By Dr. Becker
No one knows how many different species exist on planet Earth, although some estimates put it at 10 million to 30 million. Only about 1.5 million species have been scientifically described so far, so there are many new species yet to be discovered.
What is known is that the tropical rainforests are the most biodiverse regions in the world, and likely contain one-half to three-quarters of the different species on the planet.1
It behooves scientists to scientifically document new species not only for research purposes but also because conservation efforts are predicated on knowing which species live where and in what numbers. You can’t save a species if you don’t know it exists, after all.
Yet, for all of their wondrous biodiversity, rainforests are not the most hospitable places to conduct research. The forests are dense and difficult to travel, especially when lugging along heavy equipment (like camera-traps).
Further, many of the animals are rare and hide from humans, making observations often impossible. Even after recording with a camera-trap for 2,000 nights, for instance, the rare Annamite striped rabbit was never seen. But it turned up in the most unusual of places – a leech.2
Analyzing Leeches Helps Researchers Census the Jungle
Leeches feed on the blood of mammals, inadvertently collecting DNA samples along the way. By analyzing the DNA in leeches, scientists can easily gather information about the animals they feed upon, gathering a “surprisingly comprehensive snapshot of the jungle’s fauna,” The Atlantic reported.3
Mads Bertelsen, a veterinarian from Copenhagen Zoo, and Professor Thomas Gilbert of the Center for GeoGenetics, Natural History Museum of Denmark, were the first to discover that leeches act as reservoirs for their meals’ DNA.
After leeches fed on goat blood, the goat DNA survived in the leeches for at least four months.4 The team then collected leeches from tropical Vietnamese rainforests and identified “cryptic, rare, and newly discovered mammalian species.” They noted:5
“We propose that DNA from leeches represents a quick, cost-effective, and standardized way to obtain basic data on mammalian biodiversity and species occupancy, facilitating efficient use of limited conservation resources.”
Indeed, when the team sequenced DNA from 25 leeches, they found DNA from pigs, cows, the small-toothed ferret-badger, the goat-like serow, and the aforementioned elusive Annamite striped rabbit. The Atlantic continued:6
“ … [T]hey even detected DNA from the Truong Son muntjac, a small deer that was discovered in 1997 and has never been seen in the flesh. ‘That suggests that these animals aren’t as rare as we think or that the leeches are very good at finding them,’ says Gilbert.”
Hundreds of Leeches Can Be Collected in a Day
It’s often the simplest tools that turn out to be most effective, and this certainly seems to be the case with leeches. “Just by standing around in a rainforest for a day, a lucky collector (or unlucky, depending on how you look at it) can attract hundreds of leeches,” The Atlantic reported.7
If you’re squeamish this may seem like a nightmare, but researchers can simply pluck leeches from their bodies to analyze as they spend time in the jungle. This sounds like a picnic compared to researchers looking for new species in the Philippines’ deep sea regions, who have only about 30 minutes to explore, before an hour’s long period of decompression on the way back to the surface begins.
In another example, scientists had to take photographs and blood samples from 324 panther chameleons in Madagascar in order to discover 11 new chameleon species – so you can see how potentially time-saving this leech method could be.
The Forestry Department of Yunnan Province in China even has 200 department rangers who regularly collect leeches to be analyzed (some 20,000 have been collected to date).
When the DNA from the leeches is sequenced, it’s compared against existing databases. The greatest challenge comes when a sequence isn’t found, leaving the researchers to speculate whether the DNA is from a new species or simply one that has yet to be sequenced. The Atlantic continued:8
“Inevitably, the leech data will also have biases, depending on how easy different leeches are to catch, how they react to changes in climate, and which animals they prefer to feed from. But, every census method suffers from bias, and few are as cost-effective or informative.”
Leeches (and Insects) Could Help Save Threatened Species
Leeches have already proven beneficial in finding endangered species, like the saola, an antelope-like creature known as the Asian unicorn. While the saola has proven elusive by traditional research methods, researchers have collected leeches in Vietnam in hopes of uncovering its whereabouts.
DNA sequencing has proven to be accurate and “many times quicker” than traditional methods. In a study published in Ecology Letters, experts took more than 2,500 hours to identify 55,000 specimens. Using DNA sequencers, the same biodiversity information was revealed in a fraction of the time.9
DNA data derived from leeches and insects like mosquitoes, ticks, and carrion beetles are already being used to monitor species in protected areas of China and Canada. Simply speaking, by collecting leeches and blood-sucking insects, the DNA sequenced from them can reveal whether the same species are present month after month.
The technology is advancing rapidly such that researchers may soon be able to collect leeches and insects and analyze their collected DNA right in the field. As Cosmos magazine explained:10
“ … [G]enetic monitoring is rapidly becoming faster and less expensive. A British company, Oxford Nanopore Technologies, is developing a portable sequencing device.
So researchers will soon be able to go into the field, whip up their insect [or leech] soup, plug the sequencing device into their laptops, and almost instantly see a panoramic view of the species hidden all around them.”