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Fish infected with parasitic worms. (Photo: University of Leicester)

Global warming benefits parasites, disadvantages fish: study

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Tuesday, December 06, 2011, 04:20 (GMT + 9)

Scientists from the University of Leicester’s Department of Biology have found that higher water temperatures allow parasitic worms that infect fish to grow four times faster and influence the fish to prefer warmer temperatures. This study is one of the first to prove that global warming affects how parasites and their hosts interact.

The results, supported by funding from the Biotechnology and Biological Sciences Research Council (BBSRC) and the Centre for Environment, Fisheries and Aquaculture Science (Cefas), were published in the journal Global Change Biology.

The researchers saw behavioural changes in infected fish, suggesting that parasites may manipulate host behaviour to make them move to warmer temperatures, where the host's growth rate slowed.

"What we witnessed was that fish infected with the largest worms showed a preference for warmer water, suggesting that these parasites also manipulate the behaviour of host fish in ways that benefit the parasites by maximizing their growth rates," said Dr Iain Barber, who carried out the study with doctoral student Vicki Macnab.

Macnab noted that the size these parasites attain in their fish hosts determines how severely fish reproduction is affected, such that, according to the study, parasites will have a more serious effect on fish reproduction if temperatures rise.

"This research shows that global warming could shift the balance between parasites and their hosts with potentially serious implications for fish populations," she stated.

Parasitic worms infecting stickleback fish grew four times faster in waters of 20 °C compared to 15 °C, and the fish grew more slowly, suggesting that fish parasites handle higher temperatures much better than the fish they infect.

"The results are important because the size these parasites attain in their fish hosts also determines their infectivity to fish-eating birds like kingfishers and herons – the next hosts in the parasite's life cycle – and also the number of parasite eggs that they will go on to produce. Bigger larval parasites in the fish go on to become larger adult worms in birds, which produce more eggs,” Barber said.

Barber pointed out that after the eight weeks of the study, all of the worms held at 20 °C were mature enough to infect fish-eating birds, whereas none of those held at 15 °C had reached that size.

Moreover, a follow-up study showed that fish infected with the largest worms showed a preference for warmer water -- suggesting that these parasites also maneuver the behaviour of host fish to benefit the parasites and maximize their growth rates.

The results provide some of the first evidence that increasing environmental temperatures can accelerate the speed at which parasites complete their life cycles, which could lead to a higher overall level of parasitism in natural animal populations.

By Natalia Real


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