Mollusks (Phylum Mollusca): Characteristics and Facts

Snails, Sea Slugs, Octopuses, and Oysters

Octopus in aquarium

Stephanie Hager/HagerPhoto/Photolibrary/Getty Images

Mollusca is a taxonomic phylum that contains a diverse array of organisms (referred to as "mollusks"), and the taxonomic classes that include snails, sea slugs, octopuses, squid, and bivalves such as clams, mussels, and oysters. From 50,000 to 200,000 species are estimated to belong to this phylum. Imagine the obvious differences between an octopus and a clam, and you'll get an idea of the diversity among mollusks. Mollusks also belong to the kingdom Animalia.

Physical Characteristics

Mollusks have a shell and soft body and usually have a distinguishable head and foot region. They use their muscular foot to move. Some may have a hard covering, or exoskeleton. Mollusks also have a heart that pumps blood through their blood vessels, digestive system, and a nervous system. As Fact Monster describes:

"As well as a shell, most mollusks have a muscular foot for creeping or burrowing. Some also have a head with sense organs. The soft body includes lungs or gills for breathing, and digestive and reproductive parts, all enclosed by a skinlike organ called the mantle."

Mollusks have bilateral symmetry—that is, one side is a mirror image of the other—and can have one or two shells. Their organs are in a fluid-filled cavity; indeed, the very word "mollusk" in Latin means "soft." Most mollusks have a body made up of three parts: a head, a soft body mass and a foot, says Jeffrey Hays, writing on the website, Facts and Details.

The foot, which extends from a mollusk’s shell, helps it move. The upper body, or the mantle, is a thin, muscular sheet that covers the internal organs, says Hays. Most mollusks, particularly those with shells, also have gills in the central part of their body cavity. Despite looking fragile, mollusk shells are quite hard. Scientists are even studying nacre, a material found in mollusk shells, to develop materials that are stronger and lighter than steel, Hays says.

Feeding

Many mollusks feed using a radula, which is basically a series of teeth on a cartilage base. The radula can be used for complex tasks, from grazing on marine algae or drilling a hole in another animal's shell. The radula scrapes tiny plants and animals off rocks or tears food into chunks.

The adoption of different feeding habits appears to have had a major influence on molluscan evolution, according to the University of California Museum of Paleontology:

"The change from grazing to other forms of food acquisition is one of the major features in the radiation of the group. Based on our current understanding of relationships, the earliest mollusks grazed on encrusting animals and detritus."

Since mollusks are such a wide-ranging phylum, it's helpful to look at how one of the organisms that belong to this group feeds, and how it captures its prey. Take the deadly blue-ringed octopus. This mollusk hunts small crabs and shrimp during the day, but it will eat bivalves and small fish if it can catch them. The octopus pounces upon its prey, using its tentacles to pull its catch toward its mouth. Then, its beak pierces the crustacean's exoskeleton and delivers the paralyzing venom. The venom is produced by bacteria in octopus' saliva. It contains tetrodotoxin, histamine, taurine, octopamine, acetylcholine, and dopamine.

Once the prey is immobilized, this mollusk uses its beak to tear off chunks of the animal to eat. The saliva also contains enzymes that partially digest flesh, so that the octopus can suck it out of the shell. The blue-ringed octopus is immune to its own venom.

Reproduction, Distribution, and Human Uses

Some mollusks have separate genders, with males and females represented in the species. Others are hermaphroditic, which means they have both male and female reproductive organs. Mollusks may live in salt water, in fresh water, and even on land.

Thanks to their ability to filter large quantities of water—up to 10 gallons per hour—mollusks are important to a variety of habitats. The Encyclopædia Britannica explains:

"This filtering activity, however, may also seriously interfere with the various populations of invertebrate larvae (plankton) found suspended and free-swimming in the water."

Still, mollusks are important to humans as a food source—especially in the Far East and the Mediterranean—and have contributed in numerous ways to human civilization. The shells of cowries (a type of small mollusk that belongs to the gastropod family) were used as money by Native Americans, and the pearls that grow in oysters, as the result of irritation by sand grains, have been treasured for centuries. Another type of mollusk, the murex, was cultured by the ancient Greeks for its dye, known as "imperial purple," and the cloaks of some rulers were woven from long threads secreted by the bivalve species (twin-shelled mollusks) called Pinna nobilis.

Effect of Global Warming

Increasing levels of carbon dioxide are raising raising the pH level of the world's oceans, which in turn, increase the acidity of these bodies of water, says Hays of Facts and Details. This greatly weakens Mollusks' otherwise strong shells and even makes it difficult for them to produce shells in the first place, threatening their survival, Hays says, adding: "If populations of these organisms collapse then populations of fish and other creatures that feed on them could also suffer."

Northeastern University marine biologist Brian Helmuth gives the example of the common mussel, a member of the family of bivalve mollusks. In addition to the problem of increased acidity in the oceans, which—as noted—makes it harder for these mollusks to produce shells, the increasing temperatures of the oceans and even the sand and air on the surrounding beaches can mean a death sentence for mussels.

“You are sitting there in the blazing sun, you’re not going to be able to move,” Helmuth says. “You can’t escape the heat, you can’t escape the sun, you can’t go into a crevice like ... a crab.” Mussels can literally start to cook on the rocks if they get too hot, Helmuth told Carolyn Beeler for an article titled "To Understand Climate Change, Look at It From a Mussel's Perspective" published on Public Radio International's website.

Helmuth adds that global warming is shrinking the habitat in which mussels and other mollusks can live. And since mollusks are such an integral part of the food chain, that could eventually affect many other animals that depend on them for sustenance.

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