Mollusks don't ring a bell ? The term mollusk literally refers to animals with a « soft body ». You probably know our friends, the snails, which belong to the class of terrestrial mollusks !

Now let's focus on marine mollusks, the phylum that interests us. Here, you'll find all the shells you come across at the beach during your vacations ! But this phylum isn't just made up of shells, it also includes the octopus, squid, and cuttlefish, which belong to the class of cephalopods.

Let's take a closer look at shells! You've probably noticed that their shapes vary, but so do their structures. Indeed, the shell is made up of:

• either a single monolithic part, like the snails: these shells belong to the class of gastropods
• or two parts that close up as soon as you touch them. It's this latter feature that gives the class of bivalves its name !

As for the shape of their shells, it also varies for gastropods – remember the shells that are made of one single piece – such as the periwinkle, whelk, or limpet with their conical shape that resembles a Chinese hat, and the abalone shaped like an ear. For bivalves, we have the elongated shape of the knife, which resembles the object of the same name, and the more classic, squat shapes of clams or cockles, as well as those of mussels, oysters, and finally, the very characteristic shape of the scallop shell and the rough penshell.

Ultimately, the marine mollusk phylum is divided into 6 main classes, namely:

• the bivalves with shellfish whose shell consists of 2 parts called valves
• the monoplacophorans, which are mollusks with a single-piece shell that is either conical – like a Chinese hat – or flattened.
• the polyplacophorans, more commonly known as chitons, are mollusks whose shell is composed of eight articulated plates
• the scaphopods, which are mollusks whose shells are either elongated and curved or shaped like a tooth and open at both ends.
• the cephalopods with the common octopus, squid, and cuttlefish
• the gastropods with shellfish like 'snails' with a single-piece shell but not exclusively, this class also includes sea slugs – mollusks without an external shell.

The mollusk phylum totals around 100,000 species, but with an uneven distribution among the different classes.

Mollusks, as indicated by the origin of their name, are animals with soft bodies and lacking a skeleton.

The body of mollusks consists of a muscular foot and an external covering called the mantle, which houses the various organs. It is primarily the mantle that contributes to the formation of the shell.

For mollusks with shells, the mantle forms a fold around the shell's perimeter, creating a cavity called the pallial cavity. This cavity houses the respiratory organs or gills, known as ctenidia.

Almost all mollusks, except for bivalves, possess a tongue called the radula, equipped with a multitude of small teeth, much like a rasp.

Mollusks can be found at all latitudes and across all the oceans on the surface of the globe.

Although some mollusks live fixed to a substrate, they are generally free-moving animals. Mollusks primarily live on the seafloor, making them benthic animals, but some species are pelagic and capable of moving freely in the water column.

Mollusks can be found at all depths, including abyssal depths.

Dietary habits vary depending on the class. Bivalves are filter feeders, consuming particles and plankton suspended in the water. Gastropods, equipped with their radula (a tongue shaped like a rasp), "graze" algae on the surface of rocks or polyps from gorgonians. Finally, cephalopods capture fish, crustaceans, or other mollusks.

Mollusks exhibit sexual reproduction.