According to the Encyclopedia of Life (EOL), backbone-lacking snails and slugs comprise 61,995 terrestrial and aquatic species in the class Gastropoda, in phylum Mollusca, and are widely distributed planet-wide. (Source)
The number of this invertebrate species fluctuates somewhat owing both to discoveries of species, particularly “microsnails” (Source), and to species extinction, like that of “Lonely George” the Hawaiian tree snail, Achatinella apexfulva, in 2019. (Source)
Some snail characteristics make them difficult to study with regard to their sleep (that is, quiescence) or other habits—e.g., their size, location/habitats, susceptibility to climatic or other changes such as those related to ocean acidification, and difficulty in keeping in the lab; other aspects, including their relatively simple CNS (central nervous system) and ability to be sequenced in genome analyses, make certain species good research models, as seen with the great pond snail Lymnaea stagnalis.
Of the so-far very few snail species that have been more rigorously studied with regard to their apparent rest or rest-and-locomotion habits, it has been hypothesized that land and water snails of various species do experience a sleep-like state and that its length and character varies from species to species.
- For instance, among L. stagnalis pond snails, frequent, brief quiescence of approximately 7 “sleeps” of only 22 minutes’ duration (plus/minus 1 minute) per 13- or 14-hour period is seen. (Source)
- Echinolittorina malaccana studied on artificial seawalls “typically spend more than 90 percent of their lifetime exposed to air, in a state of rest” with complex metabolism in relationship to temperature fluctuations (with both shade-seeking and heat-avoiding behaviors in past observational studies; Source).
- Nine species of warm-water pelagic, or open-ocean, snails displayed nocturnal swimming and feeding in counterpoint to daytime “nearly vertical” sinking trajectories that can be conjectured as relating to quiescence and predator evasion (Source).
Snails, in short, are difficult to study and species volume so numerous and, therefore, much more research needs to be conducted to better understand how closely their seeming quiescence approximates what we would call sleep and to measure its cause, effects, and characteristics.
How is Sleep or Dormancy Defined?
“Hibernation isn’t the only type of dormancy an animal can experience, and winter isn’t the only time of year animals might go to sleep or reduce their activity.” (Source)
Terrestrial and aquatic snails are believed to be no different with regard to yielding to periods of dormancy to conserve energy. Snails are cold-blooded (ectotherms), and so, do not undergo hibernation.
On the contrary, ectotherms such as snails do enter into brumation or estivation (also spelled aestivation).
“Brumation” is the terminology modern biologists use for a state akin to hibernation, but for cold-blooded animals. “Estivation is when animals are dormant because weather conditions are very hot and dry.” (Source)
Snails in particular are very sensitive to periods of cold or dryness/lack of moisture. “During the heat of the day and the dry hours of noon, snails remain in a dormant state called aestivation or dryness sleep. Their behavior during this dormancy is quite different between species,” but it often involves sealing their shell-mouth with dried mucus. (Source)
Perhaps most fascinating, however, is the story of the snail that “slept”—really, it estivated—for almost 4 years.
The tale began in 1846, when a Helix desertorum land snail (today they’re usually called Eremina desertorum) was mounted to a board in a museum in Egypt. (Source)
About four years later, a zoologist discovered the snail was, in fact, alive—it had sealed itself against the heat by using a mucous membrane called the epiphragm.
This desert snail was “resuscitated” with tepid water and fed a cabbage-leaf diet, whereupon it lived for another couple years before falling into torpidity and dying (for certain, this time) in 1852. (Source)
What is a Snail Anyway?
It’s easy to misunderstand biologic or other complex terminology.
After all, how many among us are malacologists—itself a somewhat disagreed-upon term describing a scientist who studies the phylum Mollusca, which we now know includes class Gastropoda, home to both snails and slugs but also intervertebrates with a more complicated CNS, such as octopuses and squids? (Source)
Gastropods are “by far the largest group of molluscs . . . compris[ing] about 80% of living molluscs” (Source).
Snails are nestled among the muscular-footed gastropods, and they glide along using that mucousy muscular foot or else their locomotive ability is adapted for swimming or burrowing; unlike the, by comparison, “naked” slugs, snails carry a single, usually spirally coiled shell whose operculum (door) they can close after sealing their soft bodies inside. Many have a well-developed head with eyes and a spiky tongue called a radula. (Source)
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How Long Do the Snails Under Study Sleep?
- Lymnaea stagnalis (Great Pond Snail): As the name implies, you’re most likely to see the great, or giant, pond snail in stagnant bodies of freshwater (not saltwater). It is “widely used for the study of learning, memory, and neurobiology” (EOL). A landmark study on L. stagnalis, by Stephenson and Lewis in 2010, concluded that these model organisms exhibit quiescence, that their quiescence is a sleep-like state, and that they, therefore, should join a “growing list of invertebrates” in the sleeping (versus the nonsleeping) category. Further, these aquatic snails slept about 7 times across a 13- or 14-hour period. Each “nap” was relatively brief, of only 22 minutes’ duration, plus or minus a minute.
- Echinolittorina malaccana snails: Studied on artificial sea walls in Brunei, these marine snails were found to be sensitive to the temperature conditions of their environment, and it has been posited that they spend more than 90% of their lives attached to a rock wall and, thus, non-mobile. Were they sleeping? More studies could lead to a scientific consensus on that—but only if these snails can survive the ravages of ocean acidification brought on through dramatic climate change.
- Nine species of warm-water pelagic (open-ocean) snails—were studied, and the results were reported as recently as September 2020. The authors say that “Swimming and sinking behavior by pelagic snails is poorly studied but is important in their ecology, predator-prey interactions, and vertical distributions.” The sinking behaviors usually seen in these marine species during the heat of the day could indicate some sort of torpor or quiescence, possibly coupled with predator evasion and avoidance of light/heat extremes. The study, which did not explicitly study snail sleep, positively correlated “swimming speeds, sinking speeds, and glide angles . . . with shell size” (Source).
Do Snails Sleep at Day or Night?
The research consensus is that snails are highly attuned to their environment, especially with regard to moisture, heat, and food. They generally feed nocturnally or in the early morning, when it is coolest and dampest, and “rest” during the daytime—even the aquatic snails, whether they live in ponds, puddles, streams, lakes, or oceans (Source)!
Do Snails Sleep Inside or Outside Their Shells?
Depending on environmental conditions, snails can choose whether to rest for shorter periods or for longer time frames inside their shell—especially if their torpor/quiescence is more prolonged—or outside. Some land snails in particular, “ . . . crawl up plant stems and fall in-to a state of dormancy. They then close off the entrance to the shell with a seal made of slime, with which they also stick themselves to the surface they have chosen” (Source).
Do Snails Sleep Upside-Down?
Of the snails that can be more easily observed/studied or kept as pets, ”Pond snails use things like rocks or the side of their aquarium as their bed, attaching themselves while they sleep.” (Source)
The creator of Fishkeeping World has this interesting take on aquarium snails: “They can sleep pretty much anywhere. A sticky mucus lets them climb over all surfaces in the tank. This mucus is used to hold them in place while they sleep. So snails can sleep in any position—sideways or even upside-down.” (Source)
Do Aquarium Snails Sleep? (Or is the Snail in My Tank Dead?)
If all observations are correct, aquarium snails do seem to experience torpor/quiescence, which we interpret as sleep. Sometimes, however, they bury themselves in substrate and are difficult to observe! If you don’t want to make the mistake of the Egyptian museum in 1846—placing a living snail on a display board, thinking it’s dead—one expert (Azron21) in Reddit’s snails forum says to give it the good, old-fashioned smell test: “If they float much, remove them from the water and give them a sniff. . . . If they smell fishy, as in having an odor of fish, they are deceased.” (Source)
Will Snails Survive Climate Change?
Why are snails important? Although we can glean historic environmental data from the calcium carbonate shells of dead or fossilized snails, living snails could also be bellwether species—giving us warnings before irreversible damage occurs.
Snail researcher Rebecca Rundell sums it up: “If snails in the ocean that make their shells. . . are having trouble building them, then that means the ocean is in big trouble.” (Source)
Even land snails provide clues that could save another important species—Homo sapiens sapiens. “[If land snails are dying off] it might give you a chance to change course, to detect subtle changes that humans might not otherwise be able to see until it is too late,” Rundell continued.
Let’s hope snails—and the people who love and research them, will stick around for a lot longer than the many species who have already gone extinct.
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