Parasitic worms that infect fish and have a devastating effect on fish reproduction, grow four times faster at higher temperatures, have provided the first evidence that global warming affects interactions between parasites and their hosts.
According to researchers from the University of Leicester, global warming had the potential to change the balance between parasite and host, with potentially serious implications for fish populations.
The researchers also observed behavioural change in infected fish, suggesting that parasites may manipulate host behaviour to make them seek out warmer temperatures.
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,” Iain Barber, the lead researcher, said.
The scientists found that parasitic worms infecting stickleback fish grew four times faster in experimentally infected sticklebacks raised at 20 degrees than when raised at 15 degrees.
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.
“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,” Barbar said.
“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,” he added.
The study has been recently published in the influential journal Global Change Biology.