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[photo, Oyster shells, Shady Side, Maryland] Molluscs (phylum Mollusca) include clams, mussels, oysters, and snails, as well as numerous other families. With over 50,000 recorded species in the world (estimates of total species exceeds 200,000), this phylum is second only to arthropods in number of living species in the world. Mollusca is an incredibly diverse grouping of tiny (under 1 centimeter) and gargantuan (over 50 feet) species (though the largest species are not found in Maryland). Included species may be sexual or asexual, stationary or mobile, or even migratory. Most species are aquatic, yet molluscs may be found nearly anywhere.

Oyster shells, Shady Side, Maryland, October 2011. Photo by Diane F. Evartt.

In order Mollusca, there are nine recorded classes: seven currently in existence (with two extinct). In Maryland, three classes are found: Bivalves, Cephalopods, and Gastropods.
BIVALVES (class Bivalvia)
Though named for their shell, which actually consists of two shells connected at a hinge, bivalves are not the only creatures with this trait. The family Juliidae, for example, within class Gastropoda also bears a double shell. Bivalvia, however, is the only class with this trait found in Maryland. Like most molluscs, bivalves greatly benefit their environment, and generally are found in streams or rivers with sandy (or rocky) and muddy bottoms in sections with slow current. In Maryland, bivalves also historically have favored the Chesapeake Bay, leading to a large market in seafood and aquaculture.

Arks are clams. They have white, sometimes brown, ribbed shells with an opening for food, a foot that anchors them into place, and a straight hinge line. Living in salt water, they burrow into sediment and eat detritus and plankton. Their name comes from their passing resemblance to a ship.

Clam, Asian (Corbicula fluminea)
Asian Clams have an outer shell that is yellow-green with white spots, while the inner shell is purple. There are concentric rings on the outside. They grow up to 1.75 inches in length. They eat algae which they filter from the water. Asian Clams live on the muddy or sandy bottoms of freshwater lakes and streams. The Asian Clam is an invasive species. Also known as Golden Clam.

Clam, Hard-shell (Mercenaria mercenaria)
Hard-shell Clams have white or gray shells, sometimes with brown patches. Dark growth rings extend from the middle. A muscular foot enables them to move through mud. The inside of the shell is white and usually has a purple patch. They can grow up to 4 inches wide. Hard-shell Clams are filter feeders, absorbing algae and plankton along with water. Living in mud flats, bays, and estuaries, they burrow into sand. Also known as Quahog or Round Clam.

Clam, Macoma
Macoma Clams are very small with thin, fragile shells. They have one foot, which anchors them into place, and two siphons: one drawing food and water in and another expeling waste. These stick up through the sand or mud. Macoma Clams live in shallow water, where they burrow into sand or mud, moving when necessary.

Clam, Soft-shell (Mya arenaria)
Soft-shell Clams have oval-shaped shells that are thin and very fragile, which is how they got their name. They are white, but have a brown or gray covering. Their one foot serves as an anchor, while two siphons, which stick up through the mud, draw in and expel seawater. Soft-shell Clams can grow up to 4 inches in length. They live in mud flats, burrowing up to a foot under the surface.

Mussel, Atlantic Ribbed (Geukensia demissa)
Atlantic Ribbed Mussels have dark, ribbed shells that may be brown, black, or green, with an interior of white or blue. They can grow up to 4 inches in length. They live in marshes and mud flats, where they attach themselves to solid surfaces. Using their byssal threads, they also can embed in sediment. At high tide, they open their shells to draw in water and expel waste.

Mussel, Dwarf Wedge (Alasmidonta heterodon)
Dwarf Wedge Mussels have brown or olive trapezoid-shaped shells. The interior is blue or white, while the posterior is iridescent. They grow up to 1.78 inches in length. Dwarf Wedge Mussels live in clean, freshwater streams and rivers. Their larvae attach themselves to a host fish in order to develop, dropping off when they become juvenile mussels, leaving the fish unharmed. Their shells formerly were used to make buttons and were vital in the Japanese cultured pearl industry. The Dwarf Wedge Mussel population has declined due to the degradation of its habitat and the absence of its host fish. The Dwarf Wedge Mussel is an endangered species.

Mussel, Zebra (Dreissena polymorpha)
Zebra Mussels have "D"-shaped, dark- and light-striped shells, thus the origin of their name. They can grow up to 2 inches in length. They draw in and expel water, with each mussel filtering up to one gallon of water a day. Generally, they live in fresh water, but can be found in brackish areas, clinging to sediment or to solid surfaces with their byssal threads. The Zebra Mussel attaches itself to native mussels and clams, and builds colonies on top of them, eventually killing them. The Zebra Mussel is an invasive species.

Oyster, Eastern (Crassostrea virginica)
Eastern Oysters have gray-brown shells and white interiors. They can grow up to 8 inches in length. They live on the sea bottom in areas known as oyster beds. Eastern Oysters draw in water, feeding on algae and expelling waste. One oyster can filter up to 50 gallons of water each day. Eastern Oysters can make pearls, but they have no value in today's market. They are important for their shell deposits create beds and reefs, forming habitats for other creatures. Some deposits, due to continuous layering, can be as much as 50-feet thick. In fact, the city of Crisfield in Somerset County is built upon a foundation of oyster shells. Also known as American Oyster.

Scallop, Bay (Argopecten irradians)
Bay Scallops have thin, fan-shaped shells with 13 and 22 ribs radiating from the middle. Their top or left valves may be brown, black, gray, yellow, or red, while the bottom or right valves are lighter in color. The interior of the shell is white and can have a purple hue. With eighteen pairs of blue eyes along the edge of their shell, they see movements and predators. They can grow up to 3.5 inches. From the water, they filter plankton. They live on the bottom of saltwater bays, harbors, and estuaries, especially among eelgrass, and can swim short distances.

Shipworm, Gould's (Bankia gouldi)
Gould's Shipworms have long worm-like bodies, with the anterior end covered by a ridged shell that is used to burrow into untreated wood. They have two siphons that are used to ingest oxygen and food, including wood and plankton. With lime that they secrete, they line their burrow. They live in pilings, docks, driftwood, and ship hulls. Shipworm tunnels never intersect.

CEPHALOPODS (class Cephalopoda)
Cephalopods are aquatic molluscs that possess a prominent head, and a set of tentacles. Often referred to as octopus or squid, two species of Cephalopod are found in Maryland's waters.

Squid, Atlantic Brief (Lolliguncula brevis)
Brief Squids have large mantles, two large eyes, beaks, and long, soft bodies with five pairs of appendages or tentacles extending from their heads. They are red-brown or yellow-brown, but can change color. In order to move, water is pushed through the funnel that is located under the heads. If a predator approaches, the funnel will shift forward and propel the Brief Squids backwards very quickly or the squid will emit an ink cloud. They can grow up to 5 inches in length. They eat fish and crustaceans. Brief Squids live in schools in warm, shallow water.

Squid, Longfin Inshore (Loligo pealeii)
Longfin Inshore Squids are red, orange, or pink with brown spots, but may change their color. They have large mantles, beaks, two large eyes, and ten appendages, including eight arms and two tentacles. They can grow up to 18 inches in length. Depending on their size, they eat plankton, crustaceans, fish, and other squids. Near the continental shelf, they live in schools and migrate offshore to warmer waters during winter months. To distract predators, they squirt ink into the water. Also known as the Longfin Squid and North Atlantic Squid.

GASTROPODS (class Gastropoda)
Class Gastropoda is comprised of 611 recorded families of snails and slugs (with some 202 extinct). Of classes within the Animalia kingdom, Gastropoda is second only to Insecta in total number of recorded species. While many gastropods are aquatic, most humans encounter them on land for many gastropods aid in recycling of forest nutrients. Preferring cool damp environments, snails and slugs live on a diet of wood detritus, leaves, and other biologic debris. Gastropods aid the decomposition process, and expedite the reintroduction of minerals into the soil. An extinct species of sea snail, Ecphora gardnerae gardnerae is Maryland's State fossil.

Drill, Atlantic Oyster (Urosalpinx cinerea)
Atlantic Oyster Drills are snails. Their pointed, oval-shaped shells can be brown, gray, yellow, or purple with vertical ribs and multiple whorls. They grow to 1 inch in length. They drill through the shells of oysters and eat them, hence their name, but they also will eat other mollusks. Atlantic Oyster Drills live on oyster reefs and among grasses in shallow water.

Periwinkle, Marsh (Littorina irrorata)
Marsh Periwinkles are snails. They have grooved shells that are gray-white with brown-red spots and from eight to ten whorls on the shell. They grow to 1 inch in length. Living in salt marshes and wetlands, they eat algae and fungi that grows on marsh grass.

Slug, Sea
Sea Slugs resemble garden slugs. They are shell-less, have tentacles on their heads, and various growths on their soft bodies. They can grow up to 1.5 inches in length. Sea Slugs live in shallow water, including marshes and rivers, and in solid structures, such as reefs and buoys.

Snail, Chinese Mystery (Cipangopaludina chinensis)
Chinese Mystery Snails have shells that are green on the outside and white or blue inside. The smooth shell is large and conical with six or seven whorls. They have gills and a plate on their shell which shuts like a door when the snail is fully retracted inside. Their shells can grow up to 2.5 inches in length. They eat algae. They live in freshwater, such as lakes, irrigation canals, and ponds, where they settle into the mud. Chinese Mystery Snails carry parasites that can infect humans, namely the intestinal fluke. The Chinese Mystery Snail is an invasive species. Also known as Trapdoor Snail.

[photo, Knobbed Whelk (Busycon carica) shell, Baltimore, Maryland] Whelk, Knobbed (Busycon carica)
Knobbed Whelks are snails. Their dextral (right-sided opening) shells are ivory or gray with orange interiors bearing six coils with multiple knobs in a ring around the widest one. Knobbed Whelks have soft bodies, two pairs of tentacles, a foot, and a long proboscis for eating clams and oysters. They can grow up to 9 inches in length. During summer and winter months, they dwell in deep water, returning to shallower areas, such as mud flats or estuaries, in Spring and Fall.

Knobbed Whelk (Busycon carica) shell, Baltimore, Maryland, July 2014. Photo by Sarah A. Hanks.

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July 1, 2015

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