We return today to the rivers of China for an update on Helicostoa, the Lost Snail of the Yangtze. Helicostoa is a bizarre little snail known only from two chunks of limestone taken from the rapids of the Yangtze River about 1900. As I wrote in my 2021 post "The Lost Snail of the Yangtze," the French biologists who studied the dried specimens of Helicostoa on those chunks of limestone had no idea what they were looking at – they weren’t even sure the animals were snails – except to note that the rocks appeared to contain two forms (male and female?) that were cemented to the limestone. This cementing is almost unique among freshwater snails, and suggests that Helicostoa had highly unusual habits and adaptations for living in torrential river rapids.
But we didn’t know anything about the habits or origins of Helicostoa, because the entire primary scientific literature on this enigmatic snail consisted of a pair of short papers written in 1926 and 1937 by baffled French biologists, and the rapids where Helicostoa was collected were destroyed by the Three Gorges Dam about 20 years ago. I concluded that “No one has seen [Helicostoa] again, and probably no one ever will,” but that “… perhaps some enterprising biologist will yet discover a living population of Helicostoa in some unexplored Chinese river.”
Remarkably, two recent scientific papers lift the veil of mystery that has obscured Helicostoa for more than a century.
Last summer, a team led by Thomas Wilke of Justus Liebig University succeeded in recovering and analyzing DNA from those dried-out specimens that have been sitting in a Paris Museum for almost 120 years. Analyses of ancient DNA have become more frequent, though they are hardly routine, but when I read about the new, sophisticated biotechnology and statistics used in this paper, it was hard not to think of Arthur C. Clarke’s famous law that “Any sufficiently advanced technology is indistinguishable from magic.”
Wilke’s team had to contend with analyzing a small specimen (the snail weighed a whopping 1/300 of an ounce) whose DNA was badly fragmented, as well as contaminated with other DNA it had encountered travelling from China, lying in a museum drawer in Paris, and being handled by multiple ungloved people and their microbiomes. Using methods modified from the analysis of fossil DNA, they washed the snail to reduce external contamination, ground it up, and extracted one-half of one-billionth of an ounce of highly fragmented DNA (for you DNA fans, the average fragment length was 147 base pairs). These tens of millions of DNA fragments were sequenced, subjected to what can only be described as a heroic amount of data processing (requiring computing power that would have been unavailable just a few years ago), including the removal of any sequences that looked like those of people, dogs, cats, chickens, and E. coli (indicating contamination), then reassembled into a probable genome for Helicostoa.
This analysis showed that Helicostoa was a member of the gastropod family Bithyniidae, although not very closely related to any other living species. This result makes sense: there are lots of bithyniids living in southeastern Asia, and some of them can filter-feed (a snail that is cemented in place can’t move around to find its food, but could survive by filter-feeding on particles that the current brings to it). The large evolutionary distance between Helicostoa and other bithyniids suggests that it began to evolve away from its cousins a long time ago, and is probably an ancient inhabitant of the river rapids. We finally know what kind of snail the apparently extinct Helicostoa was.
The other news about Helicostoa is even more surprising. On July 20, 2022, I received an email from China from a biologist who had read my post (!), containing the startling news that Chinese naturalists had found a living population of Helicostoa. Earlier this year the scientific paper describing that discovery appeared. In addition to announcing the rediscovery of Helicostoa in the Pearl River basin, a group led by Le-Jia Zhang from Humboldt University in Berlin was able to make the first observations on living animals.
They were able to confirm that Helicostoa is a filter-feeder, which as I suggested is not too surprising for an animal that cements itself in place. But in cementing itself to rocks, Helicostoa completely blocks off its shell aperture (the opening through which a snail comes out into the world), and has to build a secondary opening to get out of its shell. This strikes me as a little like nailing your front door shut, then using a Sawzall to cut a hole in the wall so you can get in and out of the house. The question of why Helicostoa cements itself to rocks is still unsolved.
Zhang et al. were surprised to find that all of the cemented Helicostoa that they found were females, but were associated with many much smaller, uncemented snails, which the researchers guessed were probably another undescribed species. To find out what they were, Zhang’s team analyzed their DNA, and were astonished to find that these little snails were the males of Helicostoa, which do not cement to the rocks, and are only about 1/8th the size of the females.
But if the males of Helicostoa don’t cement to rocks, then what were the two kinds of snails that the French scientists reported from those rocks on the Yangtze? It turns out that those two forms were females of two different species of Helicostoa, the big ones being what the French named Helicostoa sinensis and the little ones being what Zhang et al. named Helicostoa liuae.
Finally, Zhang et al. raised the possibility that the Lost Snail of the Yangtze didn’t come from the Yangtze at all. They doubt that this little snail would have occurred both in the Pearl River basin and in the distant Yangtze (and intensive surveys done before the Three Gorges Dam closed found no trace of Helicostoa in the Yangtze). They suggest that the name “Kouei-Tchéou” on the label of those chunks of limestone in Paris may have referred to Guizhou Province (which includes parts of the Pearl River basin) rather than Kuizhou, a now-flooded city along the Yangtze. So the Lost Snail of the Yangtze may really have been lost!
I draw three lessons from the remarkable new chapters to the Helicostoa story. First, it’s hard to be sure that a species is extinct, even if its prospects look black, so we should be slow to give up the last hope for a vanished species. We’re not going to find all of these missing species, or even most of them, but we will find a few, and that is a bright ray of hope in a darkening world.
Second, it is striking how humble low-tech approaches (simple natural history studies) were combined with the most sophisticated cutting-edge technologies (DNA sequencing, computer-intensive statistics) to advance our understanding of the ecology and origins of Helicostoa. It is easy to get swept away by the newest, shiniest scientific techniques, but the Helicostoa story reminds us that the best science usually relies on a combination of techniques, including both good, old-fashioned natural history and innovative technology.
Finally, if we are to conserve Helicostoa and the other remaining species that are specialized to live in the rapids of great rivers, we must redouble our efforts to protect these increasingly rare habitats. I noted in my original essay that most of these rapids have been destroyed (mainly by hydropower dams). Dams now under construction or in planning stages will destroy many more rapids habitats in the big biodiverse rivers of Southeast Asia, South America, and Africa, and Zhang et al. noted that the remaining rapids in the Pearl River basin are threatened by dams and growing cities. Unless we can preserve the remaining river rapids, we will lose Helicostoa and other rapids specialists, and this time, for good.
