Mollusca

What are Mollusca?	Mollusca is a group of animals that includes snails, clams, octopuses, and more.
How many kinds are there?	There are over 100,000 different types of mollusks.
Where do they live?	Mollusks live in oceans, lakes, rivers, and even on land.
Special feature	Many mollusks have a hard shell to protect them.

Mollusca is a large group of invertebrate animals, called molluscs or mollusks. There are over 86,600 known living species. It is the second-largest animal group after the arthropods. Molluscs live in many places, such as the ocean, freshwater, and on land. They include snails, clams, octopuses, and squids.

Cornu aspersum (formerly Helix aspersa)—a common land snail

These animals have soft bodies and often have shells. Many parts of their bodies help them breathe and move. Some molluscs, like squids and octopuses, are very smart for animals without a backbone. Others, like snails and slugs, are very common.

Molluscs have been important to humans for thousands of years. Their shells have been used as decoration and even as money in the past. Pearls and a beautiful material called mother of pearl come from some molluscs. Some molluscs can be dangerous. For example, the blue-ringed octopus has a strong bite, and some cone shells can be harmful to humans. Even so, molluscs are an important food source and are studied for their special qualities.

Etymology

The words mollusc and mollusk come from the French word mollusque. This French word came from a Latin term mollusca, meaning "soft." The Latin word was first used by a scientist named J. Jonston in 1650 for sea animals like octopuses. Later, another scientist named Linnaeus also used this name when grouping living things. The Latin word mollusca was inspired by an older Greek phrase meaning "the soft ones."

Diversity

Diversity and variability of shells of molluscs on display

Molluscs, or mollusks, are one of the most diverse groups of animals on Earth. Scientists think there are between 50,000 and 120,000 different kinds of living molluscs, with even more still unknown. They are the second largest animal group, just after arthropods, which include insects and spiders.

Molluscs come in many shapes and sizes. They include snails, slugs, clams, squids, octopuses, and many other creatures. Most molluscs live in the ocean, but some live in freshwater, and others live on land. Many molluscs are found in warm tropical areas, but they exist everywhere, from the edges of the sea to the deepest parts of the ocean. One famous mollusc is the giant squid, which is one of the largest invertebrates in the world.

Anatomy

Molluscs, or mollusks, are a large group of animals without backbones. They come in many shapes and ways of living. Scientists often talk about a "general" mollusc to show the most common features. This general mollusc usually has a single shell on its back. The shell is made by a layer of tissue called the mantle.

Under the shell is a muscular foot that helps the animal move. The mantle also makes a space called the mantle cavity. This space holds important parts of the mollusc’s body, like its gills and reproductive organs. Most molluscs have a shell made of special materials, but some, like nudibranchs, do not have a shell at all. The foot can be used for walking, digging, or even to help the animal move quickly in some species. Molluscs also have different ways to move their blood, breathe, eat, and reproduce, which makes them very interesting to learn about.

Ecology

Most molluscs eat plants. They graze on algae or act as filter feeders. Some use a special tool called a radula to comb up tiny algae from the sea floor. Others feed on larger plants like kelp by rasping the surface. Filter feeders, like many bivalves, strain food particles from water using their gills.

Cephalopods, such as squids and octopuses, are hunters. They use their jaws and tentacles to catch food, with the radula playing a smaller role. Some sea-slugs drink sap from algae. Others eat sponges or hydroids.

Classification

There are many different kinds of molluscs, such as snails, clams, and octopuses. Scientists are still learning about how to group these animals. Some think there are seven groups, while others think there are more or fewer. A few of these groups are only known from fossils.

Scientists are still studying the family tree of molluscs. They wonder if some ancient animals were molluscs or close to them. They also discuss how the living groups are related. Some think the first mollusc had a body made of repeating parts. Over time, many molluscs grew shells. These shells started as a stiff coating and later became hard with minerals like aragonite. Scientists keep researching, and our understanding changes with new information.

Class	Major organisms	Described living species	Distribution
Gastropoda ^: 300	all snails and slugs including abalone, limpets, conch, nudibranchs, sea hares, sea butterflies	70,000	marine, freshwater, land
Bivalvia ^: 367	clams, oysters, scallops, geoducks, mussels, rudists†	20,000	marine, freshwater
Polyplacophora ^: 292–8	chitons	1,000	rocky tidal zone and seabed
Cephalopoda ^: 343	squid, octopuses, cuttlefish, nautiluses, vampire squids, Spirula, belemnites†, ammonites†	900	marine
Scaphopoda ^: 403–7	tusk shells	500	marine 6–7,000 metres (20–22,966 ft)
Aplacophora ^: 291–2	worm-like molluscs	320	seabed 200–3,000 metres (660–9,840 ft)
Monoplacophora ^{: 298–300}	ancient lineage of molluscs with cap-like shells	31	seabed 1,800–7,000 metres (5,900–23,000 ft); one species 200 metres (660 ft)
Rostroconchia†	fossils; probable ancestors of bivalves	extinct	marine
Helcionelloida†	fossils; snail-like molluscs such as Latouchella	extinct	marine

Evolutionary history

Main article: Evolution of molluscs

Human interaction

Main article: Molluscs in culture

For many years, people have used molluscs for food and for useful items such as pearls, mother of pearl, and Tyrian purple dye. Their shells have also been used as a kind of money in some places before modern currency was common. Some molluscs, like certain snails and slugs, can sometimes harm crops and change the environment when they are moved to new places.

Uses by humans

Further information: Seashell and List of edible molluscs

Molluscs, especially clams and mussels, have been important food for humans for a very long time. Today, China catches more of these animals than any other country. Some molluscs make valuable pearls, especially oysters in warm ocean waters. Pearls can also be made by putting small objects into oysters. Molluscs also give us special materials like Tyrian purple dye from certain sea snails and sea silk from some bivalves. Their shells have been used as money in the past.

Harm to humans

Some molluscs can sting or bite, but these events are rare and usually not serious. A few types of octopuses and cone snails can be harmful if touched. Some snails can spread diseases to people, and others can damage crops and change the balance of nature in places where they do not usually live.