Without the benefit of medication, the only thing that most creatures can depend on for protection from infection is their immune systems. Fighting off pathogens and parasites, the immune systems of vertebrates are remarkably versatile; however, it wasn't clear how much of a toll fighting some infections takes. Dave Matthews from Harvard University, USA, explains that threespine sticklebacks (Gasterosteus aculeatus) are often infected with a tapeworm, Schistocephalus solidus, which slows the fish down, making them more likely to be caught by fish-eating birds, the tapeworm's next destination. But the stickleback's immune system fights back by scarring the digestive system to entrap the parasite and prevent it from taking hold. Yet, some stickleback populations actively prevent the immune system from developing protective scaring, leaving Matthews and his colleagues to wonder why some sticklebacks would rather put up with the ill effects of a parasite infection. If the fish's bodies become too stiff as result of the internal scarring, impairing their ability to escape from hungry predators, might the cost of fighting the infection be too high? Matthews, Daniel Bolnick (University of Connecticut, USA), George Lauder (Harvard University, USA) and colleagues decided to find out.
‘We needed to cause scarring in some fish without all the other consequences of having a parasite’, says Matthews, so Meghan Maciejewski (University of Illinois at Urbana-Champaign, USA) injected the animals with a solution of aluminium salt to produce the internal scarring usually associated with the parasitic infections, but with none of the additional complications. Two weeks later, the team tested the fish's reflexes by dropping a plate into a tank of water so that it just contacted the surface, causing a pressure wave that petrified the fish into curling into a tight C-shape and beating a hasty retreat. Would the fish's internal scarring hinder their ability to flee?
The team was surprised when they realised that the scarred fish were actually better at escaping the attentions of hungry birds than fish with no scarring; the scarred fish were able to flee 12.3% (10 cm s−1) faster than unscarred fish. And when Matthews and Greta Wong (Harvard University) checked the fish's bendiness, the scarred fish were relatively stiff, but were somehow able to take advantage of their inflexible bodies to trigger a speedy get away. In contrast, the least flexible unscarred fish were unable to capitalise on their stiffness during the final tail flick that drives their escape, leaving them vulnerable to attack. ‘Somehow, the scarring allows the fish to better escape their predators, despite the fact that it also caused increased stiffness’, says Matthews.
So how does the scarring caused by a parasitic infection help sticklebacks to escape their predators? The team suggests two possibilities. They suspect that the fish's body muscles may become stronger, to counteract their increased stiffness, allowing them to brace themselves to bend less when they need to escape. Alternatively, the fish may choose to bend less when beating a retreat because the internal scarring is uncomfortable, rather than directly increasing their stiffness.
Whatever the reason, the fish that embrace internal scarring caused by their immune systems for protection from tapeworm infections seem to have a head-start over the fish that actively suppress their immune systems, allowing them to become infected. Why some fish don't bother to protect themselves is still a mystery and Matthews says, ‘The functional implications of immune responses may be more complicated than we initially predicted’.
