Scaphinotus bilobus (Cychrini), Canadian Biodiversity Info Facility

A variety of predators eat land snails and slugs, as they are small and slow-moving invertebrates. Abundant and nutritious prey, land snails might be considered "low" on the food chain. In response to this predation, land snails have evolved an array of sophisticated defenses.

Invertebrate predators of land snails include beetles and their larvae, millipedes, flies, mites, nematodes, and other snails. Vertebrate predators of snails and slugs include shrews, mice and other small mammals, amphibians, reptiles, and birds, especially ground-foragers such as thrushes, grouse, blackbirds, and turkey. By far the most comprehensive account of these land snail predators is found in Natural Enemies of Terrestrial Molluscs, edited by G.M. Barker (2004).

Land snail defenses against predators include cryptic coloration and texture; thickened shells and aperture barriers; increased mucus production; irritating smells and tastes; hiding behaviors, and rapid withdrawal or dislodging movements.

How predators eat land snails differs according to their size and capabilities. For shelled land snails, beetle larvae may enter the aperture of a snail's shell, while larger beetles can crush small snails. Predatory snails can rasp a hole in a victim's shell, then insert their head to feed. Small mammals will break a hole in the shell to pull out the snail's body, sometimes leaving the empty shell on a rock or log. Larger predators such as ruffed grouse can swallow even big snails whole. Slugs are attacked in a variety of ways, but effective predators attack quickly before much defensive mucus is produced.

While most land snails are herbivores or detritivores, some are specialized predators. The gray-foot lancetooth Haplotrema concavum is a large predatory land snail that attacks nematodes and other snails, and is a widespread native in Pennsylvania. This predator will insert its head into the aperture of a prey snail's shell, or can drill a hole in the shell to gain access. Pearce and Gaertner (1996) were able to predict which snail prey were most preferred by the lancetooth using information about how well-armed those prey were. The oval ambersnail Succinea ovalis has a large aperture and thin shell, and was attacked most often, but the small maze pinecone Strobilops labyrinthica with its thick shell and narrow aperture, was least preferred.

Another predator and prey relationship of note is an apparent "arms race" between land snails and Cychrine beetles (Carabidae), which feed mostly on land snails (Symondson, 2004) - while the beetles have evolved narrower heads to extract snails from their shell aperture, the snails have evolved more obstructed apertures (which would also help in defense against the lancetooth).

Cychrine beetles have an elongated head, thorax, and mouthparts that help them gain entry through the aperture of snail shells, while less-specialized beetles may simply crush snail shells when possible (e.g., Digweed, 1993). Mouthparts of the Cychrini have been described as having "hooks" or "spoons" that help extract mollusk flesh. Some snails in turn, have barriers of shell calcium, called denticles or lamellae, that partially obstruct their aperture (e.g. Solem, 1972). The plasticity of the land snail head and foot allow their protrusion through the aperture despite the obstacles. However, these "teeth" obviously thwart the entry of beetles and other small predators having hard exoskeletons. Some snails in the slitmouth genus (Stenotrema: Polygyridae) are so well armed - with a hard round shell, covered by hair-like processes, and a slit-shaped aperture blocked by lamellae - as to appear impervious to attack.

Further research is needed to demonstrate whether barrier adaptations have evolved specifically for defense purposes. Other hypothesized functions for barriers include storing calcium for other physiological needs, trapping air if the animal becomes immersed in water (Emberton, 1995); or providing leverage "handles" on the shell so it can be moved and balanced by the animal (Suvorov 1993, 1999).

Other land snail defenses include cryptic coloration that hides them from predators that search by sight. Few land snails in Pennsylvania are brightly colored, most are brown or tan, and many have finely textured shells that do not reflect light. For example, the tiny pinecone snails (genus Strobilops) are almost indistinguishable from the fruiting bodies of fungi on rotten logs. Some species such as the velvet wedge Xolotrema denotata have hairlike processes on their periostracum that hold onto dust and spider webs, making an effective camouflage.

Slug slime trail at log refuge Photo: Ken Hotopp

Another typical land snail defense mechanism is the production of mucus. Although mucus normally aids in maintaining a snail's skin and promoting locomotion, it is exuded in large amounts when a snail is attacked. The smothering or confusing effect of the sticky secretions can help to stop predators (e.g., Eisner and Wilson, 1970; Parkarinen, 1994). Many slugs have more copious and stickier mucus than shelled snails, suggesting an anti-predator function compensating for the lack of a shell. Land snail mucus may also contain substances repellent to an attacker, as this phenomenon is known for marine snails. Land snail mucus is also known for antibacterial properties, with some constituents used to treat acne.

During periods of inactivity land snails exhibit behavior that suggests they are hiding from potential predators. Land snails that live among rock talus or cliffs tend to remain immobile on ceilings, vertical surfaces and within cracks, rather than in locations that might be more accessible to small mammals. Many larger land snails move from leaf litter to sticks, logs or rocks for aestivation, though in addition to predator avoidance, this behavior may also aid in retaining moisture, avoid pooling water, or other functions.

Many shelled land snails will rapidly withdraw into their shell when attacked, or sometimes when merely sensing wind or movement. For example, the land snail Webbhelix multilineata frequently drops from low vegetation when disturbed. Large land snails (Neohelix sp.) kept in a terrarium at home will sometimes be seen rapidly twisting their shell when another snail is crawling upon it. This behavior appears to be an attempt to dislodge the attached snail, so it would be useful to look at whether that snail has actually rasped the shell or otherwise attacked the twisting animal.

Finally, if a land snail does survive a predator's attack, it has impressive powers of regeneration. Damage to shells can be repaired by the snail's shell-building mantle organ, although it often appears that repairs are most complete when the break is in the final "body" whorl of the shell. Field collectors are familiar with this phenomenon, as recovered shells often show irregularities where "patches" have been made. Land snails are also able to regenerate some tissue that has been lost, including parts of the foot and tentacles (e.g., Bobkova, et al., 2004).

Ken Hotopp and Tim Pearce 4/3/06