Mollusks

Mollusk Facts

• Classification:
• Geographical location: Every ocean and many bodies of freshwater on every continent except Antarctica
• Habitat: Mostly salt water, although many live in freshwater and on land
• Life span: Some live for under a year, others for one to five years; a few species live much longer
• Special anatomy: Foot (for locomotion), siphons (for jet propulsion), a shell of calcium carbonate and protein, gills or lungs, a tonguelike radula for drilling or grinding food

Mollusks first appeared in early Cambrian times, 650 million years ago. They are the largest marine animal phylum and the second largest animal phylum, Mollusca (from Latin mollis, “soft”). The only larger animal phylum is Arthropoda. According to some estimates, there are approximately 86,000 mollusk species. All mollusks have soft, boneless bodies. Most have shells, though some shells are poorly developed and are even absent in some cases. Mollusk shells are like coats of armor.

Depending on the classification, mollusks form seven or eight classes. Best known are gastropods, including approximately 65,000 species of snails and slugs; bivalves or pelecypods, approximately 15,000 species of clams and scallops; cephalopods, approximately 800 species of squid and octopuses; and polyplacophora, approximately 600 to 1,000 species of chitons. The other classes contain under 1 percent of mollusk species and are not discussed.

Mollusks are present in all of Earth’s habitats. However, most of them, which comprise the greatest species diversity, are in the oceans. There, the largest number are various species of gastropods (snails). All gastropods are univalves—possessing one shell. Many other mollusks are the familiar pelecypod—bivalve—mollusks (clams), which have two shells. Mollusk bodies look undeveloped, and many lack apparent heads. However, all have well-developed nervous, circulatory, respiratory, and sensory systems. Except for cephalopods, mollusks are slow-moving, sluggish, or immobile (sessile). In many cases, they spend their adult lives attached to rocks or dug into sand or mud, awaiting the approach of prey.

Some mollusks, mostly gastropods, can be found in freshwater and on land. For example, snails and slugs are seen at the bottoms of ponds, rivers, and lakes or under fallen trees and decaying logs. Some land snails even live in tree branches. Characteristic features of gastropods are a true head, a creeping surface (or foot), eyes, and tactile feelers. The foot gives gastropods their name, which means “foot on belly” in Latin. Most gastropods have a univalve shell, but some species have no shell. Others, such as garden slugs, have tiny internal shells.

Physical Characteristics of Mollusks

The largest mollusk is the giant squid, which is also the largest invertebrate. It can be fifty-five feet long and weigh several tons. Most mollusks, however, range from five-tenths to ten inches in length and only weigh up to a few pounds. One exception to the size rule is the tropical giant clam, with a maximum diameter of four and a half feet and a maximum weight of five hundred pounds.

There are many mollusk variations. However, some features can be generalized in mollusk bodies based on gastropods and bivalves. In motile forms, locomotion is due to crawling on flat, muscular, snail-like feet, though jet propulsion occurs in cephalopods. Also, the mollusk body has a head at one end and an anus at the other. Furthermore, much of the body is covered by a shell (internalized in cephalopods) of calcium carbonate and protein. Shells are made by mantle tissue. Inside a shell is a large part of the body, fragile parts of cardiovascular, digestive and excretory, nervous, reproductive, and respiratory systems.

Mollusk shells thus function protectively, like the rib cages and pelvic bones of vertebrates. The shells of bivalves are divided into two valves, which are opened or tightly closed by strong muscles. These muscles must be cut to open the shell for examination. The gastropods possess one asymmetrical shell, often of spiral shape, into which they retreat from the world. In cephalopods, the shell is internalized but still yields organ protection.

The organs and their positions in mollusks, especially gastropods, are as follows. At the anus end of the body are two or more gills (lungs in land forms) for breathing. At the front end of the body are jaws and a tonguelike, toothed radula. It grinds food or procures it by drilling into shells of prey or ship timbers (in shipworm snails). Around the jaws are tentacles, used for sensation in gastropods. Cephalopod tentacles are used to capture prey. Between the radula and the anus are the stomach and the gut. A heart, near the hind end of the body, receives blood and sends it into the body cavity. The mollusk nervous system—comparable to that in fish—is composed of nerves surrounding the gut, a brain, and sense organs. Mollusks also have kidneys and gonads.

Many mollusks are herbivores, grazing on underwater plants. Some terrestrial gastropods, such as slugs, eat cultivated plants and are serious garden and agricultural pests. Scaphopoda feed on organic matter deposited on ocean and lake bottoms. Bivalves filter protozoa, eggs of sea animals, and diatoms out of the water in which they live. Many gastropods are hunter-carnivores, eating slower-moving or sessile animals, such as other mollusks. Cephalopods are very active predators and prey on animals, such as crabs.

The Life Cycles and Senses of Mollusks

Most mollusks have separate sexes, and a few exhibit courtship behavior. With or without courtship, mollusks usually spawn sperm and thousands of eggs into the waters around them. There, fertilization and development of offspring occur. When eggs hatch, offspring undergo a larval stage. Larvae of most mollusk species are, at first, free-swimming, using for locomotion a ring of cilia. Most settle to lakes, oceans, or stream bottoms as crawlers who mature into adults.

Many mature mollusks, such as oysters and mussels, are sessile, permanently affixed to rocks. Gastropods are capable of slow crawling on a muscular foot. Cephalopods, such as squid and octopuses, are an exception to the mobility rule for mollusks. They are very mobile at all stages of their lives. Adult cephalopods move very quickly by expelling jets of water from mantle cavities using an organ called a siphon. Jets are expelled in the direction opposite to movement. Jet propulsion makes cephalopods the fastest-moving mollusks. It also suits them to a life of vigorous predation. In cephalopods, the foot differentiates to ten arms (squid) or eight arms (octopuses) that seize prey.

In some cases, fertilization in mollusks is internal, with protective coverings secreted around eggs. In others, fertilization and development are internal in females, and offspring are delivered alive, as with live-bearing snails. Slow-moving snails and related mollusks are often hermaphrodites or intersex (both male and female). This doubles the number of mates available to each such organism, and often self-fertilization occurs. Both options have great survival value. A few species reproduce by parthenogenesis without fertilization. Also, mother mollusks may protect developing eggs, and some oysters raise their offspring within the mantle cavity.

The mollusks live for periods ranging from under a year to two years for many small to medium-sized varieties and four to five years for some snails and shellfish. Giant squid (Architeuthis dux) live an average of four or five years, while wild giant clams (Tridacna gigas) may live for up to 100 years. The oldest known mollusk is the ocean quahog (Arctica islandica), which is estimated to have a life span of around 500 years. The geoduck clam (Panopea abrupta) and the freshwater pearl mussel (Margaritifera margaritifera) also have long life spans, averaging 160 to 190 years.

Sensory ability varies in mollusks. Most mollusks see poorly or not at all. However, cephalopods have eyes like vertebrates, complete with lenses and retinas. Squid eyes reportedly lack eyelids but otherwise look like those of humans. Although it is not ubiquitous in mollusks, some gastropods have well-developed abilities to smell and find food at considerable distances. Predators are often detected in this way, too.

Conclusive studies of intelligence have not been carried out on most mollusks. However, Cephalopods, including squids and octopuses, claim the ability to learn from experience and are among the most intelligent invertebrates. These animals have large centralized brains and learn by trial and error to solve complex problems. They also demonstrate spatial, mapping, and planning intelligence. For example, octopuses are notorious for escaping their enclosures in aquariums.

Beneficial and Destructive Mollusks

Mollusks are very abundant. On the benefit side, they are important foods for bottom-feeding fish and whales. Numerous mollusks are also valuable foods for humans, including clams, mussels, oysters, octopuses, and scallops. More exotic food mollusks include land snails. Mollusks (especially oysters) also produce pearls, the only gems made by living organisms.

On the debit side, gastropod and cephalopod carnivores do serious damage. Gastropods prey on slow-moving or sessile organisms, wreaking havoc on clam and oyster beds that provide shellfish for human consumption. Also, cephalopods are active predators, diminishing the sea’s yield of meats sought by humans.

Another mollusk problem is foreign invasion, exemplified by zebra mussels (Dreissena polymorpha). These pistachio-nut-sized European saltwater mussel invaders were released into the Great Lakes from the hulls of ocean-going vessels in the late 1980s. They flourish in the Arkansas, Hudson, Mississippi, Ohio, and Saint Lawrence Rivers. Superabundant reproduction (a female lays a million eggs per year) yields myriad larvae that, as adults, clog power plant and factory water intakes, requiring costly cleanup. More problematically, they eat microbes in the water, damaging the lifestyles of indigenous wildlife that also use this food. It is estimated that the mussels cost the industry and consumers over $500 million annually.

Other invasive mollusks include Giant African land snails (Lissachatina fulica), which carry a lungworm parasite that can infect humans and cause meningitis, and Quagga mussels, which often eat all the phytoplankton in the ecosystem they invade, leaving no food for indigenous species.

Principal Terms

Foot: bottom portion of gastropods, on which they walk; it subdivides into tentacles in cephalopods

Hermaphrodite: an organism having both male and female reproductive systems; often referred to as intersex in the twenty-first century

Mantle: the outermost living tissue of mollusks; it makes shells, mother-of-pearl, and pearls

Radula: a tonguelike, toothed organ used to grind food or drill holes in shells of prey

Sessile: an organism incapable of moving from its point of origin

Bibliography

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