Carnegie Museum of Natural History

One of the Four Carnegie Museums of Pittsburgh

land snails

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Tornadoes, Snails, and Sample Sizes

woman collecting snail species specimens
Abbey collecting snails after braving unkind vegetation. Photo by Tim Pearce.

In 2012, a tornado felled trees in four places at Carnegie Museum’s field station, Powdermill Nature Reserve in southwestern Pennsylvania, about 1-hour drive East of Pittsburgh. Each blowdown was 3-6 ha (8-15 acres), within 3 km (2 miles) of each other. These blowdowns provide natural replicates to examine land snail response to habitat change. Given that some snail species are known to occur in forests and others in meadows, we might expect the snail species composition to shift when the wind turns part of a forest into a meadow.

Samples taken in 2016 showed differences in snail species community composition between the blowdown areas and the adjacent, intact forest. However, other statistical tests did not show differences that were significant, but they were nearly significant.

A good scientist should readily accept “no difference” when statistical results show that the differences are not significant. However, when the differences are tantalizingly close to significant, one might wonder whether “no difference” is real, or if a larger sample size might have demonstrated a significant difference.

So, we sampled again this year and took more samples. We are still processing the samples, so results are not in yet, but with the larger sample size, we will accept “no difference” if that is what the statistics tell us.

In the photo, Abbey is collecting leaf litter (containing snails) at the Laurel Run blowdown. The sample she collected contained 23 snails, of five species: Glyphyalinia indentata, Punctum minutissimum, Striatura ferrea, Striatura milium, and Zonitoides arboreus.

Timothy A. Pearce, PhD, is the Head of the Section of Mollusks and Abbey Hines is a Gallery Experience Presenter at Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.

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Snail Extinction – Bad Situation Getting Worse

By Timothy Pearce

Move Aside Rosy Wolf Snail, the New Guinea Flatworm Wreaks Greater Devastation

Another species of land snail went extinct on January 1, 2019. George, the last member of his species, Achatinella apexfulva, died in a captive breeding facility at the University of Hawaii. The loss of this snail, and this species, is sad from many perspectives, I’ll mention two: first, George’s species is the first land snail ever described from Hawaii; second, this loss contributes to the largely overlooked extinction crisis of land snails around the world.

Achatinella apexfulva shell
Achatinella apexfulva from the Carnegie Museum of Natural History collection.

George was named after Lonesome George, the last Galapagos tortoise of the subspecies Geochelone nigra abingdoni, who died in 2012. Like most land snails, George the snail was hermaphroditic (having both male and female parts), so either male, female, or androgynous names would have been appropriate.

News outlets including New York Times, National Geographic, and National Public Radio, as well as various blogs (e.g., https://www.shellmuseum.org/curators-corner) have well-covered the story of George’s passing, so look there for more details that I won’t repeat. Those outlets mentioned threats leading to the demise of tree snails, including the introduced rosy wolf snail, a snail-eating snail credited with causing snail extinctions on some Pacific Islands. However, none of those news outlets mentioned the New Guinea flatworm, which is already showing itself to be a much greater threat to snail-kind than the rosy wolf snail.

New Guinea flatworm
The New Guinea flatworm (Platydemus manokwari) eats land snails so efficiently that it is causing snail extinctions. Photo from Wikimedia Commons.

The New Guinea flatworm (Platydemus manokwari), which eats mostly snails, has been categorized as one of the 100 worst invasive species. Originally found in New Guinea, human activity has introduced it to many tropical and temperate regions of the world where it has had significant negative impacts on the rare endemic land snail fauna of some Pacific islands. Evidence indicates that predation by the New Guinea flatworm is the greatest cause of the extinction or drastically reduced numbers of several native snails. Up to 65 mm (2.5 inches) long, it can follow snail mucus trails to catch prey, sometimes even into trees, so its presence in Hawaii seriously threatens the remaining Hawaiian tree snails.

In 2015, the New Guinea flatworm was found in Florida, from which it poses a threat to land snails on the mainland of the USA. A colleague told me that in some of the Everglade hammocks where the flatworm has reached, all you can find now are dead, empty shells of the colorful tree snails that were gobbled by the flatworm. The flatworm does not survive in colder climates, so for the time being, the northern United States might be spared from this scourge. The flatworm survives best at 18 to 28 C (64-82F) and nearly ¼ of them survived in an experiment down to 10°C (50F) for 2 weeks.

Timothy A. Pearce is Curator of Collections, Section of Mollusks at Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.

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Speculation: Glowing Snails and Jumping Genes

By Tim Pearce

Only one species of land snail is known to glow in the dark: Quantula striata, albeit very faintly. A glow organ under its chin produces yellow-green light, and the rest of the body glows very faintly. The snail occurs in some areas of Southeast Asia including Malaysia and Singapore. The snail uses the same system to glow as fireflies, two chemicals: luciferase reacts with luciferin to produce light.

Scientific papers, including those by Yata Haneda, have characterized the wavelength of the light, the interval of the flashes, which part of the body glows, and differences in glowing between juvenile and adult snails. However, none of the papers has addressed why the snails glow. Given that light production is energetically costly, there must be some evolutionary advantage to glowing. How does glowing help the snail in its daily life?

There are five known reasons that organisms glow: (1) attract mates (as in fireflies [originally for larval defense, see Branham and Wenzel 2003, Cladistics, 19:1-22]), (2) attract prey (as lures in deep sea fish), (3) attract dispersers (insects attracted to light disperse spores from glowing mushrooms), (4) escape predators (deep sea octopus create glowing clouds and slink away unnoticed), (5) burglar alarm (some ocean microorganisms glow when copepods try to eat them; the glow attracts fish that then eat the copepods).

I speculate that Quantula striata glows to escape predators.

Quantula striata, land snail that glows in the dark
Quantula striata, the only species of land snail known to glow in the dark.

Larval fireflies eat land snails and larval fireflies occur in Southeast Asia where this glow snail lives. Perhaps a glowing snail could fool a hungry firefly larva by falsely conveying that the snail is already occupied, so glowing might ward off an attack by a firefly larva. Thus, the evolutionary advantage is that glowing snails might experience less predation.

One way to test this hypothesis would be to expose glowing and non-glowing snails to larval fireflies to determine which kind of snail gets eaten more. I haven’t tried this experiment yet, because I don’t have glow snails available in my lab.

More speculation: could the genes for the light-producing system have moved from a firefly to this snail? It is a remarkable coincidence that the snail and the fireflies both produce light using the luciferin and luciferase system. What are the chances of that! One possibility is that the genes to produce luciferin and luciferase were somehow transferred from a firefly to an ancestor of the snail, then spread over time throughout the species. While such horizontal gene transfer is thought to be relatively rare, the transfer of genes from one species to another is known in single celled organisms (e.g., the spread of antibiotic resistance among bacteria species), and evidence exists that it has occurred in some multi-cellular organisms.

One way to test whether horizontal gene transfer could explain the luciferin and luciferase lighting system in Quantula striata would be to sequence the DNA of the snail and the DNA of fireflies living in Southeast Asia. If both genes for luciferin and luciferase were transferred from the firefly to the snail, there is a good chance that additional DNA on either side of those two genes was transferred as well. If additional firefly DNA exists near the luciferin and luciferase genes in the snail, that would be strong evidence that the snail’s ability to glow came from a firefly.

It could have happened!

Relevant Snail Joke: 

Q: What happened to the glow-snail that lost its glowing organ?

A: It was de-lighted.

Timothy A. Pearce, PhD, is the head of the mollusks section at Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.