The bond between spiny mountain crayfish and their tiny, flatworm friends was forged some 100 million years ago on the thickly forested super-continent of Gondwana. It endured the dominance of the dinosaurs and the catastrophe that killed them. It survived isolation on Australia as the continent broke away from its neighbors and sailed northward. The climate warmed, the air dried out, the earth was pushed upward into mountains and eroded by streams.

All the while, this ancient association thrived, with the worms living out their lives on the backs of hospitable crayfish. The creatures diversified and spread, adapting to every available ecological niche so that researchers today might find a unique species of flatworm living on just one kind of crayfish in only a certain stream in all of Australia.

"They've been evolving and interacting while all these massive changes have been occurring," said evolutionary biologist Jennifer Hoyal Cuthill. "... that's now at increasing risk due to climate change."

Hoyal Cuthill, a researcher at the Tokyo Institute of Technology and University of Cambridge, is the lead author of a study published Wednesday in the Proceedings of the Royal Society B on the long — and probably doomed — relationship between Australia's 37 species of spiny mountain crayfish (members of the genus Euastacus) and their 33 species of flatworm symbionts (called temnocephalans). 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.

In many parts of Australia, that would put an end to the million-century-old partnership between the two groups of species. They are ectosymbionts, meaning that one member — the flatworms — lives on the body of the other. It's obvious what the flatworms gain from their crustacean hosts — 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. And the crayfish may benefit too; Hoyal Cuthill said that 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 percent 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 color. 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."

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