Using DNA analysis, she and her colleagues traced both animals back to their beginnings and mapped their co-evolution through space and time. Then, using computer models built to gauge the likelihood of extinction, she simulated the effects that a warming environment will have on their futures.
The findings don't look good. The death of all currently endangered euastacus species would lead to the extinction of more than half the studied temnocephalans. The most distinctive lineages - the ones that migrated with their crayfish hosts to remote streams amid the mountaintop rain forests in Australia's north - would be the worst affected.
ABOUT THE PARTNERSHIP
1 They are ectosymbionts, meaning that one member - the flatworms - lives on the body of the other
2 From their perches on the animals' shells and gills, the flatworms can munch on particles of leftover food and microscopic organisms that float around them
3 It's possible that the flatworms help protect their hosts from potential pathogens, though that aspect needs further study.
As the continent warms, crayfish need to migrate to higher and higher altitudes to escape the high temperatures. But, as Hoyal Cuthill pointed out, at some point mountains have peaks.
Trapped between the sky and the rising heat, endangered euastaceans - 75 per cent of the entire genus - will likely perish. With no place for them to live, the flatworms will quickly follow.
This partnership is private and obscure - if crayfish and flatworms vanish, their ecosystem isn't going to crash down around them. But their fate resonates beyond the cool forest streams where they live.
Thomas Lovejoy, a professor of environmental science and policy at George Mason University who was not involved in the study, said that symbionts like crayfish and flatworms are the "canary in the coal mine" when it comes to climate change.
"They're more sensitive so they're the ones that first show an effect," he said. "... They're early warning signals of bigger changes to come."
The most infamous example of this is the bleaching of the world's coral reefs, which are actually a symbiotic relationship between colonies of reef-building ployps and the photosynthetic algae that provide them with food and their dramatic colour.
Stressed by warming oceans, corals are expelling their symbionts by the billions, a phenomenon that renders reefs far less hospitable to the life that typically thrives around them and leaves the corals themselves vulnerable to disease.
In general, Lovejoy and Hoyal Cuthill agree, species in close symbiotic relationships are most sensitive to change; a system is only as strong as its weakest link.
"The closer the interdependency the higher the risk of extinction," Hoyal Cuthill said.
She let out a grim chuckle. "But obviously all species are interdependent and we all depend on other species for survival," she added.
"You have to wonder how many extinctions can be tolerated before it starts a larger cascade."
