Fifty years ago, the rosy wolf snail was introduced to the South Pacific Society Islands to control a pest population, but the plan backfired. The predator ended up wiping out most of the known 61 species of tree snails native to the island, with the exception of the white-shelled Partula hyalina.
For years, scientists wondered why P. hyalina was able to survive the onslaught when none of its cousins could. Now, thanks to the world’s smallest computer and an interdisciplinary team of biologists and engineers, we know the answer lies in the stark color of their shells.
In 2015, Diarmaid Ó Foighil, professor of ecology and evolutionary biology at the University of Michigan, and colleague Cindy Bick hypothesized that P. hyalina's distinctive white shell might give it an important advantage in forest edge habitats, by reflecting—rather than absorbing—light radiation levels that would be deadly to its darker-shelled predator.
To test their theory, the researchers wanted to track the light exposure levels experienced by P. hyalina and rosy wolf snails during a typical day. However, given the size of snails, that was easier said than done.
Doing their due diligence, the team discovered that most commercial, off-the-shelf sensors were at least 12 mm—much larger than the 3 mm shells of adult rosy wolf snails. But then, Bick read an article about a recently developed sensor that—at 2 mm—was being billed as the world’s smallest computer. In a stroke of luck, the developers happened to also work at the University of Michigan.
Establishing an interdisciplinary team, Foighil and Bick collaborated with David Blaauw and his team, who pioneered the Michigan Micro Mote (M3) in 2014. Not only would this be M3’s first field application, but it would also require modifications to the world’s smallest computer.
"It was important to understand what the biologists were thinking and what they needed," said Inhee Lee, who helped adapt the M3 for the snail study.
The first step was to figure out how to measure the light intensity of the snails' habitats. At the time, the team had just added an energy harvester to the M3 system to recharge the battery using tiny solar cells. But then, Lee realized they could measure the light level continuously by measuring the speed at which the battery was charging.
Now, with a way to sense and measure light—that didn’t add to the instrument’s overall size—the computer was ready for the snails.
After testing local Michigan snails to ensure the system worked, the research team sent 50 M3s to Tahiti. Working with local conservation experts, the sensors were glued directly on to the shells of the rosy wolf snails. However, P. hyalina are endangered, meaning experts couldn’t legally glue sensors to their shells.
P. hyaline are nocturnal snails who sleep by attaching themselves to the underside of leaves. Employing an indirect monitoring approach, the team used magnets to secure M3s on the tops and undersides of leaves frequented by resting P. hyalina. At the end of each day, Lee wirelessly downloaded the generated data.
According to the team’s paper, recently published in Communications Biology, between noon and 1 p.m. local time, the P. hyalina habitat received an average of 10 times more sunlight than that of the rosy wolf snails. Thus, the researchers suspect P. hyalina have been and continue to be relatively safe from rosy wolf snails because the species won’t venture far enough into the forest edge to catch the white-shelled snails. Even under cover of darkness, rosy wolf snails run the risk of being unable to escape to shade before the sun becomes too hot for them to survive.
"We were able to get data that nobody had been able to obtain," said Blaauw, professor of electrical engineering and computer science. "And that's because we had a tiny computing system that was small enough to stick on a snail."
The success of this project has already facilitated a subsequent collaboration between engineers and biologists at the school interested in tracking monarch butterflies.
“The M3 really opens up the window of what we can do with invertebrate behavioral ecology and we're just at the foothills of those possibilities," Ó Foighil said.
Photo: An aestivating specimen of P. hyalina attached to the underside of an Alocasia macrorrhiza leaf. Credit: Bick, C.S., Lee, I., Coote, T. et al. Millimeter-sized smart sensors reveal that a solar refuge protects tree snail Partula hyalina from extirpation. Commun Biol 4, 744 (2021). https://doi.org/10.1038/s42003-021-02124-y