The Search for the Perfect Snail Snack

For generations, Hawaiians hiked up the lush, towering peaks of O‘ahu to harvest snail shells. Over 759 native species inhabited the Hawaiian archipelago, and their shells—empty after the animals died—were littered among the bright ko’oloa’ula flowers atop O‘ahu’s cliffside forests. Speckled or striped, pale or vivid, long or squat, ranging in size from 2 to 22 millimeters, the shells were strung into leis and bracelets, a testament to their beauty and bounty.

Hawai’i’s ground- and tree-dwelling snails were once so abundant that John Thomas Gulick, a 19th-century missionary and naturalist, reportedly collected more than 44,000 live snails in just three years. Today, things are different. “I haven’t seen 40,000 snails alive in my lifetime,” says Kenneth Hayes, a snail conservation biologist at the Bishop Museum in Honolulu.  

Pressured by habitat loss and invasions by non-native species, more than half of Hawai’i’s terrestrial snail species have been extinguished; those that remain cling on in such small numbers that one particularly nasty storm could wipe them out forever. In fact, 100 more of these distinctive snail species are in danger of going extinct in the next decade, according to Hayes. 

To try to stanch Hawai’i’s precipitous gastropod decline, Hayes is working alongside researchers at the University of Hawai’i, the Honolulu Zoo, and various government agencies, on the Snail Extinction Prevention Program (SEPP). Their goal is to pull some 60 species of ground- and tree-dwelling snails back from the brink of extinction through a captive breeding program. After more than a decade of work, SEPP team members are still struggling with one of the most fundamental snags in conservation biology: They’re not sure exactly what their animals eat. They are, however, inching closer to an answer.

Left to their own devices, Hawai’i’s terrestrial snails seek sustenance on a variety of plants and trees. ‘Ōhi‘a lehua, a common tree with a bright flower like a tassel, is a favorite, as is kanawao, a hydrangea relative with sharp-edged leaves. Three types of nettles are also popular. But it’s not usually the foliage the snails are after. Instead, says Solomon Champion, SEPP’s snail diet research specialist, the snails use their rake-like tongues to scrape microbes off the leaves. “They’ll essentially do a window cleaning on the surface,” Champion says.

To keep their captive snails fed, the SEPP team helicopters to Hawai’i’s high-altitude native forests to source leaves. The process is labor-intensive and expensive, and it restricts who can raise captive snails to those local enough to receive shipments of fresh flora. Bringing in wild foliage can also pose serious problems, says SEPP coordinator David Sischo. “We periodically bring in pathogens or parasites. That causes mortality in some of our colonies, which is really horrifying.” 

The struggle to source familiar, nutritious foods for captive animals is a common one. For other species of conservation concern—everything from songbirds to black-footed ferrets—scientists routinely feed them manufactured diets.

“We just can’t get wild-type diets in captivity,” says Erin Kendrick, a clinical animal nutritionist for the Smithsonian’s National Zoo and Conservation Biology Institute in Washington, D.C. “It’s not practical, and when you start talking about individual items, we don’t really have a good catalog of that. Even something as simple as a zebra—it’s naturally eating hundreds of plants, and we don’t know all of them.”

When she presents a diet to a captive creature, Kendrick says she focuses on “replicating nutrients, not foods.” The goal is to make sure her wards get enough amino acids for embryo development, for example, and the right mix of vitamins and minerals to stave off disease. 

For the past 40 or so years, snail experts have had access to a passable manufactured snail food: a cultivated form of fungus related to air conditioner mold. Hayes, though, equates this to french fries. “Snails eat it, but it’s not necessarily nutritionally good,” he says. 

Hayes’s research shows that the manufactured food leads to a breakdown in the health of snails’ gut microbiomes—a frequent occurrence for animals in captivity. In general, captive animals go from having a “really bacterially rich microbiome that has stuff that’s only found in the forest to a reduced microbial community that contains mostly common, nasty pathogens,” he says.

To home in on what snails might need to eat instead, the SEPP team has been sequencing the DNA of the microbes that live on the snails’ preferred leaves. But even here there are big, lingering questions.

Sischo says that although they’ve found hundreds of different microbes growing on the leaves, the researchers still aren’t entirely sure what their significance is relative to the snails. Do the snails browse the microbial diversity “like a cow would graze in a meadow?” Or are they targeting specific microbes? “We just don’t know,” Sischo says. It doesn’t help that all of these snail species have life histories that are really variable, he adds.

Based on the research to date, however, one group of microbes may play a particularly important role: cyanobacteria. Some species of cyanobacteria can be toxic, but others are nutritious. “They have protein, some of them have calcium, and certain ones have all the essential amino acids a snail might need,” Champion says. This finding has been something of an aha moment for the researchers. It’s possible, they now believe, that the decline in snails across the Hawaiian countryside has led to fewer “cleaners” to eat potentially troublesome cyanobacteria off of plants. If they can increase the snails’ prevalence in the forest, maybe that would help restore the also-declining plants and trees the snails require—reversing a negative ecological feedback spiral.

So far, Champion has isolated a dozen varieties of cyanobacteria from the microbial scrum. Now, he’s growing them on agar, dosing them with light and a nutrient broth similar to Miracle-Gro, and feeding them to the snails—an experiment he and the SEPP team are watching with great hope and no small amount of desperation.

Hawai’i’s snails, Hayes says, have been persisting in captivity, not thriving. Unless the food question can be cracked, the snails “are going to disappear.” 

The SEPP team’s real mission, of course, is to restore the snails to their place in Hawaiian forests. To that end, since 2020 the group has released 20,000 snails from its collection into seven field sites that are protected from predators like rats and non-native snails. As conditions allow, they hope to release many more.

To Hayes, that means the pressure is on. The SEPP team needs to figure out how to grow food for snails faster, and in larger volumes, so it can ramp up production and outsource some of the breeding to other facilities. “Because really,” he says, “the goal is to move from getting them into lifeboats and slowing extinction to getting them back [to the wild].”