by Timothy A. Pearce
Some species of slugs and snails can thrash their tail from side to side, twitching with such vigor that the creatures seem to jump. In some cases, they can become airborne briefly. I don’t know whether this behavior can properly be called jumping, but given that slugs are the quintessential slow-moving animals (slugs gave their name to the word sluggish), the vigorous twitching is certainly an un-slug-like behavior.
In contrast to slugs, snails keep their internal organs (guts) within the shell on their back and they have a strong, nimble, muscular foot. Slugs, which evolved from snails, have hollowed out their foot to accommodate their guts, since they no longer have a convenient shell for that purpose. Because the slug’s foot contains the guts, it is no longer as nimble as the foot of a snail.
The transition from snails (with external shells) to slugs (with internal or no shells) goes through an intermediate stage called a semi-slug, in which the animal has an external shell too small to accommodate the body. The guts are partly in the shell and partly in a hump on the semi-slug’s back. In the semi-slug form, the foot is still strong, nimble, and muscular. Many semi-slugs persist around the world today; in the United States, we have one species in the Smokey Mountains and several species in the Pacific Northwest.
The semi-slugs in the Pacific Northwest, in the genus Hemphillia, are commonly known as jumping slugs, although they are not commonly seen. The yellowish shell is visible through a slit in the mantle, and the internal organs are contained in a hump on the back. When I have found them, sometimes they will thrash the tail from side to side or twist it into a corkscrew shape and flop about like a fish out of water (Figure 1). In my experience, the Hemphillia slugs will “jump” for a second or two, then they crawl away at a normal slug’s pace (i.e., sluggishly) (Figure 2).
When I was in Madagascar (off the east coast of Africa), I saw a semi-slug of an unknown species on a leaf about a meter above the ground. When I reached to grab the semi-slug, it vigorously thrashed its tail, propelling itself off the leaf and safely into the vegetation below, not to be found.
A jumping snail (Ovachlamys fulgens) originally from southern Japan, arrived in North America in the past few years.The jumping snail sustains its vigorous jumping for a longer period of time than do the Hemphillia jumping slugs I saw in Washington State, and it covers more ground with its antics. See a video of the snail jumping here.
Why do they jump? First, let me say “why” questions are some of the hardest to answer in science. Science can never prove something to be true, we can only prove some things to be false (falsifying). To answer “why,” we try to think of all the possible answers, then set about testing each one, falsifying as many as we can. The remaining possibility (or possibilities) is our best guess at the truth, but we don’t know for sure because we are not guaranteed to have thought of all the possibilities.
The answer to why they jump has not yet been thoroughly studied, but people have speculated. The most common thought is that the slugs and snails likely jump to startle predators. A hungry predator that saw a tasty morsel flopping about might want it for lunch, but when the gastropod stops flopping, the predator might not be able to find it (and meanwhile the slug or snail surreptitiously crawls away). The jumping snails in the video jumped in response to prodding, and the semi-slug on a leaf evaded my grasp by jumping; both consistent with the idea that jumping could be an adaptation against predation.
Why don’t more snails jump? There are way more species of snails than semi-slugs, and although some semi-slugs jump, I am aware of only one snail that jumps. Jumping is therefore more common in semi-slugs than in snails. If jumping is an anti-predator adaptation, and given that the reduced shells of semi-slugs offer less protection from predators, I speculate that semi-slugs benefit from an additional anti-predator strategy.
Here is another mystery that I believe has not been studied: how can these gastropods jump If their mucus sticks them to the substrate? Snails and slugs are famous for their tenacious slime, by which they stick so firmly that they can crawl upside down on the undersides of objects. The answer might be that the jumping species have less slimy mucus, but I suspect that the answer involves variability in the mucus itself. Mucus changes its stickiness depending on how much pressure is applied. That is how snails can move (when they are stuck to the surface). My guess is that the jumping species can rapidly reduce the stickiness of their mucus when it is time to jump.
After the past year, when time sometimes seemed to crawl slowly by, it seems appropriate to write about leaping slugs and snails. And here is a bonus joke:
A jumping slug could jump higher than the Empire State Building.
That’s because the Empire State Building can’t jump.
Tim Pearce 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.
Carnegie Museum of Natural History Blog Citation InformationBlog author: Pearce, Timothy
Publication date: May 27, 2021