Mollusks of Great Bay Estuary

Mollusks [Mollusca]

Their name is derived from the Latin word mollis, meaning soft, because all of these animals have soft bodies. They either have one shell (most gastropods), two shells (bivalves), or no shell (cephalpods). Most have a head, a foot, and a visceral mass that holds their organs.

Two thirds of all mollusks are gastropods (meaning stomach-foot), and most of these are snails. Nearly all gastropods have a single shell, or valve, so they’re also called univalves. They also have a ribbon-like tongue with minute teeth, called a radula, for collecting food.

A snail sees with two tentacles projecting out front, and touches with another set of tentacles beneath them. Its mouth is just below. These are parts of the head. The snail crawls and climbs with its foot. A spiral shell protects its internal organs, including the stomach, the digestive tract, the heart and blood vessels, the kidneys, and the gonads. A thin membrane, called the mantle, protects these organs and also secretes the shell. If you pick up a snail, it withdraws into the shell, sealing itself off with a little disk called an operculum.

Nudibranches, also known as sea slugs, are gastropods without a shell.

Clams and mussels are types of bivalves, since they have two shells, or valves. The mantle of a bivalve surrounds its body and has two lobes. Each secretes a shell. Though bivalves have no head, their mantle sometimes has eyes or another sensory organ. They also have feet. Some species have a foot for only a portion of their life cycle. Oyster larvae, for example, attach themselves to hard surfaces by excreting cement from a pore in their foot. After this their foot falls off and they become spat, or juvenile oysters. The razor clam, by contrast, burrows extremely quickly down through the sand, using its powerful foot to pull itself along.

Most bivalves eat by filtering food out of the water column. The oyster, the mussel and the scallop lie on sediment and rock surfaces, opening their shells to let water in. But the clam lies beneath the surface of sediments and extends a siphon to the water above. In many clams, the siphon has two openings. The incurrent draws water in and over the clam’s gills, where oxygen diffuses into the respiratory system. Mucus covering the gills traps plankton, and microscopic hairs push this food along into the clam’s mouth. Waste is discharged from the excurrent siphon.

The squid and the octopus are cephalopods, another type of mollusk. Cephalopods swim by taking water into their mantle and forcing it out of their siphon. The squid has an internal shell, and the octopus has no shell. These are offshore marine animals, and so are not found in Great Bay Estuary.

Bivalves

Gem Shell Clam
Gemma gemma
This tiny estuarine clam is one of the most common intertidal infauna (animal life in the sediments), particularly in Little Bay and Great Bay. Usually no larger than one-eighth of an inch, gemma’s white shells are smooth, glossy and sometimes have a purple tinge (hence their other common name, Amethyst Gem Shell). These filter feeders strain diatoms and detritus out of the water, typically at night. In spring and fall the males release into the water column their gametes, which the females take in through their siphons.

View of some gemma shells magnified through a 4x hand lens

Females can produce two broods, the second of which is often carried through the winter. The females hold about 300 developing eggs in their mantle before releasing them as juveniles from June to August. Waves and currents carry the young away. They have a maximum two-year life span. Predators of gem shell include horseshoe crabs, green crabs, mud crabs, and a variety of shore birds and ducks.

Blue Mussel
Mytilus edulis
These long, dark blue bivalves usually live in clumps along the middle to lower intertidal areas of rocky shorelines. Blue mussels can tolerate salinities ranging from 5-18‰, but they are most abundant in the lower estuary where salinities are usually around 32-35‰. Mussel larvae settle out of the water and attach to hard substrates like rocks, pebbles or other mussels by secreting tough, fibrous byssal threads from their feet. These threads keep the mussel anchored to the shoreline despite the impacts of waves and currents. Though usually sessile, they can relocate by releasing the byssal threads, moving with their foot, and secreting new threads.

The blue mussel is a dominant organism on the rocky shore, and competes with barnacles for space to settle on. Sea stars, crabs, dog whelks, seagulls and oystercatchers prey on them. They can live up to 12 years in the northern Atlantic. Mussels are most heavy and tasty in spring and early summer, after phytoplankton blooms have fattened them, but before they have spawned.

Atantic Jackknife Clam, Common Razor Clam
Ensis directus
This clam filter feeds near surface sediments in intertidal and subtidal areas, but it is difficult to catch since it burrows swiftly into mud and sand. The clam uses its large, muscular foot to pull its long, thin, slightly compressed shell away from predators. These include humans, lobsters, crabs, moon snails and seabirds.

Eastern Oyster
Crassostrea virginica
Once oyster larvae settle out of the water and cement themselves on rocks or other shells, they remain there for life and are dependent on currents for food. One benefit of oyster beds is that they clean the water of plankton and other organic matter so light can better penetrate to plants in the water.

Optimal salinity for oysters is 10 to 28‰ (ocean water is 35‰), which makes them estuarine animals. In fact, oysters have been found to grow better with fluctuating, rather than constant, salinity levels. Settling in lower salinities also sometimes allows oysters to escape predators that are unable to tolerate brackish water. These include oyster drills, sea stars and some crabs. Oyster spat, or juvenile oysters, are growing on the rock pictured at left. But these larvae prefer to settle on other oyster shells, forming oyster beds. The beds provide hard substrate and shelter for algae, protozoans, nematodes, hydroids, amphipods, isopods, worms, snails, barnacles, mud crabs, and other animals.

Macoma Clam
Macoma balthica
Macoma’s thin white valves have a grayish, flaky periostracum (protein layer), and grow to one and a half inches wide. They lie in intertidal and subtidal sediments, tolerating salinities as low as 5‰.

Like other clams, the Macoma filters food out of the water column. But this estuarine clam actually eats more when it switches to deposit feeding at low tide. It extends its siphon across sediment surfaces, sweeping decaying organic matter. This is transported down the incurrent siphon to the gills, which sort out the diatoms and the bacteria that feed on the detritus. In the process it exhales rejected detritus, sediments and fecal pellets.

When they are suspended by waves and tides, these materials benefit filter feeders like the oysters. The oysters ingest them and grow faster. Macoma clams turn over surface sediments in Adams Cove 36 times each year.

If the Macoma is unable get enough nutrition by feeding on the deposits, it switches back to filter feeding. In sandy sediments it is primarily a filter feeder.

Winter flounder, swimming in with the tide, nip off the siphons of macoma clams. Other predators include plovers, gulls, and oystercatchers.

Ribbed Mussel
Geukensia demissa
This estuarine mussel is most common in the middle intertidal zones in the upper part of the estuary. At low tide, they can be found partially buried in marsh peat amid cordgrass stems. Its hard to see what their shells actually look like, since they are covered by muck. Clean shells are greenish or yellowish, with dark growth rings. The ribbed mussel in this image was found on a sandy northwestern shore of Little Bay. This species is not as common here as it is from Cape Cod south. Biologists suspect that winter ice damage is one of the main factors keeping it from growing large populations on northern coastal shores.

Sea Scallop
Placopecten magellanicus
Sea scallop shells grow to nearly eight inches wide, and have much finer ridges than bay scallops. The right shell is an off-white color, and the left is usually reddish brown, purplish or yellow. Most sea scallops are found in oceanic habitats like Georges Bank, which runs along the outer continental shelf about 280 miles off Cape Cod. But they can also grow in inshore, as long as salinities remain above 21‰. Several sea scallop beds in the lower Piscataqua River are commercially harvested from November to April.

Scallops lie recessed on sand and gravel, filter feeding on plankton and detritus. They move around by a kind of jet propulsion, rapidly clapping their valves to squirt water out of their mantles near the shells’ hinge. While on the move they can be carried by currents. Larger scallops, though, don’t move unless prompted by prey or some other physical disturbance. They control their shells with a single, large adductor muscle. This muscle is what is served to us as scallop meat. Lobsters, rock crabs, moon snails, sea stars, and Atlantic Cod also like to eat scallops.

Soft-Shelled Clam
Mya arenaria
Some soft-shelled clams grow offshore in water as deep as 100 feet, but they are more abundant along the coast and are common in estuaries. Adults burrow as deep as eight inches in intertidal and subtidal sand, mud and gravel. Their long, oval shells have a slightly pointy tip on one end. This is where the clam’s siphon extends up to draw diatoms, flagellates, bacteria and detritus from the water column. Though such a diet might not sound very appealing, they convert this food into market size (two-inch) bodies in about three years. Though they are recreationally harvested in Great Bay Estuary, they are relatively sparse and variable. Most clammers prefer Hampton Harbor.

Gastropods

Atlantic Dog Whelk
Nucella lapillus
This carnivorous snail preys on hard-shelled animals in rocky intertidal areas. Using its radula, the dog whelk drills a hole in the shell of its prey, and extends its proboscis (an oral tube) to eat the soft-bodied animal inside. The color of the dogwinkle varies, depending on what it eats. Barnacles turn their shell white; blue mussels give the dogwinkle a brown or purple tinge. Other identifying marks of the dogwinkle are its wide aperture (shell opening) and flared lips (aperture edges).

Dog whelk eggs on a rock at Pierce Island. The tip of a periwinkle shell is at far left.

Dog whelks avoid low salinity, crabs and winter ice by staying in the lower parts of Great Bay Estuary. These snails secrete a purple dye, which was used by the American Indians, as well as the Phoenicians of ancient times.

Eastern Mud Snail, Mud Dog Whelk
Ilyanassa obsoleta
These are the snails you see by the thousands on mudflats at low tide. They are grazing mostly the layer of diatoms (photosynthetic cells), bacteria and blue-green microalgae covering the mudflats. They are also feed on macroalgae like sea lettuce, and scavenge dead fish when available.

Tortoiseshell Limpet
Acmaea testudinalis
These gastropods have a shell about an inch in diameter that looks like a flattened cone. This shape, combined with the suction hold of their foot on lower intertidal rocks, helps them to withstand crashing waves. Their radulae are equipped with strong teeth, which can scrape microscopic algae and even coralline algae. The latter is a calcareous crust avoided by other gastropods because it wears down their radulae. This is good for the limpet, since there is no significant competition for coralline and it is available year round. It is also good for the coralline, which actually needs to be scraped by limpets to grow and reproduce.

Periwinkles
Littorina spp.
These snails are a littoral (intertidal) family of animals, which graze on microalgae and macroalgae. Because common periwinkles are air breathers, they are able to live in the upper parts of the intertidal zones.

Common Periwinkle
Littorina littorea
True to their name, these periwinkles are so abundant it is practically impossible not to step on them in the middle and lower intertidal surfaces all the way up the estuary to where salinities are 13‰. Their relatively smooth, brown shells grow to just over one inch wide and do not have pronounced spiral whorls. Mucus secreted from its foot helps cement the common periwinkle to rocks to prevent dehydration.

Rough Periwinkle
Littorina saxatilis
The shell of this periwinkle is a black-brown color, and grows to nearl one-half inch wide with a pointy spire. It lives higher up in the intertidal zone than the common periwinkle.

Smooth Periwinkle, Yellow Pseriwinkle
Littorina obtusata
The round shell of this periwinkle grows to nearly one inch wide, and can be yellow, orange, brown or black. Its rounded shells resemble the bladders of the seaweed knotted wrack, Ascophyllum nodosum. Biologists suspect they are camouflaging themselves by taking shelter in this seaweed. They feed on microscopic algae and rotting plants.

Hydrobia spp.
The hydrobia snails pictured at left have been magnified 4x by a hand lens. These could be swamp snails (Hydrobiidae), as they were found in a tidal marsh pool on the Squamscott River.

Another kind of snail, which has been identified in the estuary, is called Hydrobia minuta. It is a deposit feeder, consuming microalgae and diatoms on the surface of sediments in brackish water. It has a translucent, yellowish-brown shell that grows to one-eighth of an inch, with a deeply sutured spiral.