What is a mollusk?
In general, mollusks are soft bodied animals living in many different environments: from the depths of the oceans to the heat of the desert, and from small backyard ponds to the tops of mountains. They are one of the most diverse groups of organisms on the planet.
The Phylum Mollusca is estimated to include 100,000 species. The study of mollusks is called malacology (mal-uh-col-oh-gee) which means “the study of animals with soft bodies.”
The seven living classes of mollusks are Aplacophora, Bivalva, Cephalopoda, Gastropoda, Monoplacophora, Polyplacophora and Scaphopoda. All seven classes are represented in the scientific collections, where there are a total of two million mollusk specimens, the tenth largest collection in North America. Discover more about mollusks here!
Aplacophora: shell-less and worm-like
Known as the naked mollusks, they are worm-like in body form and lack shells but have many calcareous spicules. They can be found in oceans all over the world in very deep water. Some of these features are found in certain subgroups within the Aplacophora. Molluscan features: Narrow foot, mantle, radula.
Shell composed of 8 plates, most live in shallow water. Molluscan features: shell, ventral foot, mantle, radula.
These tiny single shell animals were only known as fossils until 1952 when a live monoplacophoran was discovered more than 11,000 feet deep off the coast of Costa Rica.Very rare; only a few specimens known. Found in very deep water. Molluscan features: shell, ventral foot, mantle, radula.
Bivalvia: Clams, Mussels, Oysters
Second largest group of mollusks. Possibly the most important group to our own region in terms of economics and the health of our
aquatic ecosystems. As efforts are underway around the region to restore the fisheries and waterways, the Museum’s historic records of the
bivalves that occurred in the area provide conservationists a glimpse of the fauna once found in the state’s streams, bays, and coastline. Molluscan features: shell, foot, mantle.
Gastropoda: Slugs and Snails
The largest and most diverse group of mollusks, found in largest number of environments. Great indicators as to the health of an ecosystem, as they are susceptible to changing environmental pressures. Molluscan features: shell, foot, mantle radula.
Scaphopoda: tusk-shaped shells
These mollusks are tooth or tusk shaped. They live in sediment offshore. Entirely marine group, conserved morphology. Molluscan features: shell, foot, mantle, radula.
Most active group, containing some of the largest, most mobile, and most intelligent mollusks with a highly developed visual sense. They also have a long fossil history and include the belemnite, the Delaware state fossil. Molluscan features: shell (internal), foot, mantle, radula.
There are several features mollusks share, separating them from other soft bodied animals such as worms and insects. Each group has a combination of these features:
- Mollusks make a hard external shell made of a mineral called calcium carbonate.
- The shell of mollusks is secreted by an organ called the mantle. As the mollusk grows bigger the mantle makes more shell to keep pace.
- A muscular foot used to dig or crawl.
- All mollusks, except the bivalves, have a strip of raspy teeth in their mouths called a radula they use when feeding.
In addition to these features, most mollusks have eyes and a well-developed nervous system, a circulatory system and a heart, gills, and a digestive system.
Color: Pattern and Structure
Camouflage, Warning, Display, Disguise
The color of mollusk shells is due to the presence of different amounts and types of pigments in the shell. As specialized cells in the mantle produce the liquid calcite, that hardens to form the shell of the mollusk, other color producing cells in the edge of the mantle produce pigments that produce the colors and patterns you see.
Colors are due to many factors
Most of the cells that secrete pigments are located along the edge of the shell. Patterns of color are created by the movement of groups of these cells as the animal grows, or by stopping and starting the secretion of the pigments to produce dots or dashes.
The basic colors and patterns are under some degree of genetic control, but diet and environment can also influence the end result.
For example, research has shown that the color and pattern of certain snails changes depending on the salinity of their immediate environment.
Some colors, such as the iridescent blue-green of abalone shells are not due to pigments but to the refraction of light off the different layers of nacre.
Bright colors send a message
In addition to their brightly colored shells many mollusks have beautiful markings on their bodies. Some species use these colors as camouflage whereas others seem to be using bright colors to advertise their presence.
Mollusk shell shapes and sculpture
Many shells have spines or other types of sculpture that appear to have a definite function. Spines are probably used as protection from predators and may also make the mollusk bigger and therefore harder to eat.
In addition to spines, many shells have thickened ribs that add strength to the shells, whereas others have knobs or bumps that probably help the animal to stay embedded in the sediment.
Some mollusks, like the carrier shells in the family Xenophoridae, don’t produce the sculpture themselves but instead create a unique sculpture by cementing rocks and the shells of dead snails and clams onto their shells.
Predators, Herbivores, Omnivores
Mollusks are a biologically diverse group of animals that exhibit an astounding variety of feeding habits. There are herbivores that graze on algae covered rocks and aggressive predatory squids and octopus that jet through the water in search of fishes and other prey.
The dietary habits of gastropods runs the gamut from grazers and scavengers to carnivores and parasites.
Whelks scavenge just about anything they can find and also prey on clams and oysters by prying them open with the edge of their shell and sucking out the animal inside! Some snails, like cone shells, use toxins to paralyze their prey and then consume it whole. Members of the family Muricidae are experts at drilling through the shells of other mollusks. They secrete digestive enzymes through the opening and then slurp up their meal through the hole. Other snails, like the worm shells, are filter feeders that are permanently anchored in place and use long sticky tendrils that drift in the water and entrap food particles.
Hover over the images below to learn how different mollusks eat.