Snails
See how they move!
SCIENTIFIC NAME: Gastropoda
Number Of Species/Distribution
There are about 70,000 different species in the Gastropod family worldwide. These include all terrestrial, marine, and freshwater snails and slugs.
Of those 70,000 species, about 5,000 live in the seas, oceans, brackish waters, and freshwaters; the remaining species live on land.
Gastropods can be found living everywhere from the tropics to the Arctic and Antarctic.
Diet/Feeding
Most freshwater gastropods are herbivorous; eating only plant material.
Freshwater snails feed mainly on algae growing on the surface of the substrate.
Snails and other gastropods feed by using a mechanism called a 'radula'.
The radula works to collect the food from the substrate surface, acting like a conveyer belt in order to bring the bits of algae into the back of mouth.
The radula may have hundreds of rows of teeth and over a hundred teeth per row.
If it breaks off a piece of food too big, the gastropod can further break it down into smaller pieces by rubbing the piece of food along a hard plate on the roof of its mouth.
Different snail species have radulas that are adapted specifically to collect and break down the species' preferential food items.
Habitat
Snails require very specific environmental conditions in order to survive. The most critical are the pH level of the water (also known as the acidity), and the calcium carbonate concentrations in the water.
Snails need calcium carbonate in order to develop shells.
45 percent of freshwater snails can be found in water that has a calcium concentration of 20 mg/litre or more.
95 percent of snails require a concentration of at least 3 mg/litre.
Snails and other gastropods have become adapted to every medium except air (there are no flying snails that we know of).
Where there is not enough calcium carbonate in the water to build shells, there are slugs.
Freshwater snails like a dependable and consistent habitat.
As a result, regions in which water is prone to drying up or freezing during summer and winter months contain fewer snail species.
Aquatic snails only move great distances if they need a better food source, but they frequently get swept away downstream when there is a strong current.
SNAILS: INTERESTING FACTS
The name gastropoda comes from the Latin words 'Gasto', meaning stomach, and 'poda', meaning foot.
The body of a snail has two main parts: the mantle and the foot. The foot is the fleshy muscle that comes out from the bottom of the shell in order to help the snail move. The mantle is the part of the snail that secretes the shell.
The foot muscle of the snail moves in a wave-like motion to pull the snail across the substrate.
Snails that live on land have eyes on stalks, whereas aquatic snails have eyes directly on the head.
Many different species of snails can live in the same habitat without competing against each other. This is possible because different species have different feeding preferences.
Aquatic snails usually move very slowly. When under attack from predators, however, they can move very quickly by 'walking' on top of the water's surface!
Some species of snails have a second 'door' on the shell, called an operculum, in addition to the main exit from which the foot muscle protrudes.
During development, the body of the snail gets twisted 180 degrees counter-clockwise inside the shell. This causes the anus of the snail to become situated directly over top of its head!
The use of 'love darts' during mating appears to have evolved not for reproductive reasons, but rather as courtship behaviour.
Life Cycle
Young freshwater snails look like smaller versions of adults. They have shells throughout their development, and are never without one.
Some snail species, such as those in marine environments, lay fertilized eggs.
Others, such as freshwater snails, give birth to live young.
Aquatic snails develop quickly and mate while they are young in order to make up for not having a larval stage.
Most aquatic snails have separate male and female sexes.
The males often have a mating organ, a penis, or a tentacle, that is used to deliver the sperm to the female.
Gastropods often take part in courtship to win the affection of a mate.
Some terrestrial gastropods 'shoot' what are called 'love darts' at their potential mate.
Females of some species can also reproduce by a process called 'parthenogenesis', which means they fertilize their own eggs.
Scientists believe this mating behaviour evolved so that the organism could still reproduce in regions where mates were scarce.
Role in Food Chain
Snails are a highly sought-after food source for many predators. The predators can be divided into two different groups: the shell crushers and the shell invaders.
Shell-crushing predators include some bigger species like pumpkinseed sunfish and mud minnows.
These species are strong enough to use their teeth to crack open snail shells and eat the snail inside.
Some crayfish can use specialised appendages, called 'mandibles', to break the shells.
Shell-invading predators include smaller crayfish, leeches, and giant water bugs.
These organisms are not strong enough to crush shells. Instead, they eat the snails by pulling them out of their shells.
To avoid being eaten, snails will build the hardest shells possible. This is why they require high levels of calcium carbonate to be present in the water.
Sources
Absolute Astronomy (2010). Gastropoda. Available here.
Brusca, R. C, and Brusca, G. J., 2003. Invertebrates, 2nd ed. Sinauer Associates, Inc., Publishers, Massachusetts. Page 703.
Canada's Aquatic Environments (2002). Ephemeroptera. Available here.
University of Alberta, Department of Biological Sciences (2010). Gastropoda (Snails).