The mollusks (American spelling) or molluscs (British spelling) are the large and diverse phylum (Mollusca) of invertebrates that includes a variety of familiar animals well-known for their decorative shells or as seafood. These range from tiny snails, clams, and abalone to the octopus, cuttlefish, and squid (which have complex nervous systems and are considered the most intelligent invertebrates).
Mollusks are characterized by having a true coelom; a body typically divided into the three parts of head, visceral mass, and muscular foot; organ systems for circulation, respiration, digestion, excretion, nerve conduction, and reproduction; and most mollusks have one or more shells and are bilaterally symmetrical (Towle 1989). Unlike the closely related annelids, mollusks lack body segmentation.
With more than 100,000 recognized species (Feldkamp 2002), mollusks are the second most diverse animal phyla after Arthropoda. The giant squid, which until recently had not been observed alive in its adult form, is the largest invertebrate although it is likely that the colossal squid is even larger.
Mollusks are renowned as a source of food, including clams, scallops, calamari (octopus), mussels, abalone, oysters, and other shellfish (a term that also includes some crustaceans). They likewise are invaluable to food chains, serving as a major source of food for fish, and their sensitivity to pollution makes them excellent monitors of water quality.
However, mollusks also appeal to people's internal nature that seeks beauty, as their diversity of form and colors provides joy, and their images are used to decorate walls, jewelry, paintings, rugs, and so forth. The pearl of an oyster or a fresh water mussel, and the mother-of-pearl from an abalone, may have little practical value, but are prized as jewelry.
On the negative side, snails and slugs damage crops, and snails serve as alternative hosts of schistosomes, a parasite that infects people.
The term "mollusk" comes from a Latin term meaning "soft," referring to the bodies of these invertebrates, although most have a shell covering. Octopuses and slugs are among those that lack such a shell. The scientific study of mollusks is called malacology.
Anatomy and characteristics
While most mollusks are marine, there are also many freshwater and terrestrial species. Some are predators, such as octopuses and squids, which capture prey with tentacles and attached suckers, and use a sharp chitinous beak and radula to kill and eat the prey. Others, such as oysters and clams, are sessile filter feeders. While some mollusks are tiny, measured in millimeters, the giant squid may grow to 20 meters long and weigh more than 3,360 kilograms (Towle 1989).
Mollusks are triploblastic (having three primary germ layers: the ectoderm, mesoderm, and endoderm) protostomes. The principal body cavity is a blood-filled hemocoel. They have a true coelom (eucoelom; fluid filled body cavity within the mesoderm). However, any coelomic cavities have been reduced to vestiges around the hearts, gonads, and metanephridia (kidney-like organs).
The body of a mollusk is generally divided into three distinct parts: a head, with eyes or tentacles, a muscular foot, and a visceral mass housing the organs. The muscular foot is used for locomotion and the head contains the sense organs, cerebral ganglia, and mouth (Towle 1989). The heart and organs of reproduction, digestion, and excretion are located in the visceral mass.
Mollusks have a mantle, which is a fold of the outer skin covering the visceral mass. In most species, this mantle secretes a calcium carbonate external shell.
In this phylum's level of organization, organ systems from all three primary germ layers can be found:
- Nervous System (with brain)
- Excretory System (nephridium or nephridia)
- Circulatory System (open circulatory system)
- Respiratory System (gills or lungs)
All species of the phylum Mollusca have a complete digestive tract that starts from the mouth and goes to the anus Many have a feeding structure, the radula, mostly composed of chitin. Radulae are diverse within the Mollusca, ranging from structures used to scrape algae off rocks, to the harpoon-like structures of cone snails. Cephalopods (squid, octopuses, cuttlefish) also possess a chitinous beak.
The gills of mollusk extract oxygen from the water and dispose waste.
Development passes through one or two trochophore stages, one of which (the veliger) is unique to the group. These suggest a close relationship between the mollusks and various other protostomes, notably the annelids.
Mollusks are generally organized into nine or ten classes, with one that is known only from fossils, but the others with extant (living) representatives.
- Class Aplacophora - solenogasters, deep-sea wormlike creatures; 250 species.
- Class Caudofoveata - deep-sea wormlike creatures; 70 known species; now generally recognized as a subclass of Aplacophora.
- Class Polyplacophora - chitons; 600 species, primarily animals that live on rocks on marine shorelines; shells divided into eight separate plates.
- Class Monoplacophora - deep-sea limpet-like creatures; 11 living species
- Class Bivalvia (also Pelecypoda) - clams, oysters, scallops, mussels; mostly sessile and filter feeders; 8,000 species; have shell with two valves (bivalvia means “two valves”) and a muscular foot that can swell and pull animal down in the substrate.
- Class Scaphopoda - tusk shells; 350 species, all marine
- Class Gastropoda - nudibranchs, snails and slugs, limpets, conches, sea hares; sea angel, sea butterfly, sea lemon; largest and most diverse mollusk class with an estimated 75,000 to 150,000 species; most have a single shell, but slugs and some others have no shell; gastropoda means "stomach-foot."
- Class Cephalopoda - squid, octopus, nautilus, cuttlefish; 786 species, all marine; have a large well-developed head and prominent foot with many tentacles (cephalopod means "head-foot").
- Class † Rostroconchia - fossils; probably more than 1,000 species; probable ancestors of bivalves.
- Class † Helcionelloida - fossils; snail-like creatures such as Latouchella.
History of mollusks
Mollusk fossils are some of the best known and are found from the Cambrian onwards. The oldest fossil seems to be Odontogriphus omalus, found in the Burgess Shale. It lived about 500 million years ago.
Based on comparison of living specimens, it is generally held that mollusks and annelids (segmented worms, Phylum Annelida) are closely related and share a common ancestor (Towle 1989). They have similar patterns of embryological development and share a characteristic larval form (the first stage of larval development, the trochophore), among other factors (Towle 1989). These were probably the first groups to have a true coelom.
Within the mollusks, Brusca and Brusca (1990) suggest that the bivalves and scaphopods are sister groups, as are the gastropods and cephalopods, so indicated in the relationship diagram below.
The development of a shell of calcium carbonate would have been of significant adaptive value, but would also reduce the surface area for gas exchange, necessitating a structure such as gills (Towle 1989).
All major molluscan groups possess a skeleton, though it has been lost evolutionarily in some members of the phylum. It is probable that the pre-Cambrian ancestor of the mollusks had calcium carbonate spicules embedded in its mantle and outer tissues, as is the case in some modern members. The skeleton, if present, is primarily external and composed of calcium carbonate (aragonite or calcite). The snail or gastropod shell is perhaps the best known molluscan shell, but many pulmonate and opistrobranch snails have internalized or altogether lost the shell secondarily. The bivalve or clam shell consists of two pieces (valves), articulated by muscles and an elastic hinge. The cephalopod shell was ancestrally external and chambered, as exemplified by the ammonites and nautiloids, and still possessed by Nautilus today. Other cephalopods, such as cuttlefish, have internalized the shell, the squid have mostly organic chitinous internal shells, and the octopods have lost the shell altogether.
- Brusca, R. C., and G. J. Brusca. 1990. Invertebrates. Sunderland, MA: Sinauer Associates. ISBN 0878930981
- Feldkamp, S. 2002. Modern Biology. Austin, TX: Holt, Rinehart, and Winston. ISBN 0030565413
- Starr, C., and R. Taggart. 2002. Biology: The Unity and Diversity of Life. Pacific Grove, CA: Thomson Learning. ISBN 0534388019
- Towle, A. 1989. Modern Biology. Austin, TX: Holt, Rinehart, and Winston. ISBN 0030139198