The phylum Molluska is one of the largest of all animal groups. Not only does it contain about 110,000 living species, the f ossil record indicates a long and extensive history.
The major mollusk groups, called classes by most taxonomists (scientists who study the relationships of plants and animals), are the Gastropoda (snails and slugs), the Bivalvia (clams, mussels, and shipworms), the Cephalopoda (squid, octopuses, cuttlefish, and nautilus), the Polyplacophora (chitons), the Scaphopoda (tooth shells), the Monoplacophora (a single-shelled animal), and the little-known Aplacophora (a questionable mollusk).
Of all animal phyla , the mollusks are perhaps the most difficult to describe in terms of a "typical" mollusk. In fact, no one characteristic is unique to the mollusks and shared by all species. Their body shapes are immensely different. In terms of feeding and behavior, mollusks range from docile, grazing herbivores to stealthy and aggressive predators. Most mollusks are marine, except for a few snails and clams that are found in damp terrestrial or freshwater environments.
Some characteristics of mollusks are unique to the group, but many are characteristic of invertebrates in general. However, researchers have identified a suite of characteristics that are combined in some generalized way within the mollusks. Mollusks do not have a central rod or backbone-type support. They do not have an exoskeleton .
Perhaps the most important characteristic of mollusks is the presence of a specialized tissue called the mantle . The mantle is part of the epidermis , or skin, of the mollusk. It is filled with mucous glands and is very slippery to the touch. The mantle contacts the shell only at the outer edges. The inner epidermis where the shell is no longer in contact with the mantle is filled with fluid and acts as a sort of protective cushioning for the mollusk's soft body.
The major function of the mantle is to secrete a substance that hardens into a three-layered shell. The outermost layer of the shell is a tough, solid deposit of calcium carbonate and is called the periostracum. The middle layer is a strengthening sheet of more calcium carbonate crystals. The innermost layer is made of organic chemicals that, when seen alone, capture light and appear to be multicolored. This layer is called the nacreous or prismatic layer. The beautiful appearance of this layer as seen in the inner shell of an abalone. The nacreous layer is also responsible for pearls. When a foreign particle penetrates the mantle, the shell-secreting cells in the mantle attach to the particle and build up layers of pearl around it.
The influence of the mantle and the resulting shell development is one of the most distinctive characteristics of the mollusks. Most living mollusk species have shells, although many have adapted to life without it. Remnants of the primitive shell can be found in the bodies of mollusks like the slugs and squids.
The gastropods , the snails and slugs, are one of the most familiar groups of mollusks. The name comes from a misunderstanding. Early biologists believed that the stomach of the gastropod was located in the muscular single "foot" on which the animals move, and that the animals ate with this foot. As a result, they named them "stomach-foot" (gastro-pod). Today we know gastropods do not eat with their "foot," but the name remains.
The body of a gastropod body rests on this single muscular "foot." The foot looks somewhat like a pedestal, and is the organ responsible for locomotion, or movement from place to place. Many species can withdraw their foot into their shell for protection, as gastropods are considered a tasty food item for many predators, including humans. Often, a gastropod will simply pull its shell over the foot for protection from predators.
The head end of the foot contains the sensory organs and often a set of retractable antennae. In most snails the mouth is lined with a sharp, zipperlike structure called a radula. The radula is used for scraping algae off rocks. However, there are many predatory species in which the mouth is located on the end of a proboscis, or long snout. This proboscis is used to bore into the shells of other animals. Some species also inject poison into the soft bodies of their prey before using the mouth to consume it.
The shells of gastropods are the most spectacular part of the gastropod body. Over millions of years, gastropods evolved elaborately coiled and brightly colored shells as protection for their soft internal organs. The diversity of coiling shapes, from plain spirals to intricate towers, makes these animals valuable for shell collectors. Unfortunately, this has led to severe population reductions in many species. The beautiful shell of the sea conch, which is often depicted in pictures and movies as a type of musical instrument, is a prized collector's item, and the conch is rapidly becoming hard to find.
The gastropods are one of the successful groups of mollusks, so much so that they are considered to be pests in many areas. Pulmonates, a group that includes slugs and snails who have evolved lungs , can survive in almost any temperate climatic region. They have been so successful, in part, because of their reproductive strategies. Most mollusks are either male or female. One onate, the familiar garden snail, is hermaphroditic , with both male and female reproductive organs. Being hermaphroditic allows the snail to reproduce anywhere at any convenient time.
Many snails live in freshwater ponds and lakes. The rest of the mollusks, with the exception of some clams, are all marine. The fact that snails live on land is important for many researchers who study climatic fluctuations, as many species are intolerant of even slight climatic changes. Snail fossils are often studied to interpret temperature and humidity in the past.
Slugs—gastropods that do not have shells—are also pulmonates. The pulmonates are considered to be the most derived of the mollusks. The most primitive mollusks are the Prosobranchia. These gastropods are the largest and oldest group of gastropods. They have an operculum , which is a small shell flap used to cover the foot when withdrawn into the shell cavity. They usually have a pair of gills and the sexes are mostly separate. The Opisthobranchia are a small group of gastropods who are very diverse in body form and function. In the opisthobranchs the body shell is greatly reduced. Some are small and round giving rise to the name bubble shell. All are marine. Many of the members of this group are burrowers who cause a lot of damage to rocks and the wood of many sailboats.
Many opisthobranchs have lost their shell entirely. Some of these animals are called the nudibranchs, who are probably the most beautifully colored animals on Earth. They have colorful projections on their backs that can be used for respiration, digestion, or protection if they contain stinging cells.
The bivalves, or clams, are another highly successful mollusk group. Bivalves have two shells, or valves, that are compressed on either side of the body. The shells are made from the excretions of the mantle as in gastropods, but are very different in shape and some functions.
The two shells enclose most of the body, and strong muscles inside the shell keep the two valves tightly closed. The shells open only to take in and release water or to allow the foot to extend into the sand.
Like gastropods, bivalves have a strong muscular foot that protrudes from the valves. However, bivalves do not use their foot as much for moving from place to place. Many bivalves do use the foot for moving through the sand, but it is more often used to anchor the animal in the fine sediments where it lives.
The body plan of bivalves is similar to that of gastropods but has many basic differences. The gills are paired and very large, and gas exchange occurs on the surface of these structures. The gills actually evolved primarily for the purpose of food gathering and their gas exchange function is secondary. The circulatory system is open, which means that blood is pumped by the heart through arteries into openings or sinuses. It is recaptured by veins and pumped back to the heart. The blood simply diffuses over the organs at the sinuses. Bivalve blood is poor in hemoglobin , the oxygen-carrying chemical in blood, but this deficiency is compensated for by the large surface area of the gills. Fortunately, the sedentary lifestyle of the bivalve does not require great amounts of oxygen.
Bivalves are filter feeders who take in water through a front opening, called an in-current siphon, and release it through an ex-current siphon. The digestive system includes an esophagus, stomach, long intestine, and an anus. Once the water is taken in, it passes over the gills, and mucus that coats the gills captures organic debris in the water. This organic material is transported to the esophagus by cilia .
Bivalves do not have a very evolved sensory system, but are limited to a few ganglia that respond to environmental stimuli. They have no antennae. Pectens, or scallops, are the only bivalves that have eyes complete with lenses, corneas, and retinas .
Bivalves have separate sexes. Reproduction occurs by releasing sperm and eggs in the water. In some species, the gametes (sperm and egg) are taken into the body and fertilization occurs within. In others, fertilization takes place in the external environment of the water.
All bivalves are aquatic . Some forms live in freshwater, but it is more common to find them in marine environments. Their diversity is largely based on the way they feed, the structure of their gills, and where they live.
Cephalopods include the squid, octopus, cuttlefish, and the survivor of an ancient lineage—the nautilus. Their variety of forms and functions is so varied that entire books have been written about specialized groups of cephalopods.
Shells, which are so typical of mollusks, appear to be absent in the cephalopods. In both squid and cuttlefish, an internal vestige of the shell remains inside the soft body. Anyone who has ever seen the "cuttlebone" used in parakeet and parrot cages may not realize that this is the remnant of the ancient mollusk shell. The squid have an even more reduced shell that exists as a simple clear plasticlike film that runs the length of the body. The octopus has the vestige of a shell that has been put to use as mouthparts that are quite effective for biting prey.
The sensory systems of the nautilus, octopus, and squid are highly developed. These animals are extremely intelligent and have shown that they have memory and can solve problems of a simple nature. They are acutely aware of their environments and can respond to threats with a wide array of defense tactics. The octopus and squid are well-known for squirting dark colored "ink" into the water to confuse a predator, then darting away in the flash of an instant.
Cephalopods are the swiftest mollusks. While most mollusks are sedentary or sluggish, cephalopods can swim quite fast at times and are masters of deep-ocean living. Locomotion is accomplished by means of jet propulsion in which a strong ex-current siphon spurts out water to propel the animal away from danger. Cephalopods sneak up on their prey, using their tentacles to stealthily reach out towards the victim or to delicately slide along the ocean bottom.
Squid, octopus, and cuttlefish are very unlike the nautilus. The relationship between an octopus and a nautilus is hard to see unless the animals are closely examined, but both animals have similar tentacles, which they use for grasping prey. The tentacles can wrap around a prey item such as a shrimp and pull it to the mouth.
Scientists know little about the soft body evolution of the cephalopods, but the fossil record of coiled shell species is well documented. The ancestors of the nautiloids are the ammonites , animals with spirally coiled shells that, unlike gastropod shells, have separate chambers. These chambers functioned in gas regulation and allowed the animal to move up and down in the water column. Some ammonites are believed to have dived so deeply down into the water searching for prey that, as a result of increasing water pressure, they developed very intricate infoldings of the shell. The infoldings increased the surface area of the fragile shell, which in turn increased its strength. These shells could have withstood enormous underwater pressures. The graceful and highly intricate shells of the ammonites are valued by people all over the world.
Other Mollusk Groups
It is easy to find general mollusk characteristics in other mollusk groups. The polyplacophorans, or chitons, are shelled grazing animals found on rocks in the intertidal zone. The scaphopods have shells that look like long pointed teeth. They live in the sand and filter food from the water. A rare, recently discovered monoplacophoran called neopilina is a single-shelled animal that is considered to be a "living fossil" as scientists had thought they were extinct. Scientists believe the monoplacophorans represent the primitive form of the mollusks.
Brook Ellen Hall
Barth, Robert, and Robert Broshears. The Invertebrate World. Philadelphia: Saunders College Publishing, 1982.
General Biology. New York: Grove and Newell, University Tutorial Press, 1969.
Pechenik, Jan. Biology of the Invertebrates. Boston: Prindle, Weber & Schmidt, 1985.