As the current swine flu pandemic attests, infectious diseases are a fact of life for all creatures. Atlantic blue crabs are no exception and play host to numerous bacteria. Lou Burnett, a physiologist at the Grice Marine Laboratory in Charleston, USA, explains that crabs fighting infections suffer from the crab equivalent of a stuffy nose. Bacteria mixed with hemocytes– crabs' white blood cells – form clumps that get trapped in the gills, become incorporated into the crabs' exoskeleton and are eventually sloughed off during the next molt. It's good that gills can help crabs get rid of bacteria, but this comes at the cost of interfering with respiration,'Burnett says, as the clogged-up gills can make it difficult for crabs to get enough oxygen. He decided to investigate whether crabs fighting a bacterial infection can still cope with increased physical activity without getting out of breath (p. 3428).

To investigate how crabs are affected by infections, Burnett teamed up with Lindy Thibodeaux and his wife, immunologist Karen Burnett. First, they caught Atlantic blue crabs in Charleston Harbor, took them back to the lab, and screened them for infections to identify healthy crabs. To create a port to infect the animals, they drilled a tiny hole in each crab's carapace and sealed it with rubber. After injecting the crabs with either saline or a sub-lethal dose of bacteria they waited an hour for the infection to take hold. The team then placed the crabs on a treadmill, where they scurried along at a speed of 8 m min–1 for half an hour.

With the gills of the infected crabs getting clogged up with bacteria, how did the crabs cope with exercise? By placing the treadmill in a flow-through respirometry chamber, the team was able to measure how much oxygen the crabs used before, during and after their brisk walk. To their surprise, they found that a crab's oxygen uptake plummeted by 30% to 40% shortly after receiving a bacterial injection, suggesting that there is a dramatic drop in metabolic rate when an immune response is launched to clear an infection from the crab's system. This bacterially induced metabolic depression was totally unexpected,' says Burnett.

The team expected crabs fighting off an infection to suffer from impaired oxygen delivery from their gills, causing infected crabs to switch to anaerobic respiration during exercise. To investigate this, they measured lactic acid build up by taking blood samples from the crabs before, during and after their bout of treadmill exercise. But the crabs surprised them once again: they weren't relying on anaerobic respiration. We didn't see higher lactate levels in the infected crabs' hemolymph,' Burnett says.

Do healthy and infected crabs fuel their exercise differently? To find out,the team flash-froze crabs in liquid nitrogen, pulverised the frozen crab carcasses to powder, and measured the levels of metabolites important to energetics in the pulverised crab tissue. Yet again, they found no differences between infected crabs and their healthy counterparts. The only difference was that infected crabs had metabolic depression before the exercise, which persisted during exercise,' Burnett says. But they were still able to fuel their muscle movements in the same way as healthy crabs.'

Burnett notes that they put the crabs through their paces in well-oxygenated water, and that crabs are likely to experience tougher conditions in their natural environment. We suspect that the crabs won't perform as well in real-world conditions,' he says. `We're now planning to test their resilience at different temperatures and oxygen levels.'

Thibodeaux, L. K., Burnett, K. G. and Burnett, L. E.(
2009
). Energy metabolism and metabolic depression during exercise in Callinectes sapidus, the Atlantic blue crab: effects of the bacterial pathogen Vibrio campbellii.
J. Exp. Biol.
212
,
3428
-3439.