Bivalve Mollusks: Oysters & Scallops & Clams, Oh My!

ON THE REEF, OYSTERS, SCALLOPS, CLAMS AND MUSSELS ARE PRETTY MUCH THE INACTION FIGURES. Mostly, they just sit there. If you come close, they clam up, so to speak, until you go away. Yet, environmentally, economically and, yes, culinarily, they’re big players in the oceans and in our kitchens.

Spoiler alert: The bivalve mollusks you’ll see on the reef are unlikely to be the ones seafood lovers salivate over. But, they’re likely to be more interesting to see, like this variable thorny oyster (Spondylus varius), an Indo-Pacific species. 

variable thorny oyster (Spondylus varius)
An apparently happy variable thorny oyster (Spondylus varius), an Indo-Pacific species photographed in the Philippines. 

AS FAR AS LIFE UNDERWATER GOES, BIVALVE MOLLUSKS DON’T GET THE FLASHY HEADLINES that come to their super-brainy fellow mollusks, the octopuses. This is despite the fact that, above water, oysters, scallops, clams and mussels rule seafood kitchens far more than their eight-limbed cousins, both in annual fisheries harvest and cuisine possibilities.

On the reef, however, oysters, scallops, clams and mussels assume the roles of quiet homebodies. In contrast to octopuses’ much-recounted playfulness, cleverness, curiosity and camouflage capabilities, members of Class Bivalvia basically just sit there.

In fact, oysters invariably do sit there – they stick themselves to a spot and stay put for the rest of their lives. Mussels also attach to the substrate, but they can move if they want. Clams move around a bit by digging themselves into new burrows in the seabottom. Scallops mostly lie around on the seafloor but those scallops can swim for short distances. Among bivalves, that’s Millenium Falcon stuff.

oyster bed
The supply of commercially and culinarily significant bivalves centers on extensive beds like this oyster bed, shown at mid-tide, on the South Carolina coast – areas we divers are unlikely to dive.


As unassuming as they are, bivalves are considered among the most important animals in marine and freshwater ecosystems. Why? They’re suspension feeders, filtering stuff from the passing water currents. They’re great at cleaning up waterways, removing algae and many impurities.

And oysters, at least, are considered a keystone species, providing essential habitat in their extensive reefs for hundreds of other marine species. These include barnacles, mussels and sea anemones that settle on them, crabs, shrimp and fish juveniles that hide out in them, and starfish, snails and fishes that forage on all of the above.

Farmed oysters, clams and mussels account for some two-thirds of all U.S. marine aquaculture. Oyster production alone was worth some $234 million in 2015. They thrive best in murky, phytoplankton-dense waters.

giant clam Tridacna squamosa Great Barrier Reef
Giant clams achieve their brogdinagian size and exotic colors the same way reef-building corals do – from photosynthetic algae embedded in their tissues. This guy, possibly a variation of Tridacna squamosa, was spotted on the Great Barrier Reef.


Mollusca is the second-largest phylum of invertebrate animals on the planet, encompassing some 85,000 extant species. The classes of mollusks you’re most likely to see are the gastropods (snails, the largest number of species), chitons, cephalopods (octopuses, squids, cuttlefish and nautiluses) and bivalves.

The word mollusk comes from the Latin for “soft,” and the most universal characteristic shared by the members of Phylum Mollusca’s half-dozen classes is a soft body. All mollusks have hard calcium shells that enclose those soft bodies, a “mantle” of tissue that covers the body inside the shells, a hard beak-like rasping structure called a radula and a muscular foot they use to move or dig. Unless they don’t – octopuses and nudibranchs, for example, have given up their shells for other defenses.

iridescent scallop (Pedum spondyloideum) Great Barrier Reef
Some bivalves bore their way or simply ensconce their way into rock, coral heads and the like. This iridescent scallop (Pedum spondyloideum) was nicely situated in a coral structure on the Great Barrier Reef. Iridescent scallop coloration can very significantly, including pale blues, reds and other hues.


The “valve” in bivalve refers to their dorsal and ventral shells. In proper terminology, they sport two hard shells hinged at a point called the beak. If you want, you can just call it a two-part hinged shell.

In the case of oysters, scallops and other bivalves, they encase themselves within a pair of perfectly matched hard shells – the valves, from Latin for “leaves of a door” – hinged at a point called the beak. The shells are made with calcium carbonate secreted by the mantle. Growth starts at the beak and expands as the animal matures, adding discernible growth rings. The two valves are opened and closed by ligaments and muscles.

All bivalve mollusks are described as having the basic mollusk body features – but laterally compressed to fit within the valves. The mantle, covering the body, is a soft membrane that secretes the minerals to manufacture the calcium carbonate shells.

Within the mantle cavity, bivalves stuff in a digestive system, heart, kidneys and circulatory system. Very large gills capture oxygen as the water passes through. In bivalves, the gills also filter the seawater for algae and other planktonic nutrients.


Additionally, with bivalve mollusks, it’s all in the foot, at least initially. For one thing, without heads they lack the rasping radula associated with most mollusks. They miss out on brains as well but have nervous systems, three pairs of ganglia that control movements like muscle, mantle and siphon functioning.

Instead, a bivalve’s big foot supports each group’s lifestyle. An oyster’s foot helps it get settled in place and then disappears; a clams maintains its usefulness all its life. Keep in mind that, unlike the flat-bottomed feet of snails or people, bivalve feet tend to be more spade shaped.


Northern quahog hard clam (Mercenaria mercenaria)
I believe this clam, photographed at Cape Ann, Mass., is a northern quahog (Mercenaria mercenaria), perhaps about to dig it way into the sandy bottom.

Apparently a clam is happiest when it’s well-hidden in the sediment, with siphons – tubes lined with tiny beating hairs called cilia – extended up to the seafloor surface to draw and discharge plankton-laden seawater. For a clam, the shovel-shaped foot is a digging tool. The optimum spot is in shallow water, even tidal flats or the surf line. But a species called Abra profundorum has been spotted in the Pacific Ocean at a depth of some 16,000 ft/4,800. The overwhelming percentage of clams are ocean denizens but there is a small number of families that live in freshwater habitats.

The cilia lined siphons of a prototypical clam are clearly shown in this drawing.

Clams generally can bury themselves anywhere from just under the seabottom to as deep as 3 ft/1 m. Clams like the Pacific razor clam, Siliqua patula (long and sort of jackknife shaped) can dig their way into the sediment at a rate of about one foot per 30 seconds.

They can range in size from less than 0.004 in/0.01 mm to as large as some 4 ft/1.2 m. The giant clams found in the Indo-Pacific basin achieve giant-ness with the assist of embedded photosynthetic algae. These zooxanthellae algae account for their oft-striking colors.

Some clams, or at least near relatives, are borers, drilling their way into rocks, corals and the cement of piers and walls (sometimes causing damage). The toredo worms (Teredo navalis, or shipworms) that destroyed wooden ship hulls through history are actually a form of clams in Family Teredinidae. They operate with long, soft, exposed bodies, using tiny shells at one end to do the boring work.


Atlantic thorny oyster Spondylus americanus
An algae-encrusted shell provides good cover for this Atlantic thorny oyster Spondylus americanus, photographed in the Caribbean.
Atlantic thorny oyster Spondylus americanus
The oyster’s disappearing smile: Same spot, shell closed, wouldn’t know he’s there. 








Unlike a clam, an oyster pursues a lifestyle not buried in the ocean substrate but atop it, often attached to rocks, corals or other oysters. As a spat – a new oyster– it utilizes its foot to crawl around on the seafloor to find the right spot to settle down. Once it’s found a suitable location, it secretes a glue that binds it to the attached surface. It stays in that place for the rest of its life and the foot becomes a negligible factor.

Atlantic winged oyster (Pteria colymbus)
Atlantic winged oysters (Pteria colymbus) feature wing-like shells and often position themselves on gorgonians to maximize access to the currents.

Oysters are characterized by rough, irregular-shaped shells that they keep slightly open to draw in water for the oxygen exchange and plankton filtering – and that they close tightly at the first sign of a threat. While an oyster’s two valves mirror each other for a perfect fit, they are not symmetrical. One side is more cup-shaped than the other, with the rounded side facing up, the flat side glued to the substrate.

Random but interesting: New research has concluded that oysters can hear.

Species in family Ostreidae constitute “true oysters,” including most of the edible oysters that form extensive beds, like eastern, Pacific and Virginia oysters. But divers are likely to encounter oysters in other families, like thorny oysters, that are more likely to be individually ensconced in cracks and crevices in the reef, and, in the stems of gorgonians.

Oysters are famous for their pearls, but pearl oysters are in genus Pinctada, not closely related to the edible varieties. Pearls are formed when a parasite or other microscopic irritant lodges within the folds of an oyster’s mantle. The oyster secretes a calcium carbonate “pearl sac” to isolate it.


Cornish mussels
Mussels, like these Cornish mussels, are found in extensive beds.

Mussels are found in both marine and fresh water habitats, although the term is most used to refer to varieties in the marine family Mytiliadae, like the highly edible blue mussel (Mytilus edulis) found on rocky shores, in marshes and in intertidal zones. Oysters are found in oceans worldwide, but tend to prefer cooler waters.

Although some species burrow into the sediment, most follow the oyster example of attaching themselves to a substrate exposed to the water column. But rather than gluing themselves into place forever, mussels attach themselves to their sticking place with very strong byssal threads extruded down a groove in the foot. These threads are strong but flexible, allowing the mussels some sway in the current while remaining secured.

Many mussels also have the ability, if needed, to detach themselves and move to a better location. Mussel threads have proven to be so strong that scientists have sought to replicate their properties in the laboratory.

Mussel trivia: It’s estimated that a hard-working mussel can filter more than 17 gallons of water a day. And while they often tend to occupy locations exposed to the air at low tide, they can close their shells so tightly that they don’t run the risk of drying out.


Scallops may have as many as 200 eyes at the tips of their tentacles, covering as much as a 250-degree arc.

Scallops are the action figures of bivalvedom, daring to live in the open on the seafloor, safeguarded by an ability to “fly” away when perceiving danger. For a while. A scallop swims by clapping its two valves rapidly together, creating a jet action that can propel it through the water several feet above the seabottom. They swim in spurts, typically flying for 25 feet or so before landing. And, typically, they need to rest for a while after doing this four or five times.

Scallops also have a movement often described as a “jump.” A scallop’s swimming involves a forward movement with water taken in through the front of its valves and ejected through small holes in the area of its hinge.  Its jumping, with water both taken in and expelled through the valve openings, actually moves them backwards in smaller steps.

Click on this screenshot to view the flying scallops video by Alex Dulavitz of East Coast Divers, shot at Stellwagen Bank, off the Massachusetts coast. 

Sometimes scallops swim not to escape from danger but to migrate, enmass, to more friendly feeding grounds.

A scallop’s swimming abilities are powered by its single round, meaty abductor muscle that controls its hinge. In large species, the muscle can be as broad as two inches across. This is the luscious part of the scallop that is the scallop delicacy.

With their scalloped edges, scallop shells are best known for their beauty – inspiring art and legend through history. But in fact, they’re streamlined to enhance a swimming scallop’s ease of movement through the water. The valves’ ridged architecture strengthens them. Like oyster shells, one valve is more rounded than the other. Unlike oysters, scallops settle with the flat side uppermost, minimizing visibility.

They’re filter feeders, of course, and since they live their lives exposed to the passing currents, they lack siphons. On the other hand, they have eyes, quite complex ones. that’s long been known, but new research has found that Scallops may have as many as 200 eyes at the tips of their tentacles, covering as much as a 250-degree arc.


Depending on the species, bivalves may have separate sexes or both male and female sex organs (hermaphroditic). They mostly reproduce by broadcast spawning, releasing male and female gametes into the water column for random fertilization, passing through the plankton as larvae before settling and beginning to develop shells. Some species retain their eggs inside their shells until released as more mature larvae.


Bivalves’ filtering penchant provides a great service in removing algae and other stuff from the water column, but it comes with a dark side – “bioaccumulation.” Among the stuff filtered out will be contaminates in the water, and they build up. The most prominent examples are the red tide toxins of harmful plankton blooms.

Red tide doesn’t harm the bivalves but it can harm who eats them. When you buy clams or oysters for dinner, it’s assumed that the folks who farmed them or harvested them took this into account.

Zebra mussels are invasive bivalve species in the Family Dreissenidae. Since they showed up in the Great Lakes in the 1980s, they’ve done a super job of clearing the lakes of

Zebra mussels Family Dreissenidae
Zebra mussels galore have covered this wooden wreck in the St. Lawrence River.

phytoplankton. But they’ve also upset the lakes’ food chain, sent many species of native mussels closer to the threat of extinction and clogged up industrial water intake systems. They cause enormous economic losses. They’ve been a problem in Europe since the 1800s and probably were transported to the United States in cargo ships’ ballast waters.

PRINCIPAL SOURCES:  Marine Biology, Peter Castro, Michael Huber; Marine Life, Caribbean, Bahamas, Florida, Marty Snyderman & Clay Wiseman; Reef Creature Identification, Florida, Caribbean, Bahamas, Paul Humann, Ned DeLoach; Marine Life of the North Atlantic, Andrew Martinez; Bivalves, Paleontological Research Institution; Mussels, Ocean Conservancy; Bivalves, Sam Noble Museum; Difference Between Oysters and Clams, Difference; The Benefits of Eating Bivalves,; Scallop Facts,; Bivalves: Phylum Mollusca, Class Bivalvia; Delaware Geological Survey; How Does a Scallop Swim; Clamsplaining; What East Mussels, Sciencing; Digging Razor Clams,; Oyster Reef Habitat,, National Oceanic and Atmospheric Administration; Mollusks, Bivalves, Clams,, Encyclopedia Britannica; Mollusca, Bivalves,, Wikipedia.