Substrate Attributes Determine Gait in a Terrestrial Gastropod

1. What they did

Snails and slugs usually move by “adhesive crawling”: waves of muscle pass along the foot, which glides on a thin layer of mucus and leaves a continuous mucus trail. Making this mucus is costly. Some species can also “lope”: the snail lifts its head, stretches it forward and puts it down again, creating arches in the foot. Only the parts of the foot between the arches touch the ground, so the snail leaves a broken, discontinuous trail. Loping had been thought of as rare, and as a way to move faster or escape predators.

The authors had often seen the snail Cornu aspersum loping on dry sidewalks and brick walls in southern California, leaving obviously broken mucus trails. They tested how surface type affects gait in the laboratory:

• Horizontal surfaces. 25 snails were each placed on dry acrylic, glass and concrete plates in random order. Each was filmed from above and watched from the side to see whether arches formed in the foot, and the wet mucus trail was traced. From the video and trails they measured gait, speed and the area of mucus left per centimetre of trail. Concrete soaked up a drop of water in about 7 seconds; on glass and acrylic it had not disappeared after 10 minutes.
• Vertical surfaces. 24 snails started on each horizontal plate, which was then slowly tilted upright, and their gait was recorded.
• Wood. 12 snails were placed on acrylic and on the bark of a white birch log.
• Mucus cost. Snails were weighed before and after crawling on acrylic or loping on concrete. Weight lost was used as a measure of mucus used.
• Other species. They watched five other kinds of land snails and slugs moving on concrete, in the laboratory or in the field.

2. What they found

Gait depends on the surface

• On horizontal concrete, 23 of 24 snails switched from adhesive crawling to loping within 2 to 9 cm, and kept loping for at least 6 cm. None loped on acrylic or glass. (One snail that switched back and forth on concrete was left out.)
• On vertical concrete, 14 of 23 snails loped; none loped on vertical acrylic or glass. Loping had not been recorded on vertical surfaces before.
• All 11 snails loped on wood; none loped on acrylic.

Loping is not faster

• Snails were fastest crawling on glass (mean 0.20 cm per second) and slowest loping on concrete (0.16 cm per second); crawling on acrylic averaged 0.19 cm per second.
• On concrete itself, 10 snails crawled for at least 5 cm before they started loping. Their speed was the same with both gaits (0.14 cm per second crawling, 0.15 cm per second loping).
• Snail size had no clear effect on speed.

Loping saves mucus on absorbent surfaces

• Crawling snails left a trail of about 0.85 to 0.87 cm² of mucus per centimetre on acrylic and glass. Loping snails on concrete left only 0.57 cm² per centimetre, because of the gaps in the trail.
• Even so, snails loping on concrete lost more weight than snails crawling on acrylic: 1.9 mg per centimetre of trail against 1.3 mg, and 4.4 mg against 1.2 mg per cm² of mucus left. Concrete takes far more mucus for each patch of foot contact. One loping snail lost about ten times more than the others and was left out.
• The authors calculated that a snail crawling continuously on concrete would use about 3.8 microlitres of mucus per centimetre, twice what loping snails actually used. For a 3.15 g snail moving 100 cm in a day on concrete, that would be about 380 microlitres (about 12% of body weight) crawling, but only about 190 microlitres (about 6%) loping.

Loping is widespread

All nine Rumina decollata, four Achatina fulica and many Arion ater, Deroceras and Lehmannia valentiana slugs were seen loping on concrete. In early trials, snails that were short of water did not lope more than well-hydrated ones, and snails on wet concrete did not move at all.

3. What they concluded

• In Cornu aspersum, gait depends on the surface: snails crawl on smooth, non-absorbent surfaces and lope on rough, porous, absorbent ones such as concrete and wood.
• Loping is no faster than crawling, and has never been seen in the presence of a predator, though it has been seen many times without one. Escaping predators is probably not its main purpose. Loping on old, weathered sidewalks and on wood makes avoiding irritating substances unlikely as well.
• The results fit the idea that loping saves the water or glycoproteins in mucus when snails move over absorbent surfaces.
• Loping is probably far more common in nature than the literature suggests. Most studies used glass or acrylic so researchers could see the foot, and these surfaces happen to cause crawling. Loping may be as common as crawling, or more so, in natural habitats with soil, rock, wood and dead leaves. Field studies are needed.
• Loping may have costs: extra muscular work, and weaker grip because less of the foot touches the surface. Neither has been measured.
• A gait that saves fluid could help snail-inspired robots carry less fluid or travel further.

4. How tracking was or was not used

No wildlife tracking in the usual sense. Gait was identified both by watching the foot and by reading the trail: a continuous mucus trail meant adhesive crawling, and a broken trail with gaps meant loping. For the other species, gait was judged from the arches in the foot and the form of the mucus trail. The researchers traced each wet trail with an ink marker or pencil and measured its length and area from photographs.

5. Tracker or observer certification

Not reported. The observations were made by the researchers; gait scoring was described as unambiguous for all but one snail.

6. Other technologies, skills or innovations

• A video camera above each plate, with a ruler for scale; frames overlaid in Photoshop, and trail length and mucus area measured in ImageJ.
• Weighing snails to 0.1 mg before and after moving, as a measure of mucus used.
• Testing surface absorbency by timing how long a 500 microlitre drop of water took to disappear.