Molluscs - Phylum Mollusca

Molluscs (Phylum Mollusca) are invertebrates that include animals such as squid, octopuses, cuttlefish, nudibranchs, snails, slugs, limpets, sea hares, mussels, clams, oysters, scallops, as well as many lesser known creatures. There are an estimated 100,000 species of molluscs making them second largest phylum in the animal kingdom, having fewer species than only the Phylum Arthropoda.

Molluscs have soft bodies that consist of three basic parts, a foot, a visceral mass, and a mantle. Because molluscs are very diverse and come in many different shapes and sizes it is difficult to make too many generalizations about the group's anatomical structure beyond these three basic parts.

The foot functions in locomotion and in molluscs that posses a shell, the foot can often be drawn inside. The visceral mass, located above the foot, contains the digestive system, the heart, and other internal organs. Finally, the mantle is a layer of tissue that covers the visceral mass and in many molluscs it contains glands which secrete a hard shell.

The majority of marine molluscs begin their life as ciliated, free-swimming larvae that later develop into adult form. Freshwater and terrestrial snails develop within the egg and emerge as tiny but fully-formed versions of the adult form. Molluscs are most diverse in marine habitats but also inhabit freshwater and terrestrial habitats.

Classification:

Kingdom: Animalia
Phylum: Mollusca
The Phylum Mollusca contains the following subgroups:
- Class: Aplacophora (solenogasters, deep-sea wormlike creatures)
- Class: Bivalvia (clams, oysters, scallops, mussels)
- Class: Cephalopoda (squid, octopodes, nautilus, cuttlefish)
- Class: Gastropoda (nudibranchs, snails, slugs, limpets, sea hares, and relatives)
- Class: Monoplacophora (deep-sea limpet-like creatures)
- Class: Polyplacophora (chitons)
- Class: Scaphopoda (tusk shells)

Gastropods (Class Gastropoda) include a wide variety of organisms such as terrestrial snails and slugs, sea slugs, sea hares, sea butterflies, canoe shells, conchs, whelks, limpets, periwinkles, oyster borers, coweries, nudibranchs, and many others. The Gastropoda group is the largest of all mollusc groups, with approximately 40,000 living species and fossil evidence of over 15,000 extinct species.

The unifying characteristics of gastropods include:

most have a single spirally coiled shell
undergo torsion during development
posess a mantle cavity and mantle
posess a muscular foot
most are dioecious but some (such as snails) are hermaphroditic

Classification:

Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
The Class Gastropoda contains the following subgroups:
- Subclass: Opisthobranchia (opisthobranchs)
- Subclass: Prosobranchia

Structure of a Snail Shell

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Helix Garden Snail (Helix pomatia)

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Shell Structure

A snail's shell is secreted by glands in its mantle rim and consists of three layers:

hypostracum - the innermost layer, closest to the snail's body
ostracum - middle, shell building layer, consists of prism-shaped calcium carbonate crystals and organic (proteid) molecules
periostracum - outermost layer, consists of conchin (a mixture of organic compounds), gives the shell color

Shell Function

The snail's shell serves to protect its internal organs, prevent water loss, provide shelter from cold, and protect the snail from predators.

Snail Classification

A Natural History of Terrestrial Snails

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About Snails

Snails are invertebrates, which means they lack a backbone. They belong to a large and highly diverse group of invertebrates known as the Phylum Mollusca (also known more commonly as 'mollusks'). The Phylum Mollusca includes slugs, clams, oysters, mussels, squids, octopuses, and nautiluses, in addition to snails.

Scientific Classification

In scientific terms, snails are classified into the following hierarchy of animal groups:

Kingdom: Animalia (Animals)
Phylum: Mollusca (Mollusks)
Class: Gastropoda (Gastropods)
Order: Stylommatophora (Terrestrial Snails and Slugs)

What Classification Reveals About Terrestrial Snails

Each of these levels of classification tells us a bit about snails in increasingly specific terms. For example, because snails are animals, they possess the characteristics shared by most animals: they are multicellular and are heterotrophs.
Since terrestrial snails are classified as mollusks and gastropods, they posses the typical characteristics of those groups too. They have a single, often spirally coiled shell (univalve), they undergo a developmental process called torsion, and they possess a mantle and a muscular foot used for locomotion.
Finally, because terrestrial snails are grouped with terrestrial slugs into the Order Stylommatophora, they share some similarities with terrestrial slugs. Because sea slugs and sea hares are in a different order, terrestrial snails have less in common with sea slugs and sea hares than they do with terrestrial slugs. Members of the Order Stylommatophora are characterized by having eyes on the top of tentacles (sea snails have eyes at the base of their tentacles).

Pulmonates

Terrestrial snails belong to a group within the Order Stylommatophora called the 'pulmonates'. This classification level is as yet unassigned (it is not officially a family) but the term pulmonate is used to refer to air-breathing land snails.

Snail Diet and Feeding Habits

A Natural History of Terrestrial Snails

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Diet

Terrestrial snails are herbivorous. They feed on plant material (such as leaves, stems, and soft bark), fruits, and algae.

Mouthparts

Snails have a rough tongue called a radula that they use to scrape bits of food into their mouths. They also have rows of tiny teeth made of chiton.

Calcium

Snails need calcium to build their shells. Snails obtain calcium from a variety of sources such as dirt and rocks (they use their radula to grind bits from soft stones such as limestone). The calcium snails ingest is absorbed during digestion and is used by the mantle to create the shell.

Snail Habitat / Adaptation

A Natural History of Terrestrial Snails

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Habitat

Snails first evolved in marine habitats and later expanded into freshwater and terrestrial habitats. Terrestrial snails live in moist, shady environments such as forests and gardens.

Adaptation

A snail's shell provides it with protection from changing weather conditions. In arid regions, snails have thicker shells that help them retain their body moisture. In humid regions, snails tend to have thinner shells. Some species burrow into the ground where they remain dormant, waiting for rain to soften the ground. In cold weather, snails hibernate.

How Snails Move

Natural History of Terrestrial Snails

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How Snails Get Around

Terrestrial snails move using their muscular foot. By creating an undulating wave-like motion along the length of the foot, a snail is able to push against a surface and propel its body forward, albeit slowly. At top speed snails cover a mere 3 inches per minute. Their progress is slowed by the weight of their shell. In proportion to their body size, the shell is quite a load to carry.

A Trail of Slime

To help them move, snails secrete a stream of slime (mucus) from a gland located at the front of their foot. This slime enables them to glide smoothly over many different types of surface and helps to form a suction that helps them cling to vegetation and even hang upside down.

Refs:

For a list of sources used in the creation of this visual guide, please see the Recommended Reading page.

Snail Life Cycle / Development

A Natural History of Terrestrial Snails

Photo © Milos Luzanin / Shutterstock.

From Egg to Adult

Snails start life as an egg buried in a nest a few centimeters below the surface of the ground. Snail eggs hatch after about two to four weeks depending on the weather and environmental conditions (most importantly, temperature and soil moisture). After hatching, the newborn snail sets out on an urgent search for food.
The young snails are so hungry, they feed on the leftover shell and any nearby eggs that have not yet hatched. As the snail grows, so does its shell. The oldest part of the shell is located at the center of the coil while the most recently added parts of the shell are at the rim. When the snail matures after a few years, the snail mates and lays eggs, thus completing the full life cycle of a snail.

Snail Senses

A Natural History of Terrestrial Snails

Photo © Milos Luzanin / Shutterstock.

Sight

Terrestrial snails have primitive eyes (referred to as 'eyespots') that are located on the tips of their upper, longer pair of tentacles. But snails don't see in the same way we do. Their eyes are less complex and provide them with a general sense of light and dark in their surroundings.

Touch

The short tentacles located on a snail's head are very sensitive to touch sensations and are used to help the snail build a picture of its environment based on feeling nearby objects.

Hearing

Snails don't have ears but instead use their bottom set of tentacles to pick up sound vibrations in the air.

Evolution of Snails

Natural History of Terrestrial Snails

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Early Snails

The earliest known snails were similar in structure to limpets. These creatures lived in shallow sea water and fed on algae and they had a pair of gills. The most primitive of the air-breathing snails (also called pulmonates) belonged to the Family Ellobiidae. Members of this family still lived in water (salt marshes and coastal waters) but they went to the surface for air. Today's land snails evolved not from this group but from the Family Endodontidae, a group of snails that had similar characteristics to the Family Ellobiidae.

Tendencies in Snail Evolution

When we look back through the fossil record, we can see various tendencies in how snails changed over time. In general the following patterns emerge:

the process of torsion becomes more prominant
the shell became increasingly conical and spirally coiled
there is an evolutionary tendency among pulmonates towards the entire loss of a shell

Snail Estivation and Hibernation

A Natural History of Terrestrial Snails

Photo © Alexey Biryukov / Shutterstock.

Estivation

Snails are usually active in the summer, but if it gets too warm or too dry for them, they enter a period of inactivity known as estivation. They find a safe place—such as a tree trunk, the underside of a leaf, or a stone wall—and suction themselves onto the surface as they retreat into their shell. Thus protected, they wait until the weather becomes more suitable. Occasionally, snails will go into estivation on the ground. There, they go into their shell and a mucous membrane dries over the opening of their shell, leaving just enough space for air to get inside allowing the snail to breath.

Hibernation

In late fall when temperatures drop, snails go into hibernation. They dig a small hole in the ground or find a warm patch, buried in a pile of leaf litter. It is important that a snail finds a suitably protected place to sleep to ensure its survival through the long cold months of winter. They retreat into their shell and seal its opening with a thin layer of white chalk. During hibernation, the snail lives on the fat reserves in its body, built up from a summer of eating vegetation. When spring comes (and with it rain and warmth), the snail wakes and pushes the chalk seal to open the shell once again. If you look closely in spring, you may find a chalky white disc on the forest floor, left behind by a snail that has recently come out of hibernation.

How Large Do Snails Grow?

Natural History of Terrestrial Snails

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Snail Size

Snails grow to a variety of different sizes depending on the species and individual. The largest known land snail is the Giant African Snail (Achatina achatina). The Giant African Snail has been known to grow to lengths of up to 30cm.

Snail Anatomy

A Natural History of Terrestrial Snails

Photo © Anette Linnea Rasmussen / Shutterstock.

Parts of the Body

Snails are very different from humans so when we think about body parts, we're often at a loss when relating the familiar parts of a human body to snails. The basic structure of a snail consists of the following body parts:

foot
head
shell
visceral mass

The foot and head are the parts of the snail's body that we can see outside its shell, while the visceral mass is located within the snail's shell and includes the snail's internal organs.

Internal Organs

A snail's internal organs include:

a lung
digestive organs (crop, stomach, intestine, anus)
a kidney
a liver
reproductive organs (genital pore, penis, vagina, oviduct, vas deferens)

Nervous System

A snail's nervous system is made up of numerous nerve centers that each control or interpret sensations for specific parts of the body:

cerebral ganglia (senses)
buccal ganglia (mouthparts)
pedal ganglia (foot)
pleural ganglia (mantle)
intestinal ganglia (organs)
visceral ganglia

Reproduction in Snails

A Natural History of Terrestrial Snails

Reproduction

Most terrestrial snails are hermaphroditic which means that each individual possesses both male and female reproductive organs. Although the age at which snails reach sexual maturity varies among species, it may be up to three years before snails are old enough to reproduce. Mature snails begin courtship in early summer and after mating both indivudals lay fertilized eggs in nests dug out of moist soil. It lays several dozen eggs and then covers them with soil where the stay until they are ready to hatch.

Vulnerability of Snails

A Natural History of Terrestrial Snails

Living in a Dangerous World

Snails are small and slow. They have few defenses. They must retain enough moisture so their tiny bodies don't dry out, and they must obtain enough food to give them energy to sleep through the long cold winter. So despite living in tough shells, snails are, in many ways, quite vulnerable.

Smart Ways Snails Protect Themselves

The good news is that snails are quite clever and are well adapted to deal with the threats they face. Their shell provides them with good, impenetrable protection from weather variations and some predators. During the daylight hours, they usually hide. This keeps them out of the way of hungry birds and mammals and also helps them conserve moisture.

Battling Humans

Snails aren't too popular with some humans. These little creatures can to quickly eat their way through a carefully tended garden, leaving a gardener's treasured plants all but bare. So some people leave poisons and other snail-deterents around their yard, making it very hazardous for snails. Also, since snails don't move quickly, they are frequently in danger of crossing paths with cars or pedestrians. So be careful where you step if your walking on a moist evening when snails are out and about

Snail Strength

A Natural History of Terrestrial Snails

Slow but Strong

Snails can haul up to 10 times their own weight when crawling up a vertical surface. When gliding along horizontally, they can carry up to 50 times their weight.

Longevity of Snails

A Natural History of Terrestrial Snails

How Long Do Snails Live?

Snails live on average 5 to 10 years, though some have been known to live up to 15 years.

Predators

A Visual Guide to Terrestrial Snails

Predators

Snails must try their best to steer clear of a wide variety of predators. Animals that feed on snails include birds (such as blackbirds and song thrushes) and mammals (including rats, mice, moles, rabbits, hedgehogs, foxes, and humans).

Malacology

A Visual Guide to Terrestrial Snails

What is Malacology?

Malacology is the study of mollusks. A malacologist is a scientist whose research focuses on mollusks (and this includes the study of snails).

Glossary of Snail Terms

Natural History of Terrestrial Snails

Glossary

The following list of words highlights the terms used in this visual guide used to describe different aspects of snails:

A Visual Guide to Terrestrial Snails

References:

The following books and websites were used in the creation of this visual guide:

Allen J, Humphries T. 2000. Are You a Snail? New York: Kingfisher.
Brosi A. The Trail of the Snail. March 24, 2006.
Buholzer T. 1987. Life of the Snail. Minneapolis: Carolrhoda Books Inc.
Hickman C, Roberts L, Larson A. Integrated Principles of Zoology. McGraw Hill. 2001.
Snail. Wikipedia. March 26, 2006.
A special thanks to Shutterstock.com and its photographers for the wonderful photos used to illustrate this visual guide. For information about individual image owners, please refer to the copyright information posted just below each photo.

Monday, August 16, 2010