Mollusks (Phylum Mollusca): Characteristics and Facts

This Animal Group Includes Snails, Sea Slugs, Octopuses, and Oysters

Octopus in aquarium

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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."

MollusksScience.Weebly adds that mollusks have bilateral symmatry—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," the science website adds, noting that the very word "mollusk" in Latin means "soft."

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, says 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."

Capturing Prey

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, says Encyclopaedia Britannica.—mollusks are important to a variety of habitats. The encyclopedia adds:

"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.

Sources

  • Martinez, Andrew J. "Marine Life of the North Atlantic." Aqua Quest Publications, Inc.: New York, 2003.
  • "The Mollusca." University of California Museum of Paleontology.
  • Molluscs.” Biology Education.
  • "Mollusks." Factmonster.
  • "Phylum: Mollusks." Mollusk Science.
  • Salvini-Plawen, Luitfried. “Mollusk.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 13 Apr. 2018,