Mollusks

For More Information

Infrequently Asked Questions

Find out whether your mollusk question is on the list! Click here to visit the Infrequently Asked Questions page.

What is a mollusk? 

Mollusks (Phylum Mollusca) make up a group of animals that are more similar to each other in anatomical, developmental, and evolutionary characters than they are to other animals. Along with several related phyla, they share schizocoel coelemic cavities and protostomal development.

Click on cladogram for a larger version in a new window Family tree artwork by Katherine Arisumi;sSources used in constructing the family tree: Brusca, R.C. & Brusca, G.J. 1990. Invertebrates. Sinaur Associates, Inc., Sunderland, Massachusetts. 922 pp. Haszprunar, G. 2000. "Is the Aplacophora monophyletic? A cladistic point of view." American Malacological Bulletin, 15: 115-130

 treeThis family tree shows relationships among different groups of animals, and shows the evidence that lets us know these relationships. Scientists call this particular kind of family tree a cladogram. "Clade" refers to a group of organisms that are all descended from a common ancestor. At any point on the tree, all the organisms on branches from there to the top are in a clade. One clade can contain other clades, and every time two branches diverge, they form two new clades. The list of features at the base of each branch is a list of features shared by all the animals in that clade. These features are the evidence that tells us why those particular animals belong in a particular clade. An evolutionary biologist would interpret time as moving from the base of the tree toward the top, and the shared features as new features (evolutionary novelties) that an ancestor acquired and then passed on to all of its descendants.

In this tree, for example, Bivalvia, Scaphopoda, Cephalopoda, and Gastropoda all form a clade. Find the point where the branches for all those groups come together into one place. At that point is a list of features that all those groups share. Cephalopoda and Gastropoda form another clade within that larger clade, and you can see the list of features that they share.

This family tree shows that a feature the Mollusca share with both the Arthropods (e.g., crabs, insects) and Annelids (e.g., earthworms) is spiral cleavage of the developing embryo. The clade that includes all three of those groups is often called Spiralia. A feature that the mollusks share with just the Annelids is trochophore larva.

Features that are unique to mollusks, or to just some of the mollusks are a radula, a heart with an associated hemocoel circulation, ctenidia, a circumpharyngeal nerve ring with a ladder-like arrangement of nerves.

Estimates about the number of living species of mollusks typically range from 50,000 to 200,000; the Mollusks department usually says about 100,000 species. At least 35,000 additional species are known from the fossil record. The living mollusks are placed into eight classes (some workers combine the Solenogastres and Caudofoveata into a single class called Aplacophora):

  • Solenogastres
  • Caudofaveata
  • Polyplacophora  
  • Scaphopoda  
  • Cephalopoda  
  • Bivalvia  
  • Gastropoda  
  • Mollusks eat many different types of food. Mollusks that eat plants are herbivores, mollusks that eat animals are carnivores; mollusks that eat both plants and animals are called omnivores; and mollusks that eat dead and decomposing material are known as detritovores. They are found in a wide variety of ecological niches in marine, freshwater, and terrestrial habitats. Some mollusks live thousands of meters below the surface of the ocean, while others live thousands of meters above sea level, even on the tops of mountains. They can also be found in tropical rainforests and deserts. All in all, mollusks have found a way to live almost everywhere.

    How we use mollusks 

    When in France, do as the French do: eat escargot! Snails can be used for food and medicine. Don't assume those things holding your shirt together are plastic—sometimes mollusk shells are used for making buttons.

    "Brother, can you spare a mollusk?" Believe it or not, some island nations use shells as money! There are many additional uses for mollusk shells as art, jewelry, and other useful items.