Bird tapeworms (Schistocephalus solidus) have three distinct life stages. First, they infect copepods (tiny crustaceans), such as Cyclops strenuus abyssorum. The copepods are eaten by sticklebacks – in this case, the three-spined stickleback, Gasterosteus aculeatus. The sticklebacks are then eaten a bird, in which they breed and produce eggs with which to infect the next generation of copepods.
In order to be infectious to a bird, the tapeworm larvae must grow to a size of at least 50mg. That being said, the bigger the better – larger parasites are far more fertile, producing many times more eggs – which are also larger. Larger parasites also make their hosts less able to breed and more likely to be eaten by a bird.
Parasites infecting organisms which do not control their own body temperatures (such as most fish) are more likely to be directly affected by climate change – a parasite infecting a warm-blooded mammal, for example, can rely on a temperature-controlled living space. To test what impact temperature would have on how infective the tapeworms were, Macnab and Barber (2011) kept two populations at different, static temperatures, within their normal temperature range – 15°C and 20°C respectively – and fed half of each population infected copepods (the others got non-infected copepods).
Temperature, they found, did not affect the likelihood that a fish eating an infected copepod would be infected – in both cases, about half of the exposed fish were infected. However, they found that the tapeworm larvae grew much faster in the warm-water group. 8 weeks in, every tapeworm larvae in the warm-water group had reached the 50mg size necessary to infect a bird – whereas none of the larvae infecting the cooler group had. In fact, in the warmer population, the average size of the tapeworms was twice the size they needed to infect a bird. They estimate that this difference would allow each parasite to produce at least an order of magnitude more eggs than in the 15°C group – almost 200,000 eggs each as compared to 12,000.
They also showed that once infected, the fish with infective worms preferred warmer water. A different population of infected and non-infected sticklebacks were introduced to an aquarium with cooler (~15°C) and warmer (~21°C) compartments, with an intermediate-temperature (~18°C) linking chamber. The fish were then allowed to settle in the intermediate chamber and watched for three hours.
The non-infected fish, as well as those with parasites too small to infect a bird, tended to stay in the intermediate chamber. However, fish with large, infective parasites preferred warmer waters, with a thermal preference over 1°C warmer than the other groups.
Although such a pattern might be perhaps be explained by an attempt on the part of the sticklebacks to increase the effectiveness of their immune system, the authors suggest that the tendency of fish bearing larger-but-noninfective parasites towards lower temperatures is more likely motivated by the tapeworms. Larger parasites would have increased energy demands, increasing the likelihood that the host would starve – and the parasites with it. When the parasites are large enough to infect a bird, however, all bets are off – the priority is to get large and to get eaten.
Previous studies on these species, such as Barber et. al. (2004), have found that, once a stickleback was infected by a sufficiently large parasite, the parasite would impair the fish’s abilities to flee predators. Fish infected by such parasites were less likely to make any evasive behaviour, less likely to reach cover, less likely to perform “staggered dashes” to prevent a predator from anticipating where they would move next and more likely to try and “evade” predation by simply slowly swimming away.
Fish that prefer warmer waters are probably going to end up at the surface and at the edges of lakes – right where they’d be more vulnerable to bird attacks. There is also potential for a positive feedback relationship – fish infected by larger parasites prefer warmer waters in which the parasites grow faster and the fish are more likely to be consumed by birds. It seems that one beneficiary of a warming climate is the tapeworms.
References
Macnab, V., & Barber, I. (2011). Some (worms) like it hot: fish parasites grow faster in warmer water, and alter host thermal preferences Global Change Biology DOI: 10.1111/j.1365-2486.2011.02595.x
Barber, I., Walker, P., & Svensson, P. (2004). Behavioural Responses to Simulated Avian Predation in Female Three Spined Sticklebacks: The Effect of Experimental Schistocephalus Solidus. Infections Behaviour, 141 (11), 1425-1440 DOI: 10.1163/1568539042948231 [PDF]
Other coverage
Some like it hot (if they’re riddled with parasites) – Not Exactly Rocket Science. Be sure to check out the comments, one of the authors has added information.
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