to 82°F. The metabolic rates of fed individuals, and those infected with the common blue crab parasite Hematodinium perezii, were both more than double those of unfed, uninfected individuals at 72°F. Fed crabs at 82°F exhibited a further 60% increase in metabolic rate from that of unfed individuals measured at the same temperature.

However, research caution what happens to the crabs critical oxygen levels when water temperatures in Chesapeake Bay rise with global warming. Bay temperatures have already risen 1.5 to 2.5°F since 1960, and are projected to rise another 3.5° to 9°F by as early as 2070, according to researchers.

"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," says Brill.

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