Snails are often pictured carrying their spiraled shell on their back, a common sight in gardens and along shorelines. This leads many to wonder: Do all snails possess an external shell?
The Defining Feature of Snails
Snails are classified as gastropods, a large and diverse group within the phylum Mollusca. For most commonly recognized snails, the external, coiled shell is a characteristic trait. This shell is an exoskeleton, primarily composed of calcium carbonate, and is secreted by a specialized tissue called the mantle. It serves several functions, including protection from predators and mechanical damage, and preventing desiccation, or drying out. As the snail grows, its shell also expands, with new material continually added to the shell’s opening.
Slugs: Shell-less Relatives
While most snails have a visible external shell, their close relatives, slugs, differ significantly. Slugs are also gastropods, but they have either completely lost their external shell or possess a greatly reduced, often internal, shell. This shedding of the shell has allowed slugs to adapt to different lifestyles and environments.
The absence of a large external shell brings both disadvantages and benefits. Slugs are more vulnerable to predators and are at higher risk of desiccation compared to their shelled counterparts. However, lacking a bulky shell provides slugs with flexibility and access to smaller spaces that shelled snails cannot utilize. In some slug species, a small, remnant internal shell still exists, often serving as a calcium storage site rather than a protective barrier.
Beyond the External Shell
The presence or absence of an external shell among gastropods is complex, reflecting their evolutionary diversity. Beyond slugs, some gastropods have internal shells or shells so reduced they are not immediately apparent. For instance, nudibranchs, a group of marine gastropods often called sea slugs, are entirely shell-less as adults, shedding larval shells during development. These creatures have evolved alternative defense mechanisms, such as camouflage or chemical deterrents, in place of a physical shell.
Other examples include certain terrestrial slugs that retain a small, vestigial internal shell, which is a remnant of their shelled ancestors. Additionally, some gastropods are termed “semi-slugs” because their shell is present but too small to allow the animal to fully retract inside. This spectrum of shell development highlights that the reduction or loss of an external shell has occurred multiple times across different gastropod lineages.