What Is a Mollusk?

Emily Fink, Heather Pier
  • Author
    Emily Fink

    Emily has a PhD in Molecular and Cellular Biophysics and Biochemistry from the University of Buffalo with over ten years of experience in biomedical research and mentoring students in the fundamentals of molecular biology and research practices. Emily is also a certified peer reviewer through Elsevier.

  • Instructor
    Heather Pier

    Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

Learn what a mollusk is. Read about the phylum Mollusca and see mollusk examples. Understand types and characteristics of mollusks, classes, and symmetry. Updated: 03/15/2022

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What Is a Mollusk?

A mollusk generally refers to a member of the large group of soft-bodied invertebrates within the phylum Mollusca. Examples of mollusks include snails, clams, oysters, squids, and mussels. This group of organisms varies greatly in habitats and appearance, but they have a few universal features that will be elaborated on later in this lesson.

  • Mantle is the dorsal wall of the body that protrudes and covers the visceral mass, such as the shell of a snail.
  • Radula is the part of the digestive system, along with the stomach and esophagus, that helps scrape food from various surfaces into the mollusk's mouth.
  • Nervous system coordinates actions, movements, and sensory inputs.

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  • 0:02 What Are Mollusks?
  • 1:42 Digestive System
  • 3:35 Nervous system
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Phylum Mollusca

Taxonomy is defined as the scientific method of classifying biological organisms. It uses a hierarchal system, akin to a reverse pyramid, of eight major levels, from the most general to the most specific. The levels from largest to smallest are domain, kingdom, phylum, class, order, family, genus, and species. Humans, for example, have the following taxonomic classification: domain (Eukarya), kingdom (Animalia), phylum (Chordata), class (Mammalia), order (Primates), family (Hominidae), genus (homo), and species (sapiens). We diverge from mollusks at the phylum level, which is pretty high up on the scale of taxonomy, underscoring the vast complexity and varieties of the phylum Mollusca.

The phylum Mollusca is the second largest phylum of invertebrate animals, with Arthropoda (containing insects) being the largest. The estimated number of species of mollusks alive today is estimated at around 200,000 on land and sea, with the fossil record indicating an additional 60,000–100,000 different species. In the sea, mollusks are the largest marine phylum, about 23% of all known marine organisms. Mollusks can inhabit fresh water as well as dry land, but most mollusk species live in the oceans.

Mollusca Classes

Class fits into taxonomy directly below phylum. The phylum Mollusca is divided into roughly nine different classes, although opinions vary on the exact number of classes.

The classes of Mollusca are briefly described in the table below.

Mollusk Class Examples Estimated Number of Living Species Habitat
Gastropoda Snails, slugs, abalone, conch, limpets, sea hares, and sea butterflies 70,000 land, sea, and freshwater
Bivalvia Clams, oysters, scallops, and mussels 20,000 sea and freshwater
Polyplacophora Chitons 1,000 sea
Cephalopoda Squid, octopus, ammonites, nautiluses, and cuttlefish 900 sea
Scaphopoda Tusk shells 500 sea
Aplacophora Small deep sea marine mollusks that resemble worms 320 sea
Monoplacophora Deep sea mollusks with a cap like shell 31 sea
Criconarida, Rostoconchia, and Helcionelloida Extinct Extinct Extinct

Mollusks Examples

Mollusks are found in different environments all over the world, including freshwater, saltwater, and on land. Mollusks are found throughout the ocean, from shallow, rocky tidal areas to the very deep sea. They can also be found at various terrestrial climates and altitudes. Commonly recognized mollusks in the Gastropoda class are snails and slugs. Even within the sub-categories of Gastropoda, there are different species.


A snail is a shelled gastropod in the phylum Mollusca.

snail


Within the Bivalvia class are marine and freshwater mollusks, such as clams, oysters, cockles, mussels, and scallops. They are defined as lacking a radula and possessing a shell composed of calcium carbonate enclosing a compressed body.


Examples of Bivalvia

Bivalvia


Within the Polyplacophora class are marine animals known as chitons. They have also been called sea cradles or coat-of-mail shells. Their unique shells are composed of eight different plates, providing them protection and flexibility.


A chiton

chiton


The cephalopods are a class of mollusks containing marine mollusks characterized by bilateral body symmetry, a prominent head, and tentacles. Examples of cephalopods include squid, octopus, cuttlefish, and nautilus.


A nautilus is a mollusk in the cephalopod class.

nautilus


The Scaphopoda are also referred to as tusk shells or tooth shells. They are found in oceans all over the world and can range from 0.5 to 18 cm long.


A depiction of a tusk shell

Scaphopoda


The Aplacophora are small, cylindrical, and wormlike marine mollusks with no shell and little resemblance to the other types of mollusks. They are found all over the world and inhabit the deep, benthic regions of the oceans.


An example of an Aplacophora

aplacophora


The Monoplacophora also live at the bottom of the deep sea. They have one dome-like shell.

Mollusk Characteristics


A general schematic of mollusk anatomy

mollusk anatomy


Mollusk Characteristics

There are common characteristics shared among all mollusks, including soft bodies, a mantle, a visceral mass, and the foot. Not all mollusks possess a true brain. The mantle of a mollusk is a thin tissue layer covering the outside of the mollusk. In mollusks that possess shells, such as snails, the mantle is responsible for secreting, repairing, and maintaining the shell. The visceral mass, which is also called a visceral hump, is the portion of the mollusk that contains most of the digestive, respiratory, and reproductive systems. The mantle partially surrounds the visceral mass. Using the example of a snail, the visceral mass is found inside its shell. The foot of the mollusk is a muscular organ used for movement. In shelled mollusks, such as the snail, the foot is the portion of the snail at the bottom that is used for crawling across surfaces.

Mollusk Symmetry

An additional feature of mollusk anatomy that is shared by all within the phylum Mollusca is bilateral symmetry. Bilateral symmetry is defined as being divisible into two symmetrical halves along a central axis. Humans and other species outside of the phylum Mollusca can have bilateral symmetry as well.

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Frequently Asked Questions

What are classes of phylum Mollusca?

Class is a taxonomic designation directly below phylum. The mollusk classes that are still alive today include Gastropoda, Bivalvia, Polyplacophora, Cephalopoda, Scaphopoda, Aplacophora, and Monoplacophora.

What are three examples of members of the phylum Mollusca?

There are estimated to be around 200,000 living species of mollusks, making it the second-largest phylum of invertebrates. Some examples include clams, snails, and octopi.

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