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, known as molluscs or mollusks. With over 86,600 known living species, it is the second-largest animal group after the arthropods. Molluscs are very diverse, living in many different places such as the ocean, freshwater, and on land. They include familiar creatures like 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 perform important functions, like breathing and moving around. Some molluscs, especially cephalopods like squids and octopuses, are very intelligent for animals without a backbone. Others, like gastropods which include snails and slugs, are the most common type of mollusc.

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. However, some molluscs can be dangerous. For example, the blue-ringed octopus has a powerful bite, and certain cone shells can deliver venom that is harmful to humans. Despite these risks, molluscs remain an important food source and continue to be studied for their unique qualities.

Etymology

The words mollusc and mollusk come from the French mollusque. This French word started from a Latin term mollusca, which means "soft." The Latin word was first used by a scientist named J. Jonston in 1650 to talk about a group of sea animals like octopuses. Later, another famous scientist named Linnaeus also used this name when organizing living things into groups. The Latin word mollusca was inspired by an even 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 estimate that there are between 50,000 and 120,000 different kinds of living molluscs, with many more that have not yet been discovered. 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 less familiar 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 invertebrates with many different shapes and ways of living. Because they are so different, scientists often describe a "general" mollusc to show the most common features. This general mollusc has a single shell on its back, made by a layer of tissue called the mantle. Underneath the shell is a muscular foot that helps the animal move.

The mantle also creates a space called the mantle cavity, which 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 jet propulsion in some species. Molluscs also have different types of blood systems and ways to breathe, eat, and reproduce, making them very interesting animals to study.

Ecology

Most molluscs eat plants, grazing on algae or acting as filter feeders. Some use a special tool called a radula to comb up tiny algae from the sea floor, while 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 and use their jaws and tentacles to catch food, with the radula playing a smaller role. Some sea-slugs drink sap from algae, while others eat sponges or hydroids.

Classification

There are many different ways scientists think about how to group molluscs, which are animals like snails, clams, and octopuses. Some believe there are seven living groups, while others say there are more or fewer. A few of these groups exist only in fossils.

The family tree of molluscs is still being studied and is full of questions. Scientists debate whether certain ancient animals were molluscs or close relatives. They also discuss how the living groups are related to each other. Some think the first mollusc had a body made of repeating parts, while others disagree. Over time, many molluscs developed shells, which started as a stiff coating and later became hard with minerals like aragonite. Scientists continue to research and discuss these relationships, and our understanding changes as new data comes in.

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 thousands of years, people have used molluscs for food and for valuable items such as pearls, mother of pearl, and Tyrian purple dye. Their shells have also been used as a kind of money in some societies before modern currency existed. Some molluscs, like certain snails and slugs, can harm crops and disrupt ecosystems 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 leads the world in catching these animals. Some molluscs make valuable pearls, especially oysters in warm ocean waters. Pearls are also made by placing small objects into oysters to create them. Molluscs also provide 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 incidents are rare and usually not very dangerous. 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 are not normally found.