I am so proud of our recent publication, mostly because it has been a long time coming. We received an Australian Research Council grant in the year 2000 to work on this so it has taken us some time to achieve our goal.
The characters in this story are the Australian freshwater spiny crayfish and their flatworm friends, which are called temnocephalans. These worms spend their entire lives living on the bodies of the crayfish, and in many cases, one species of flatworm is found on only one species of crayfish.
Their partnership has endured for 100 million years. To put that in perspective, our own species Homo sapiens has occupied the earth for 0.2 million years, so we are a blink compared to these ancient creatures. There are three genera discussed in our paper: one genus of crayfish and two types of temnocephalans. I will introduce them in turns.
Spiny crayfish (genus Euastacus)
The genus Euastacus are cool-climate specialists who live in leafy upland rivers, where they hide under rocks and burrow into the banks. The genus originated on the ancient continent of Gondwana and persists only in eastern Australia.
Unlike their more familiar relatives such as yabbies, spiny crayfish grow slowly and live for a long time. The larger species, such as the Murray Cray, take up to 7 years to become sexually mature and can live for 50 years or more. Nobody knows exactly how long, and really large specimens (which could be very old indeed) are increasingly difficult to find.
Fishing pressure and habitat alteration has caused serious declines in many species, while climate change puts all of them at risk. No less than 75% of the Euastacus species are endangered. Those most at risk reside on isolated mountains in northern Queensland.
We collected 37 different species of Euastacus for our study. Each one lives in a different river system or National Park, and some were collected by hand, others using nets. Patience and persistence were required in every case.
I remember one trip in particular, when David, Kim, my husband and I were in the Grampians National Park on New Year’s Eve 2003 – after a day of collecting we set up our microscopes on a picnic table in the campsite to pick worms from our crayfish and label our specimens. Members of the public were intrigued and after looking down our microscope, one young camper presented us with a drawing of a crayfish (probably Euastacus bispinosus) that has hung on my office wall ever since.
Temnocephalans – ectosymbiotic flatworms
I have written about temnocephalans before, and it is worth looking at the video in my article here to see how they move.
Basically, they look like little hands with eyeballs and they stay attached to the crayfish using large suction discs. The “fingers” are tentacles that they use to catch prey from the water.
We have no evidence that they harm the crayfish, so we say that they are symbiotic (meaning the relationship is probably of mutual benefit). It may be that temnocephalans keep the crayfish clear of other parasites and that crayfish stir up sediment providing sources of food for the worms.
There are two genera of temnocephalans in our study: Temnohaswellia have six tentacles and are usually white, while the genus Temnosewellia are brown and have five tentacles. The former genus was named after William Haswell, a director of the Queensland Museum, while the latter was named after our own co-author Kim Sewell.
Some temnocephalan species live on more than one crayfish species, a pattern common among the Temnohaswellia. Others are found exclusively on one species of crayfish, a pattern common among the Temnosewellia. This is true especially among the far northern populations, which are most at risk of extinction.
In order to complete this study, we dissected individual worms so we could send samples to London for DNA analysis and keep enough of the animal in Australian so we could identify each species.
In some cases, the temnocephalans we found did not even have names. Kim, Lester and David had to complete careful microscopic and taxonomic work and publish species descriptions before we could continue.
By analysing DNA sequences from 37 crayfish species and 33 species of temnocephalans, we were able to describe the ancient association between them.
The evolutionary history was reconstructed in matched evolutionary trees (called a co-phylogeny) allowing us to see patterns of divergence over a period of time that included extensive climate change as continents separated and drifted north.
Host-shifts are evident in the patterns revealed, meaning when one group diverged (as in when a new crayfish species formed) the other followed (a new temnocephalan species was born). There are few datasets available of this type and they are of great interest to evolutionary biologists.
Unfortunately, the close association between crayfish and flatworm species means that if one goes extinct the other is likely to follow. Our analysis suggests that if all the endangered Euastacus species go extinct, then 60% of temnocephalan species will follow suit.
Both types of organisms are so unique and special they deserve protection, but climate change will make this challenging due to their reliance on clear cold water and shady habitats.
It is our hope that by highlighting these issues, more people will care about the future of freshwater crayfish and the tiny animals (there are others) that live on them or in their burrows. In any case, we are delighted to be able to offer this unique insight into the phenomenon of co-evolution.