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Which Mollusks Have Shells?

Instructor: Danielle Haak

Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences

Mollusks are a diverse set of creatures that have been around thousands of years. Read this lesson to learn what types of mollusks have shells, the different subcategorizations of shell-carrying mollusks, and how their shells are made.

What Are Mollusks?

Mollusks are a hugely diverse bunch of organisms - there are over 50,000 species! Despite this diversity, all mollusks are invertebrates (meaning, they lack a back bone); have an unsegmented body structure with a head, visceral mass, and foot; and most live in aquatic or moist environments. You are probably familiar with a number of mollusks: they include the snails, octopuses, squids, clams, mussels, scallops, oysters, and chitons, among others.

A mollusk's head region is where its brain is located. The foot region is the part that comes in contact with the surrounding environment and the visceral mass is the tissue in between, where the internal organs are located. Though there are qualities that all mollusks share, you may be surprised to learn that not all of them have shells. This lesson will identify what types of mollusks do have shells.

Which Mollusks Have Shells?

Certain groups of mollusks have a hardened shell surrounding their tissues. Clams, mussels, scallops, and oysters collectively make up the bivalves, a subcategorization of mollusks. Bivalves have two shells that hinge together on one side, enabling the organism to open and close as necessary. The name 'bivalve' translates to 'two shells', so this is easy to remember.

Here are some pictures of different bivalves with shells:

Aquatic mussels
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Clams on display
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Scallops pulled off the ocean floor
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Oysters are often grown on oyster farms, like the one shown here.
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You are also probably familiar with snails; they have shells, too! Snails, snugs, limpets, and abalones are all found within the gastropods subcategorization of mollusks. Unlike bivalves, however, gastropods only have a single shell with an opening at the bottom. Some have a 'trap door' feature, called an operculum, that they can close, but this is not the same as a second shell.

Here are some pictures of different gastropods:

Here you can see the foot and head of the snail sticking out of the shell.
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A view of a limpet from above. Unlike the bivalves, the limpet does not have a second shell underneath.
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Multiple abalones attached to rocks at the water
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How Is a Shell Made?

In addition to the features that make them mollusks, those with shells have an additional body part called the mantle. In addition to being the outermost wall of the mollusk's body and protecting its internal organs, the mantle is responsible for secreting the calcium carbonate used to slowly build up the shell over time.

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