Art and Science of Nature
Understanding the Beauty of the Natural WorldThe Significance of Snail Shell Color and Pattern
[Adapted from an article that I co-authored entitled, "Isolation of Evolution of the Amphidromus in Nusa Tenggara" in American Conchologist magazine]
The almost 2 inch endemic Manus Island Green Tree Snail, Papustyla pulcherrima, is one of only a handful of tropical land snails that have a naturally green shell color. Photo by and courtesy of Brian Parkinson from worldwideconchology.com
The tropical biosphere is inhabited by animals of far greater color than any other zoogeographical zone on Earth. The Phylum Mollusca, and in particular the terrestrial mollusks, the land snails, develop shells that have some of the greatest diversity of color and pattern among all living organisms.
The often brilliant coloration and bold patterns attracted ancient societies who used the shells for ornamentation purposes. Today, naturalists with a bent on conchology are still amassing collections of these brilliantly colored snails for aesthetic reasons, but also for scientific study.
The question of why these molluscan species have developed a multitude of colors and patterns can be best understood through the foundations of evolutionary science.
Amphidromus adamsii, an extremely varied 25mm (± 1 inch) land snail species that may exhibit the ultimate in intra-population variability. Photo: Richard L. Goldberg.
Land snails can develop shells with myriad colors forms within one population (intra-population variability), or may vary from the norm only in separated populations (inter-population variability). Occasionally a species can exhibit both phenomena. The Darwinian concept of natural selection can best be used to explain why a snail species can vary locally or over its entire range.
Forms of the half inch-size Fly-Speckled Polymita, Polymita muscarum, a species that exhibits inter-population variability throughout its geographical range in Oriente Province, Cuba. Left - A color form found at Loma de Calixto; Right - shells from Port of Vita. The Port of Vita form also exhibits considerable variation within the population, or intra-population variability. Photo: Richard L. Goldberg.
The external appearance of a shell — color and pattern, for instance — are likely to be influenced by natural selection when determining the genetic make up of a snail. Extreme color polymorphism in a population perhaps indicates that looking different from your neighbor is advantageous, making it more difficult for predators to develop a search image, such as color and contrast, to locate their prey. The unique color form can then spread rapidly in a population.
Amphidromus laevus from Timor-Leste. An example of frequency dependent selection where the typical dark banded pattern is occasionally replaced with a unicolored shell. Photo: Richard L. Goldberg.
An example of genetic drift. Left - Amphidromus wetaranus from Wetar Island, Indonesia. Right - A form of the same species from a small satellite island off Wetar Island. The red central band found on all specimens of the satellite island form is never found on the Wetar Island form. Photo: Richard L. Goldberg.
These shells of Amphidromus columellaris from the Tanimbar Islands, Maluku Region, Indonesia show little variation within the population. Could that balance be tipped by the founder effect? Photo: Richard L. Goldberg.
Environmental factors such as the geology, climate, flora, fauna, and food are all suspected contributors to the rise of a unique shell appearance. To reduce the competition with other species which overlap in some aspect of their niches, a phenomenon called character displacement occurs. Shell, behavioral, anatomical or biochemical characters eventually deviate from the ancestral form to allow the species to survive with its neighbors. Character displacement might also manifest itself in the color, pattern, and even the form of the shell. Changes brought on by one, or a combination of, these biological concepts can eventually render an isolated population unable to interbreed with the parent population, producing a situation which makes speciation possible.
Though these concepts are often difficult to understand when evolutionary science is not your main focus, understanding that tropical snails do change and adapt to external factors and random influences is enough to appreciate just how diverse a snail species can be.
Rain at Dolphin Head
Dolphin Head in Hanover Parish is one of the most massive geological formations in western Jamaica. It is often referred to as an island within an island.
The summit of Dolphin Head
Satellite Photo of Dolphin Head, north western Jamaica. The trail to the base of the mountain is difficult to traverse. Hiking up the mountain is an extreme trip in itself!
Being A Maroon Descendant
Dowdie is a thirty-something Maroon; a modern young man who transcends the ancient traditions of his Maroon heritage and the encroachment of a modern world on his remote and insular community.
Dowdie, a modern Maroon. Photo: Richard L. Goldberg.
Maroons are a group of Jamaicans, direct descendants of escaped slaves that claimed the mountainous Leeward communities of the Cockpit Country as their own almost 300 years ago. Cutting themselves off from the outside world, the Maroons are self-governed and pay no taxes to the Jamaican government, an arrangement worked out in 1739-40 with the British after they lost the Maroon Wars.
Maroons are fiercely proud of their land and their heritage. When I met Dowdie this week in the Cockpit Country, one of his first questions to me in his English-like creole language was what do you think of “our” country? His question was aimed not at Jamaica, but at the town of Accompong, Nation of the Maroon descendants.
Now free to come and go; the main entrance to Accompong. Photo: Richard L. Goldberg
When entering the now permanently opened gates of Accompong, situated north of towns and villages with names like Maggoty and Retirement, the rural and pristine countryside transforms into a small, self-sufficient rural farming community. Neatly trimmed gardens of native plants and trees surround small concrete homes with tin roofs. A small provisions store at a three-way intersection near the center of town, a church and a local gathering place line steep, winding mountain roads. A quick wavy to the locals from the car while driving around the few roads in town receives an immediate likewise response.
Not more than 30 years ago, the gates to Accompong were closed to outsiders. Permission had to be granted to enter the community by The Colonel, leader of the Maroons. The Colonel still governs this peaceful society, but in recent years he has opened the Maroon community to tourism. Peace Corp projects and the Jamaican government encourage Jamaican visitors to see the “other Jamaica”, the Jamaica that few have ever seen. With all of this new-found openness, the Jamaican government still recognizes the Maroon’s sovereignty. No direct descendant of a Maroon living here pays taxes to the government.
The dining area at Baboo's Garden. Photo: Richard L. Goldberg
Dowdie works for Tony Kuhn, an American ex-patriot who built and operates “Baboo’s Garden”, an ecotourism enterprise on a Cockpit mountainside north of Accompong. Four years ago the Maroons granted Tony a 30-year lease on the land. Baboo’s Garden grew from here. His place is immersed in nature. Thatch-roof bungalows with tents inside are built into massive limestone outcrops. Tony’s vision and Dowdie’s Maroon sensibilities made Baboo’s Garden a most unique experience for guests like myself and my travel companions Alan Gettleman, a recently retired NASA employee and an orchid aficionado, and Homer Rhode, best known as the namesake of the “Homer Rhode knot”, well-used by fly fishermen.
Orthalicus undata jamaicensis, an arboreal molluscan species with a ± 2 inch shell, uncommonly found crawling on Gumbo Limbo trees at night. Photo: Richard L. Goldberg
Dowdie took an immediate interest in our like-minded pursuit of Jamaica’s terrestrial mollusks. And why not! For his entire life Dowdie has been exposed to the unique natural character of the Cockpit Country and its many endemic species of plants and animals. But he knew little about mollusks. The few larger species that are commonly found crawling on the coconut, mango and Gumbo Limbo trees were the only snails he was familiar with. The smaller, more difficult-to-find ground dwelling and limestone loving snails were new to him.
Dowdie joined us during one of our treks deep into the Cockpit Country north of Quickstep, a small village northeast of Accompong. He had never been this far into the Cockpit Country. Using his keen sense of observation Dowdie became an expert at looking for the small snails.
Tudora humphreysiana, a rare ± 15mm operculate terrestrial mollusk from the Cockpit Country of Jamaica. Photo: Richard L. Goldberg
The habitat-specific ground dwelling species found under leaf litter and those species living in fallen rain forest debris on narrow ledges of vine-entangled cliff faces of the Cockpits exposed Dowdie to yet another aspect of his natural world.
As the day wound down and the afternoon rain saturated the Cockpits with its daily soaking of life-giving moisture, Dowdie showed us a two-foot long, single stalk ground plant that he unearthed at the base of a limestone cliff. He did not recognize the plant species with its lance-shaped leaves, iridescent purple on the top and blood red underneath. With a smile and lilt in his voice he quipped, “could this be a new species?”
Dowdie's botanical find at Quickstep. Photo: Alan Gettleman
Over dinner that evening Dowdie relayed a story that seemed far removed from his idyllic crime-free world. Two weeks prior to our arrival he drove with a friend to a small town far west of Accompong. Call it a case of bad timing, but he was caught in the crossfire of a drug-related altercation and was wounded by three bullets to his torso.
The Cockpit Country, home to Jamaica's Maroons and one of the greatest concentrations of biodiversity in the world. Photo: Richard L. Goldberg
How could this have happened? And why did he not show the signs of such trauma? Dowdie was lifting and climbing as if nothing had ever happened to him. He did not complain or make light of his wounds. His take on the incident is that the Lord had a reason to spare him at that moment. If true, Dowdie was spared so that he can continue his Maroon heritage and ensure that he plays a part, hopefully great, in the survival of the natural world of the Cockpit Country.
The Importance of Limestone
Pleurodonte (Dentellaria) bronni (Pfeiffer, 1846) - 18mm diameter; Photo: Richard L. Goldberg.
The habitat of Pleurodonte bronni; limestone karst shaded by foliage; Photo: Richard L. Goldberg
The Significance of Shell Form
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
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.
