Art and Science of Nature

Understanding the Beauty of the Natural World

Archive for Endemism

The Significance of Shell Form

Shielded from the Rain - Adamsiella pearmanaeana - 20mm. Photo: Richard L. Goldberg

Shielded from the Rain - Adamsiella pearmanaeana (Chitty, 1853) - 20mm. Photo: Richard L. Goldberg

This is not your run-of-the-mill helical garden snail trucking along in your backyard at a snail’s pace. The species with its other-worldly form is Adamsiella pearmanaeana (Adam-see-el-la pear-man-e-ana), found only in a small area within Trelawny Parish, Jamaica, another of Jamaica’s exotic endemics.

Scientists use fancy terminology to describe the sometimes bizarre ornamentation created by snails. In this case, flaring peristome (expanded lip), strongly ribbed whorls, and deep impressed sutures are just some of the many terms that help to identify a species.

Species descriptions are an essential part of animal identification. For mollusks like the terrestrial Adamsiella pearmanaeana, close observation and comparisons of the shell and animal characteristics between specimens of the same species and similar species help scientists to better understand the relationship among and between snails. If the form and structure is unique, it is then classified as a distinct species and placed in a genus with species of similar ilk. The process of observation, description and classification spans all living organisms. It is the methodology of biological sciences.

After a snail is categorized, the next logical question to ask might be, “What purpose does the exotic ornamentation serve? Why do snails need flaring peristomes?”

At best, scientists can only speculate about the functionality of shell form through observation of a species in situ (in its natural habitat) and comparison of form and functionality with similar species. Field studies become an essential part of gaining a broader understanding of a species like Adamsiella pearmanaeana.

For instance, the flaring lip is like a wide brim of a hat. In fact, scientists believe that the lip provides the snail protection when it is sealed to a hard surface, acting as a barrier around the aperture or opening to the shell (sometimes referred to as the mouth of the shell). But wait, there’s more.

Why the notch along the inner lip? And why is the tip of the spire truncate or broken off? Here’s where it gets complicated.

Okay. Let’s gather some facts. We know that Adamsiella pearmanaeana is an operculate snail. Through observation we find that when aestivating, its shell is tightly attached to a limestone rock and the operculum is set in place in the aperture. The snail is able to slightly move out the plug-like operculum to allow air and moisture in and wastes out.

Top Arrow: opening in spire.  Bottom Arrow: notch in lip and opening to hollow column from umbilical region of shell.  Photo: Richard L. Goldberg

Top Arrow: opening in spire. Bottom Arrow: notch in lip and opening to hollow column from umbilical region of shell. Photo: Richard L. Goldberg

Now, if you view the shell from the top down into the truncate spire, it is hollow. The spiral shell is wrapped around an open column that leads to the notch in the lip at the bottom of the shell. Essentially the notch provides a pathway from the aperture to the central column leading up to the opening in the spire; the shell structure seemingly provides a protected pathway for the snail to have contact with the outside world while keeping out beetles and other insects that prey on the snail. The shell then acts as a barrier and breathing tube. Seems plausible to me!

If, in fact, Adamsiella pearmanaeana is one of thousands of examples of how shell form has had a direct role in protecting a species from its predators, then it is no wonder that these extreme forms have played a significant role in species survival for tens of thousands of years.