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Global warming changes balance between parasite and host in fish -- new study

05.12.2011
Worms infecting fish grow 4 times faster at higher temperatures and manipulate the behavior of fish

Parasitic worms that infect fish, and have a devastating effect on fish reproduction, grow four times faster at higher temperatures – providing some of the first evidence that global warming affects the interactions between parasites and their hosts.

The study from the University of Leicester revealed that global warming had the potential to change the balance between parasite and host - with potentially serious implications for fish populations.

The researchers from the University of Leicester's Department of Biology also observed behavioural change in infected fish – suggesting parasites may manipulate host behaviour to make them seek out warmer temperatures.

And they discovered that whilst parasites grew faster in higher temperatures, 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 of the Department of Biology at the University of Leicester, who carried out the study with doctoral student Vicki Macnab.

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

Vicki said: "The research shows a dramatic effect of increased environmental temperatures on the growth rates of parasites in fish hosts. The size these parasites attain in their fish hosts determines how severely fish reproduction is affected, so our results suggest that parasite will have a more serious effect on fish reproduction if temperatures rise. In addition, our paper documents behavioural changes in infected fish that suggests the parasites are manipulating host behaviour to make them seek out warmer temperatures, creating a positive feedback mechanism to exacerbate the effects of global warming.

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

The scientists found that parasitic worms infecting stickleback fish grew four times faster in experimentally infected sticklebacks raised at 20°C than when raised at 15°C.

In contrast, the fish grew more slowly at the higher temperature, suggesting that fish parasites cope with higher temperatures much better than the fish they infect.

Dr Barber said: "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.

"After the 8 weeks of the study, all of the worms infecting the fish held at 20C were ready to infect fish-eating birds, whereas none of those held at the lower temperature had reached a size at which they were ready to be transmitted."

In a follow up study, the authors also showed 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 and maximize their growth rates.

The results provide some of the first evidence that increasing environmental temperatures can lead to a shift in the delicate balance that exists between co-evolved hosts and parasites, increasing the speed with which parasites complete their life cycles that could lead to an increase in the overall level of parasitism in natural animal populations.

For more information, please contact:

Dr Iain Barber
Head of Department
Department of Biology
College of Medicine, Biological Sciences and Psychology
University of Leicester
Tel: +44(0)116 252 3462
email: ib50@le.ac.uk
Reference: 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 in press. DOI: 10.1111/j.1365-2486.2011.02595.x

http://doi.wiley.com/10.1111/j.1365-2486.2011.02595.x

Affiliations: Vicki Macnab is a PhD student funded by the BBSRC and Cefas; Dr. Iain Barber is senior lecturer and Head of the Department of Biology at the University of Leicester.

About the BBSRC

BBSRC invests in world-class bioscience research and training on behalf of the UK public. Our aim is to further scientific knowledge to promote economic growth, wealth and job creation and to improve quality of life in the UK and beyond.

Funded by Government, and with an annual budget of around £445M, we support research and training in universities and strategically funded institutes. BBSRC research and the people we fund are helping society to meet major challenges, including food security, green energy and healthier, longer lives. Our investments underpin important UK economic sectors, such as farming, food, industrial biotechnology and pharmaceuticals.

For more information about BBSRC, our science and our impact see: http://www.bbsrc.ac.uk

About Cefas

Cefas is an internationally renowned scientific research and advisory establishment. Operating as an executive agency of the Department for Environment, Food and Rural Affairs (Defra), it has been investigating marine and aquatic environments since 1902. It has two main laboratories, in Lowestoft and Weymouth, and works out of a number of port offices around the English coastline.

Cefas works alongside government and other agencies, both in the UK and internationally, to play a vital role in securing healthy marine and freshwater environments for everyone's well-being, health and prosperity. For more about its work and range of applied marine science, visit: www.cefas.defra.gov.uk

About the Journal

Global Change Biology exists to promote understanding of the interface between all aspects of current environmental change that affects a substantial part of the globe and biological systems. Studies must concern biological systems, regardless of whether they are aquatic or terrestrial, and managed or natural environments. Both biological responses and feedbacks to change are included, and may be considered at any level of organization from molecular to biome. Studies may employ theoretical, modeling, analytical, experimental, observational, and historical approaches and should be exploratory rather than confirmatory. GCB publishes primary research articles, technical advances, research reviews, commentaries and letters.

Journal URL: http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1365-2486

Dr. Iain Barber | EurekAlert!
Further information:
http://www.le.ac.uk

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