On the Trail of a Snail

Whatever the time of year, we always find something of interest on our nature walks. A few days ago we found ourselves on the trail of a snail, which crossed right in front of us. It glided slowly but stealthily over twigs and needles, toting a brown spiral shell on its back. When I got home, I pulled out my copy of “Land Snails of British Columbia” by Robert Forsyth, an excellent guide to our provinces’ slugs and snails, to learn more about this quiet but compelling creature.

In evolutionary terms, slugs and snails are very successful, with as many as 35,000 species of land dwelling slugs and snails in the world, and 94 species in BC.

Snails belong to a large group of marine and land-dwelling creatures called molluscs. The word mollusc comes from the Greek word “mollis” meaning soft, in reference to the soft-fleshed body.

Within the molluscs, the land snails and slugs are classified as gastropods, which literally means, “stomach-foot.” The stomach, called the “visceral mass” is positioned on top of a muscular foot. Wave-like contractions along the sole of the foot enable to the snail to move forward. Snails glide easily over textured surfaces by secreting mucous to make a slime-trail.

On the snail’s head are two pairs of tentacles. The longer pair are the ocular tentacles, tipped with eyespots used to detect light, shadow and movement. The second pair, called sensory tentacles, are positioned near the ground and have chemoreceptors. Some snails have a “homing” ability, and can return to a desired location or sheltering spot.

In the snail’s mouth is the radula, a ribbon-like tongue with many rows of tiny teeth. Snails eat fungi, plants and other living or dead organic material, and some will eat carrion if they happen upon some.

A fold of tissue called a mantle envelopes the snail’s visceral mass. The mantle lines the inside of the shell and forms a narrow fold along the shell’s opening. The shell has an outer organic layer, and an inner layer made mostly of calcium carbonate. Shells protect the snail from predators, and also prevent water loss.

The different colours, shapes and patterns on the shells are part of what makes snails so fascinating. They can be glossy or dull, have a uniform colour or be patterned, and have surface sculpturing, such as ribs, or be smooth. Some snail shells are flat discs, while others are spindle shaped. There are endless variations.

Each full coil of the snail’s tube is called a whorl. Flipping over a snail, one may see the umbilicus, which is the entrance to the often hollow central pillar, or columella. The spiral lines visible on the shell’s surface, which separate the whorls, are called sutures. The opening of the last whorl is the aperture, where the animal emerges.

Many aquatic snails, and some land snails (mostly tropical) have gills. In our area, the land snails all have a lung for respiration, and are called Pulmonates. The lung is positioned like a small sac in the mantle’s cavity.

Finding a mate can be a challenge for these slow-moving creatures, so most are hermaphrodites, possessing both sperm and eggs. Some snail species are also capable of self-fertilization. Snails lay eggs in a small depression in the ground, or in leaf litter. The whitish eggs are usually round and soft, and are laid in clusters, or singly.

Snails occupy a surprisingly diverse range of habitats, as long as the basic needs of moisture, shelter and food are met. Drying out is of particular concern to these soft, slimy creatures. Generally, snails don’t venture out during dry conditions, and many seek out moist microclimates, such as underneath a piece of wood or some leaves.

Birds, small mammals, amphibians, reptiles and even some insects all eat snails. For defense, snails withdraw into their shells, and some give off a repellant odour.

After finding one snail, we were able to find a few more species. Our chance-encounter snail turned out to be a Pacific sideband (Monadenia fidelis). This brown snail has a narrow, yellow band around its periphery. The animal itself is a rosy brown colour. This is a common native species in mixed forests.

We also found in an introduced species called the grovesnail (Cepaea nemoralis). The shell has striking dark spiral bands on an orange or yellowish background. The grovesnail is native to Europe and is now common in North America. There are many introduced snails and slugs in BC, which likely hitchhiked here on exotic garden plants.

The third snail, also an introduced species, was the tiny (seven millimeters wide) garlic glass-snail (Oxychilus alliarius). We found it under a piece of wood in our backyard. The shell is a glossy, translucent brown and the animal is very dark, nearly black. It emits a strong garlic odour.

Now that I know more about these creatures, I’m looking forward to my next snail encounter. To learn more about snails visit Robert Forsyth’s web site at Mollus.ca

Special thanks to Hugh Griffiths for the photograph of the grovesnail. To visit Hugh’s blog go to Rock Paper Lizard.

Filed under Nature Writing | Tags: British Columbia, Snails | Comment (1)