Truncatella caribaeensis and the other species of terrestrial snails in the family Truncatellidae have a unique mode of locomotion: they move by alternate stepping motions of the snout and foot. The specimen above (shell length = 6.6 mm) was photographed while taking a "step" forward with its pink snout extended and its foot trailing behind.
Henry Pilsbry1 gave a detailed description of the looping of Truncatella, including the stages of one step:
The progression begins by a stretching forward of the proboscis [snout], its tip is then applied to the surface moved over, and this tip flattens out until nearly the size of the foot. If the proboscis secures a firm attachment, the foot may now be released, and either drawn up to the attached proboscis by sliding the edge of the foot along the surface, or the entire animal may be supported upon the proboscis and the foot raised clear of the surface and drawn up to the proboscis , when its edge will rest on the surface moved over.
The series of pictures below show the stages from a to e constituting one step and taking place in about 3 seconds. In stage e the snail is about to or has just lifted its snout off the surface to take another step forward. The operculum is partially visible right behind the foot. Pilsbry also noted that "the entire cycle of movements comprising the 'step' is executed in four seconds or less".
According to Gary Rosenberg2, the looping of Truncatella, rather than crawling on a mucus layer like most other terrestrial gastropods, is a mucus-saving adaptation to terrestrial life.
1. Pilsbry, H.A. 1948. Land Mollusca of North America (north of Mexico ). Volume 2, part 2, p. 1066.
2. Rosenberg, G. 1996. Independent evolution of terrestriality in Atlantic truncatellid gastropods. Evolution 50:682-693.