Gastropod

Gastropods are a very large and diverse group of animals that belong to the phylum Mollusca. They include creatures known as snails and slugs. Gastropods are the second-largest animal class after insects. Their fossil record shows they have been around since very ancient times.

Gastropods live in almost every type of place you can think of. You can find them in the ocean, in freshwater like rivers and lakes, and on land in gardens, woodlands, deserts, and mountains. They live in many habitats, from areas where the ocean meets the land to very deep parts of the sea.

The word “snail” usually means a gastropod with a big shell that its body can hide inside. Slugs either have no shell or a very small one inside their bodies. Marine gastropods include animals like abalones, conches, periwinkles, whelks, and cowries, which usually have shells that spiral. Limpets are different because their shells are only conical and do not spiral. Because gastropods are so diverse, they show many differences in their anatomy, behavior, feeding habits, and reproduction.

Etymology

The word gastropod comes from Greek words for "stomach" and "foot". The name describes how a gastropod's "foot" is under its body.

Earlier, these animals were called "univalve" because they usually have one shell. This is different from bivalves like clams, which have two shells.

Diversity

Gastropods, like snails and slugs, are very diverse animals. They have more species than almost any other group except insects. Scientists think there are between 50,000 and 120,000 known species of gastropods, with even more still unknown, especially in the deep sea.

Many gastropods live in the ocean, especially near the edges of continents. Some live in freshwater, and others live on land. Sadly, some gastropod species have disappeared, and others are in danger of being lost.

Habitat

Main articles: sea snail, sea slug, Terrestrial animal § Gastropods, land snail, semi-slug, and slug

Gastropods live in many places, both on land and in water. You can find them in the deep ocean, in fresh water, and even in deserts. Most gastropods live in the sea, but many also live on land as snails and slugs.

These animals have adapted to live in many kinds of environments, from cold polar regions to warm tropical areas. Some snails have thin shells or no shells at all. Others live in extreme places like deep ocean trenches or hot hydrothermal vents. Birds can sometimes carry gastropods from one place to another.

Anatomy

Snails go through a special process called torsion. This means part of their body turns around. This helps them fit into their shells better and stay safe.

Gastropods usually have a head with tentacles and eyes. Many have shells that spiral. Some sea slugs are bright colors to warn others or hide. They have simple eyes and a nervous system. Their digestive system includes a radula, a tongue-like organ they use to eat. Most breathe with gills, but many land species have lungs.

Life cycle

Main article: Reproductive system of gastropods

See also: Mating of gastropods

Gastropods have interesting life cycles. They lay eggs, and their young grow in different stages.

The life cycle includes egg laying, embryonic development, and growth. Some gastropods rest during very hot or very cold weather, called estivation or hibernation.

Feeding behavior

Gastropods eat many different things depending on their type. Some ocean gastropods eat plants, while others eat animals. Land gastropods, like snails and slugs, can eat leaves, bark, fruit, fungi, and dead animals.

Some sea slugs eat plants, and others eat animals. Terrestrial gastropods eat plants, fungi, algae, and soil. Many of them eat tiny organisms found on decaying matter. Some slugs like to eat fungi. Their eating habits help spread fungi by carrying spores.

Genetics

Gastropods have different ways of organizing their mitochondrial genes than other animals. Big changes in gene arrangement happened when groups like Patellogastropoda and Heterobranchia first appeared. In other groups such as Vetigastropoda and Caenogastropoda, there were smaller changes, mostly tiny shifts in gene positions. In Heterobranchia, the gene order is quite stable, with most changes moving some tRNA genes.

Geological history and evolution

See also: List of marine gastropod genera in the fossil record

The first gastropods lived only in the ocean. They appeared in the Late Cambrian period. These early gastropods had a special coiled shell, but we are not sure if they were true gastropods.

By the Ordovician period, true gastropods appeared. They began to live in many different water places.

During the Mesozoic era, many modern gastropod families started to change and evolve. One of the earliest land snails, called Anthracopupa, was found in old rocks from Europe. Fossils from this time show beautiful, well-preserved shells. Many of these shells look very similar to snails we see today.

Taxonomy

Further information: Changes in the taxonomy of gastropods since 2005

See also: Taxonomy of the Gastropoda (Ponder & Lindberg, 1997)

Gastropods, like snails and slugs, have a classification system that scientists are still figuring out. They use DNA to learn more about how different groups of gastropods are related. Today, scientists know about several main groups, but some details about how they are related are still being studied.

In the past, scientists used fewer groups to sort gastropods, but new discoveries have caused many changes. Researchers keep updating how they group these animals as they find out more about their genetics and how they evolved.

Ecology and conservation

Many gastropod species face threats from losing their homes, pollution, and changes in the weather. Some species are in danger or have gone away because of these problems. Efforts to help gastropods often focus on protecting their homes, especially in freshwater and on land.

Gastropods are eaten by many different animals, depending on where they live. In the ocean, they are hunted by fish, marine birds, marine mammals, crustaceans, and other mollusks like cephalopods. On land, gastropods face predators such as insects, spiders, birds, and mammals.