Mollusk Facts

Scientific Name: Gastropoda, Bivalvia, Cephalopoda

Octopus in aquarium against the glass

Stephanie Hager/HagerPhoto/Photolibrary/Getty Images

Mollusks (Mollusca) is a taxonomic phylum that contains a diverse array of organisms, including snails, sea slugs, octopuses, squid, and bivalves such as clams, mussels, and oysters. Between 50,000 and 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.

Fast Facts: Mollusks

  • Scientific Name: Mollusca: Gastropoda, Bivalvia, Cephalopoda, Monoplacophora, Scaphopoda, Aplacophora, Polyplacophora
  • Common Names: Snails, sea slugs, octopuses, squid, and bivalves
  • Basic Animal Group: Invertebrate
  • Size: Varies from 0.04 inch (solenogasters and gastropods) to 4 feet (giant clam) 
  • Lifespan: 3 years to over 500
  • Diet: Carnivore or herbivore
  • Habitat: Oceans and coastal waterways worldwide
  • Population: Unknown
  • Conservation Status: Most are classified as Least Concern; mollusks make up nearly one-fourth of all marine animals on Earth. 

Description

Mollusks have a shell and soft body and usually have a distinguishable head and foot region. 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.

In addition to a shell, most mollusks have a muscular foot for creeping or burrowing, and some have a head with sense organs. Their soft body includes lungs or gills for breathing and digestive and reproductive parts. These are surrounded by a skin-like organ called the mantle. Mollusks also have bilateral symmetry—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."

The upper body, or the mantle, is a thin, muscular sheet that covers the internal organs. 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.

A garden snail crawls over dead leaves
Puneet Vikram Singh, Nature and Concept photographer/Getty Images

Habitat and Distribution

Mollusks—snails, sea slugs, octopuses, squid, and bivalves—are found in habitats ranging from freshwater lakes and rivers to shallow coastal waters to the deepest parts of the oceans, worldwide. Most live in the bottom sediments, though cephalopods are primarily free-swimming species; some snails and clams are terrestrial.

Species

Mollusks belong to the kingdom Animalia and there are tens of thousands of invertebrates that fit into the Mollusca phylum. New ones are still being discovered, and researchers continue to modify the number and grouping classifications. One common schema used by researchers includes seven classes:

  • Gastropoda (snails and slugs) 
  • Bivalvia (clams, scallops, oysters, mussels) 
  • Cephalopoda (squid, octopi, nautilus)
  • Monoplacophora (limpets) 
  • Scaphopoda (tusk shells) 
  • Aplacophora (shell-less, worm-like animals) 
  • Polyplacophora (chitons)  

Diet and Behavior

Many mollusks feed using a radula, essentially 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. Consider 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, a combination of 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 and Offspring

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.

The mollusk life cycle varies greatly among different classes of mollusks and among species within classifications. Squids reproduce sexually: The females lay fertilized eggs in the water which hatch into larvae and then develop on their own. Octopuses do the same, except the females carry the eggs with her until they hatch. Clams, mussels, and oysters produce larvae that drop through the sea water and attach to a host to grow to maturity on. The hosts are usually fish, but oysters prefer the shell of an adult oyster. Land snails are hermaphrodites that mate and both partners produce fertilized eggs. The eggs are deposited in the soil; they hatch with a shell but most consume calcium to make it harden. 

Mollusks and Humans

The primary use that humans have for mollusks and the rest of ocean-living animals is their ability to filter large quantities of water, up to 10 gallons each. Mollusks are also 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.

Culturing Tahitian pearls
Culturing Tahitian pearls. CampPhoto/Getty Images

Conservation Status

Over 8,600 molluscs are listed in the IUCN Red List, most classified as Least Concern or Data Deficient, although many are threatened or endangered. The phylum represents nearly one-fourth of all the species on the planet.

Threats

Increasing levels of carbon dioxide are raising the pH level of the world's oceans, which in turn, increases the acidity of these bodies of water. This greatly weakens mollusks' otherwise strong shells and even makes it difficult for them to produce shells in the first place, threatening their survival. If mollusks begin to die out in mass, then fish and other animals that feed off of them may 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 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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