Does global warming affect worms?

Global warming favors fish parasites

Global warming could deeply affect the delicate balance between animals and their parasites. British researchers have now found this in a study on fish. In warmer water, the parasitic worms in sticklebacks grew four times faster and more strongly than in colder water. The larger parasites also manipulated the behavior of their host fish: They caused the sticklebacks to stay mainly in the warmest areas of the water. The scientists report in the specialist magazine "Global Change Biology" that this creates positive feedback that enables the parasites to mature even more quickly and thus also to spread.

"Our study shows that global warming can alter the balance between parasites and their hosts," say Vicki Macnab and Ian Barber from the University of Leicester in England. This could have serious consequences for fish populations and thus also for fisheries.

Reproduction and growth of the fish are disturbed

The size of the parasitic worms affects how well the fish thrive and also how strongly they reproduce. Fish infected by large worms stay small and lay fewer spawn. “Our results indicate that the parasites could slow the reproduction of the fish more strongly in the future if the temperatures continue to rise,” says Macnab. This could result in significant losses for fish farms, but also for the fishing of wild species.

According to the researchers, the changes they have now observed in parasite growth and fish behavior are the first evidence that climate change is also changing parasite-host relationships. If the warming continues, this could even mean that wild animals will generally suffer more from parasite infestation in the future.

Three infected sticklebacks © University of Leicester

Warmer water encourages parasite growth and maturation

For their study, the researchers infected sticklebacks with worm larvae in laboratory aquariums. Some of the fish were then kept in water at 20 degrees Celsius, others at 15 degrees. In the warmer basins, the fish grew significantly more slowly, the scientists report. The worms would have grown in size four times faster than those in the sticklebacks that were kept in colder water.

At the same time, the warmer water also shortened the maturation time of the parasites: "After eight weeks, all the worms in the fish kept at 20 degrees were ready to be transmitted further," say the researchers. In the colder basin, on the other hand, none of the worms had reached this stage of development at this point in time. This result is important because it shows that the risk of the parasites spreading increases with the temperature.

Parasites influence the behavior of their hosts

A comparison of healthy and infected fish showed that the parasitic worms of the sticklebacks apparently also actively contribute to their well-being: Contrary to their usual habits, the sticklebacks infected with the largest worms specifically went to warmer water areas in the basin, the scientists report. "This suggests that the parasites are influencing the behavior of their hosts in order to optimize their growth," says Barber. (Global Change Biology, 2011; doi: 10.1111 / j.1365-2486.2011.02595.x)

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(Global Change Biology / dapd, 05.12.2011 - NPO)

5th December 2011