Differences between Slug and Snail

Differences between Slug and Snail

Slugs and snails are terrestrial gastropod mollusks. The primary visible distinction between the two is the snail's prominent external shell. Slugs either lack a shell entirely or have a small internal one. This single anatomical difference leads to variations in their physiology, behavior, and habitat requirements. Both animals move using a muscular foot that secretes mucus to aid locomotion.

Comparison Table

Category	Snail	Slug
External Shell	Possesses a visible, coiled external shell made of calcium carbonate.	Lacks a large external shell; some species have a small, internal vestigial shell.
Defense Mechanisms	Can retract into its shell for protection from predators and dehydration.	Relies on hiding, camouflage, and producing large amounts of sticky mucus to deter predators.
Moisture Requirement	More tolerant of dry conditions due to the shell's ability to retain moisture.	Highly vulnerable to dehydration and requires a consistently moist environment.
Habitat Range	Can inhabit a wider variety of environments, including drier locations.	Generally restricted to cool, damp, and dark habitats, often burrowing in soil.
Calcium Requirement	High; requires significant dietary calcium to build and maintain its shell.	Lower; calcium is still needed for physiological functions but not for a large external shell.
Body Profile	Bulkier and less flexible due to the rigid shell.	More flexible and slender, allowing it to fit into narrow spaces.

Venn diagram comparing Differences between Slug and Snail

Anatomy and Shell

The most apparent difference is the snail's external shell, which is primarily composed of calcium carbonate and grows with the animal. This structure serves as an exoskeleton, offering protection from mechanical damage and predators, and also aids in calcium storage and muscle attachment. A snail cannot leave its shell, as it is a fused part of its body containing the visceral hump, which holds most of the digestive, reproductive, and respiratory organs.

Most slugs do not have an external shell. Some species, however, possess a small, plate-like vestigial shell that is located internally, under the mantle. This internal shell can function as a storage site for calcium salts. The absence of a large external shell makes slugs more streamlined and flexible, enabling them to hide in tight crevices and burrow underground more easily than snails.

Physiology and Behavior

The presence or absence of a shell directly influences how these gastropods interact with their environment. Snails can retreat into their shells to protect themselves from predators and to avoid desiccation during dry or hot weather. They can seal the shell's opening with a dried mucus membrane called an epiphragm to retain body moisture. This adaptation allows snails to survive in a broader range of habitats, some of which may experience periodic dry spells.

Slugs are more susceptible to drying out and must remain in damp environments. They are typically active at night or on overcast, humid days and seek shelter in dark, moist places like under logs, rocks, or in soil burrows during the day. In periods of extended drought, some slugs can encase themselves in a cocoon made of mucus and soil to wait for more favorable conditions. For defense, slugs rely on their ability to hide and their production of copious amounts of sticky mucus, which can deter some predators.