Snails are a group of shelled mollusks with species found in many habitats such as freshwater, seawater and land. Land snails, although better known than other types, represent only a small group. Marine snails are the most numerous and biodiverse group.

All snail species need oxygen, food, water and adequate temperature and humidity to live. Present in human life since ancient times, many terrestrial snails are considered garden pests. Snail eating is common in some cultures and the creatures can be farmed as gastronomic delicacies.

Snails are omnivorous animals, which means they can eat food from plant and animal sources. Snail food can come from a variety of materials, such as decomposing plants and invertebrates, and any type of leaf or algae.

However, different snail species can have distinct food preferences.

The giant African land snails (Achatina fulica) -- which are popular pets in some countries but also serious agricultural plagues and illegal to keep in the U.S. -- can eat any food crop, including lettuce, cucumber and cabbage.

The wood snail feeds mainly on decaying organic matter, nettles and buttercups, while water snails eat small invertebrates and algae.

Like most animal species, snails need oxygen to survive. Most land snails, and some marine and freshwater species, have a single lung, where the exchanges between oxygen and carbon dioxide occur.

Aquatic species have to come to the surface to breathe, in order to take the atmospheric oxygen. Pond snails, bladder snails, ram's horn snails, the common land snail and freshwater limpets are examples of snails that breathe through lungs.

Some snails, such as water nerites, bithynias and mud snails, have gills instead of lungs, and can only take the oxygen dissolved in the water.

Like most living creatures, both land and water snail species need to drink water to survive.

Land snails drink from small puddles formed on leaves or on the ground, but they also get their water from the juicy leaves they eat.

Marine species take saltwater when feeding, but have an excretory mechanism to eliminate excessive salt quantities they ingest.

Optimum temperatures vary according to species, but most land snails prefer warm temperatures from 65 to 80 degrees Fahrenheit and high-humidity environments. Sphincterochila boissieri, which is found in Egypt and Israel, and can withstand temperatures of up to 120 degrees Fahrenheit.

However, this species lives in a dormant state for most of the time, becoming active only after the rain. Some species of the genera Arion and Deroceras are found in temperate climates, but are also adapted to live in polar climates.

Terrestrial snails, as their name suggests, live on land but that land must be able to meet the snail's needs. Forest floors, gardens and marshes all provide moist conditions for land snails to thrive. Hiding under leaf litter or at the base of leafy plants allows snails to avoid the drying effects of the sun.

Freshwater snails can live in lakes, rivers and even man-made ponds. The bottom sediments, or substrates, of these water bodies influences which snail species can survive there. If you are preparing a pet snail habitat, ensure that the substrate is adequate for your snail species.

From tropical coral reefs to the dark, cold oceans depths, marine snails are found in a variety of saltwater habitats. Here they can feed on algae and decaying marine plants. Some are even carnivorous.

Habitats within the ocean vary by species. Some are free floating, like plankton while others, like the purple sea snail, remain attached to various sea creatures.