Mollusks may be the most difficult animal group for the average person to wrap their arms around: this family of invertebrates includes creatures as widely divergent in appearance and behavior as snails, clams, and cuttlefish.
Fast Facts: Mollusks
- Scientific Name: Mollusca (Caudofoveates, Solanogastres, Chitons, Monoplacophorans, Scaphopods, Bivalves, Gastropods, Cephalopods)
- Common Name: Mollusks or molluscs
- Basic Animal Group: Invertebrate
- Size: Microscopic to 45 feet in length
- Weight: Up to 1,650 pounds
- Lifespan: Hours to centuries-the oldest is known to have lived over 500 years
- Diet: Mostly herbivore, except for cephalopods who are omnivores
- Habitat: Terrestrial and aquatic habitats on every continent and ocean in the world
- Conservation Status: Several species are threatened or endangered; one is extinct
Any group that embraces squids, clams, and slugs present a challenge when it comes to formulating a general description. There are only three characteristics shared by all living mollusks: the presence of a mantle (the rear covering of the body) that secretes calcareous (e.g., calcium-containing) structures; the genitals and anus opening into the mantle cavity; and paired nerve cords.
If you're willing to make some exceptions, most mollusks can also be characterized by their broad, muscular "feet" which correspond to the tentacles of cephalopods, and their shells (if you exclude cephalopods, some gastropods, and the most primitive mollusks). One type of mollusk, the aplacophorans, are cylindrical worms with neither shell nor foot.Getty Images
Most mollusks are marine animals that live in habitats from shallow coastal areas to deep waters. Most stay within the sediments at the bottom of water bodies, although a few-such as cephalopods-are free swimming.
There are eight different broad categories of mollusks on our planet.
- Caudofoveates are small, deep-sea mollusks that burrow into soft bottom sediments. These worm-like animals lack the shells and muscular feet characteristic of other mollusks, and their bodies are covered with scale-like, calcareous spicules.
- Solanogastres, like caudofoveata, are worm-like mollusks that lack shells. These small, ocean-dwelling animals are mostly blind, and either flattened or cylindrical.
- Chitons, also known as polyplacophorans, are flat, slug-like mollusks with calcareous plates covering the upper surfaces of their bodies; they live in intertidal waters along rocky coastlines worldwide.
- Monoplacophorans are deep-sea mollusks equipped with cap-like shells. They were long believed to be extinct, but in 1952, zoologists discovered a handful of living species.
- Tusk shells, also known as scaphopods, have long, cylindrical shells with tentacles extending from one end, which these mollusks use to rope in prey from the surrounding water.
- Bivalves are characterized by their hinged shells and live in both marine and freshwater habitats. These mollusks have no heads, and their bodies consist entirely of a wedge-shaped "foot."
- Gastropods are the most diverse family of mollusks, including over 60,000 species of snails and slugs that live in marine, freshwater, and terrestrial habitats.
- Cephalopods, the most advanced mollusks, include octopuses, squids, cuttlefish, and nautiluses. Most of the members of this group either lack shells, or have small internal shells.
Gastropods or Bivalves
Of the roughly 100,000 known mollusk species, about 70,000 are gastropods, and 20,000 are bivalves or 90 percent of the total. It is from these two families that most people derive their general perception of mollusks as small, slimy creatures equipped with calcareous shells. While the snails and slugs of the gastropod family are eaten the world over (including as escargot in a French restaurant), bivalves are more important as a human food source, including clams, mussels, oysters, and other undersea delicacies.
The largest bivalve is the giant clam (Tridacna gigas), which reaches a length of four feet and weighs 500 pounds. The oldest mollusk is a bivalve, the ocean quahog (Arctica islandica), native to the northern Atlantic and known to live at least 500 years; it is also the oldest known animal.Bright yellow banana slug. Alice Cahill/Getty Images
Octopuses, Squids, and Cuttlefish
Gastropods and bivalves may be the most common mollusks, but cephalopods (the family that includes octopuses, squids, and cuttlefish) are by far the most advanced. These marine invertebrates have astonishingly complex nervous systems, which allows them to engage in elaborate camouflage and even display problem-solving behavior-for example, octopuses have been known to escape from their tanks in laboratories, squish along the cold floor, and climb up into another tank containing tasty bivalves. If human beings ever go extinct, it may well be the distant, intelligent descendants of octopuses that wind up ruling the earth-or at least the oceans!
The largest mollusk in the world is a cephalopod, the colossal squid (Mesonychoteuthis hamiltoni), known to grow to between 39 and 45 feet and weigh up to 1,650 pounds.548901005677/Getty Images
With the exception of cephalopods, mollusks are by and large gentle vegetarians. Terrestrial gastropods like snails and slugs eat plants, fungi, and algae, while the vast majority of marine mollusks (including bivalves and other ocean-dwelling species) subsist on plant matter dissolved in the water, which they ingest by filter feeding.
The most advanced cephalopod mollusks-octopuses, squids, and cuttlefish-feast on everything from fish to crabs to their fellow invertebrates; octopuses, in particular, have gruesome table manners, injecting their soft-bodied prey with venom or drilling holes in the shells of bivalves and sucking out their tasty contents.
The nervous systems of invertebrates in general (and mollusks in particular) are very different from those of vertebrate animals like fish, birds, and mammals. Some mollusks, like tusk shells and bivalves, possess clusters of neurons (called ganglions) rather than true brains, while the brains of more advanced mollusks like cephalopods and gastropods are wrapped around their esophagi rather than isolated in hard skulls. Even more weirdly, most of the neurons of an octopus are located not in its brains, but in its arms, which can function autonomously even when separated from its body.The mouth of a limpet. Getty Images
Reproduction and Offspring
Mollusks generally reproduce sexually, although some (slugs and snails) are hermaphrodites, they still must mate to fertilize their eggs. Eggs are laid singly or in groups within jelly masses or leathery capsules.
The eggs hatch into veliger larva-small, free-swimming larvae-and metamorphose into different stages, depending on the species.
Because modern mollusks vary so widely in anatomy and behavior, sorting out their exact evolutionary relationships is a major challenge. In order to simplify matters, naturalists have proposed a "hypothetical ancestral mollusk" that displays most, if not all, of the characteristics of modern mollusks, including a shell, a muscular "foot," and tentacles, among other things. We don't have any fossil evidence that this particular animal ever existed; the most any expert will venture is that mollusks descended hundreds of millions of years ago from tiny marine invertebrates known as "lophotrochozoans" (and even that is a matter of dispute).
Extinct Fossil Families
Examining the fossil evidence, paleontologists have established the existence of two now-extinct classes of mollusk. "Rostroconchians" lived in the world's oceans from about 530 to 250 million years ago, and seem to have been ancestral to modern bivalves; "helcionelloidans" lived from about 530 to 410 million years ago, and shared many characteristics with modern gastropods. Somewhat surprisingly, cephalopods have existed on earth ever since the Cambrian period; paleontologists have identified over two dozen (much smaller and much less intelligent) genera that plied the world's oceans over 500 million years ago.
Mollusks and HumansWayne Barrett & Anne MacKay / Getty Images
Over and above their historical importance as a food source-especially in the far east and the Mediterranean-mollusks have contributed in numerous ways to human civilization. The shells of cowries (a type of small gastropod) 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 since time immemorial. Another type of gastropod, 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 Pinna nobilis.
There are over 8,600 species listed in the ICUN, of which 161 are considered Critically Endangered, 140 are Endangered, 86 are Vulnerable, and 57 are Near Threatened. One, the Ohridohauffenia drimica was last seen in 1983 in springs feeding the River Drim in Macedonia, Greece and was listed as extinct in 1996. Additional surveys have failed to find it again.
The vast majority of mollusks live in the deep ocean and are relatively safe from the destruction of their habitat and depredation by humans, but that's not the case for freshwater mollusks (i.e., those that live in lakes and rivers) and terrestrial (land-dwelling) species.
Perhaps not surprisingly from the perspective of human gardeners, snails and slugs are most vulnerable to extinction today, as they are systematically eradicated by agriculture concerns and picked off by invasive species carelessly introduced into their habitats. Just imagine how easily the average house cat, used to picking off skittering mice, can devastate a near-motionless colony of snails.
Lakes and rivers are also prone to the introduction of invasive species, particularly mollusks which travel attached to international seagoing ships.
- Sturm, Charles F., Timothy A. Pearce, Ángel Valdés (eds.). "The Mollusks: A Guide to Their Study, Collection, and Preservation." Boca Raton: Universal Publishers for the American Malacological Society, 2006.
- Fyodorov, Averkii, and Havrila Yakovlev. "Mollusks: Morphology, Behavior, and Ecology." New York: Nova Science Publishers, 2012.