Most clams spend their lives anchored in sand or mud and never swim a stroke, but a few bivalves are genuinely capable swimmers. Scallops are the famous exception, jetting through the water by clapping their shells together. The full picture of bivalve movement is richer than that, though, because “swimming” is only one item on a surprisingly long menu of locomotion strategies that different clam and mussel species have evolved.
Scallops Are the Swimmers of the Bivalve World
When people ask whether clams swim, the animal they’re usually picturing without knowing it is a scallop. Scallops swim by rapidly opening and closing their two shells, squirting water out through openings near the hinge. The result is a form of jet propulsion not all that different in principle from a squid’s, though considerably less graceful. A swimming scallop looks a bit like a set of chattering teeth bouncing across the ocean floor.
The mechanics are surprisingly well studied. A single clapping cycle takes roughly 0.28 seconds and can be broken into three roughly equal phases: closing (which generates the jet), gliding, and opening back up for the next clap. During the closing phase the valves can rotate at up to about 182 degrees per second and accelerate at around 1,370 degrees per second squared, producing a pressurized water jet that pushes the animal forward or upward.1PubMed. Dynamics and energetics of scallop locomotion The system is efficient in one specific sense: nearly all the mechanical energy from the adductor muscle goes into producing the jet rather than being wasted moving the heavy shells. Only about one percent of the energy runs the shell-opening-and-closing oscillator itself; the rest does hydrodynamic work.
Scallops pull this off because of a body plan that differs from most other bivalves. Their shells are lighter and more streamlined, they have a proportionally large fast-twitch adductor muscle dedicated to rapid clapping, and their hinge has elastic properties that help spring the shells open again between jets.2PubMed. Jet propulsion in the cold: Mechanics of swimming in the Antarctic scallop Adamussium colbecki To stay aloft, a scallop has to maintain a minimum speed and hold its body at a particular angle of attack, much like an airplane wing generating lift.3Canadian Journal of Zoology. Jet-propelled swimming in scallops: swimming mechanics and ontogenic scaling Once it slows down too much, it sinks. Swimming for a scallop is less like cruising and more like a series of desperate hops, usually triggered by the approach of a predator like a sea star.
Why Most Clams Don’t Swim
The typical clam, whether it’s a quahog, a geoduck, or a littleneck, has a heavy, roughly symmetrical shell optimized for life buried in sediment. That shape is the opposite of what you’d need for swimming. A streamlined, lightweight shell costs you protection against crushing predators and wave action. A massive fast-twitch muscle costs you a lot of metabolic energy to maintain. For a clam that lives in mud and filter-feeds from a fixed position, those tradeoffs make no sense. Evolution has pushed most bivalves toward staying put rather than jetting around.
There’s also the matter of body geometry. Research using burrowing robots has shown that a typical clam shell, like that of the hard clam Mercenaria mercenaria, has a blunt, prosogyrous (forward-tilting) front end whose shape is specifically adapted for rocking back and forth during burrowing. The shell doesn’t translate straight downward; it “walks” into the sediment through a clever rotational motion where the axis of backward rotation is slightly forward of the axis of forward rotation, resulting in net downward progress.4Paleobiology. Why clams have the shape they have: an experimental analysis of burrowing A shell designed to walk itself into sand is simply not a shell designed to fly through water.
Burrowing Is the Dominant Form of Clam Locomotion
If swimming is the headline talent, burrowing is the bread and butter. Most clams move by extending a muscular foot out of their shell, using it to probe and anchor in the substrate, and then contracting it to pull the body downward or forward. It’s slow, purposeful work, and different species have come up with surprisingly varied ways to do it.
Razor clams are the speed demons of the burrowing world. The Atlantic razor clam, Ensis directus, can dig itself out of sight in seconds, which is impressively fast for an animal with no limbs. The secret turns out to be that the razor clam doesn’t just push through static soil. When it contracts its valves, the motion fluidizes the surrounding sand, turning a solid substrate into something that behaves more like a thick liquid. In this fluidized state, the drag on the clam drops dramatically, enough that the animal can reach full burrow depth. In static, unfluidized soil, the clam simply isn’t strong enough to get there.5Journal of Experimental Biology. Study uncovers secret to speedy burrowing by razor clams
At the opposite end of the spectrum, the soft-shell clam Mya arenaria is one of the slowest burrowers among bivalves. Its technique is unusual: rather than relying heavily on its foot to dig, it squirts water downward through an opening at its front end, jetting sand away from below itself. The foot mostly just acts as an anchor. Individual water jets can last several seconds and are powerful enough to clear a path, making it less of a digger and more of a hydraulic excavator.6Journal of Molluscan Studies. Hydraulic burrowing in the bivalve Mya arenaria Linnaeus (Myoidea) and associated ligamental adaptations
The substrate itself matters. Studies on the surf clam Paphia undulata have shown that burial depth and sediment water content affect not just how a clam burrows but how well it feeds and how much energy it nets. Clams buried at a moderate depth in moderately wet mud tended to have higher energy budgets than those buried too deep or in substrates that were too dry or too soupy.7SpringerLink / Journal of Oceanology and Limnology. Effects of substrate on burrowing behavior, feeding physiology, and energy budget of undulated surf clam Paphia undulata Getting the depth and soil type right isn’t just about comfort; it’s a genuine survival calculation.
Crawling, Floating, and Other Tricks
Between full-blown swimming and staying buried, bivalves have evolved a grab bag of other movement strategies that don’t fit neatly into either category.
Giant clams, the massive reef-dwelling Tridacna species, look about as immobile as a boulder. But they have more mobility than their size suggests. As larvae and juveniles, giant clams can swim and glide. Even adults retain some ability to crawl, repositioning themselves on the reef by extending their foot.8PubMed Central. The behaviour of giant clams (Bivalvia: Cardiidae: Tridacninae) They’re not going to chase anything down, but the idea that they’re permanently cemented in place is wrong.
Some cockle species can use their foot to leap or “jump” off the sediment surface when threatened, flinging themselves a short distance. It’s not swimming and it’s not burrowing; it’s more like a startled hop, powered by a single explosive thrust of the foot against the ground.
The Asian clam, Corbicula fluminea, one of the world’s most successful invasive freshwater bivalves, has an especially creative repertoire. Small specimens can secrete long mucous threads through their siphons that act as draglines, catching the current and buoying the animal into the water column. The threads are produced in response to water flow and help the clam disperse downstream or even between connected waterways.9PubMed. Flotation of the Bivalve Corbicula fluminea as a Means of Dispersal On the bottom, Corbicula can also crawl using its foot, and does so more actively when it finds itself on substrates it doesn’t prefer, such as coarse gravel.10Freshwater Biology. Differences in substratum preferences and behaviour within the invasive Corbicula species complex
Young mussels have their own dispersal trick. Post-larval blue mussels (Mytilus edulis) spin long, single-filament drifting threads that are completely different from the stout attachment byssus threads they use to anchor themselves as adults. Drifting threads are incredibly long relative to the animal’s body, exceeding it by more than two orders of magnitude, and have no attachment plaque at the end. They function like a kite line, catching currents and carrying the tiny mussel to new habitats.11Marine Biology. Byssus drifting and the drifting threads of young postlarval mussel Mytilus edulis
How Bivalve Larvae Move
Every bivalve, no matter how sedentary it becomes as an adult, begins life as a microscopic larva drifting in the plankton. Bivalve larvae swim using cilia, tiny hair-like projections that beat in coordinated waves to push the larva through the water. This is true swimming in a technical sense, although the speeds involved are minuscule.
Larvae of the fluted giant clam, Tridacna squamosa, for instance, swim at speeds measured in the hundreds to low thousands of micrometers per second, roughly a millimeter per second at best.12Journal of Experimental Marine Biology and Ecology. Larval ecology of the fluted giant clam, Tridacna squamosa, and its potential effects on dispersal models That’s not fast enough to fight any meaningful current, but the larvae can adjust their depth in the water column and actively choose where to settle. High-speed video work on bivalve veliger larvae has confirmed that these organisms use their cilia both for swimming and for feeding, generating flow fields that draw in food particles while simultaneously propelling the animal.13Journal of Plankton Research. Hydrodynamic disturbances produced by small zooplankton: case study for the veliger larva of a bivalve mollusc
Once larvae settle and metamorphose into juveniles, most species lose the ability to swim entirely and transition to whatever adult locomotion strategy their species uses, whether that’s burrowing, crawling, attaching by byssal threads, or, in the case of scallops, retaining the ability to jet-propel later in life.
How Far Can a Clam Actually Travel?
Even the most mobile adult bivalves don’t cover much ground in the scheme of things. Lab experiments with Corbicula fluminea found that active foot-powered crawling would predict a maximum upstream migration of no more than about 0.15 kilometers per year. Yet field observations of the species spreading upstream have recorded rates of 0.5 to 11 kilometers per year, far faster than the clam could possibly walk.14Biological Invasions. No experimental evidence for vector-free, long-range, upstream dispersal of adult Asian clams [Corbicula fluminea (Müller, 1774)] The gap strongly suggests that most long-distance movement in clams is passive: carried by currents, hitching rides on birds or boats, or floating on mucous threads rather than actively walking there.
Scallops can cover more distance per swimming bout than burrowing clams cover in a week of crawling, but even scallop swims are short, usually a few meters at a time to escape a predator. No bivalve is covering the kind of distances that fish, marine mammals, or even sea turtles manage. The evolutionary strategy for most bivalves is not to move far but to find a good spot and stay there, letting the water bring food to them.
Climate Change and the Limits of Scallop Swimming
For the bivalves that can swim, that ability may be under threat. Research on the Atlantic king scallop, Pecten maximus, has found that ocean warming and acidification reduce the force that the adductor muscle can generate. The proposed mechanism links metabolic changes in the muscle tissue to decreased swimming performance under future climate conditions.15Frontiers in Ecology and Evolution. Exploring the mechanisms behind swimming performance limits to ocean warming and acidification in the Atlantic king scallop, Pecten maximus A scallop that can’t clap hard or fast enough to maintain that minimum speed will sink, and a scallop that can’t outrun a sea star is a dead scallop. The ecological consequences could ripple through food webs where scallops are both commercially harvested and ecologically important grazers and prey items.
Antarctic scallops face a version of the same problem in reverse. Adamussium colbecki is adapted to extremely cold water, where metabolic rates and muscle performance are naturally constrained. Even under current conditions, swimming is energetically expensive for these animals.2PubMed. Jet propulsion in the cold: Mechanics of swimming in the Antarctic scallop Adamussium colbecki As ocean temperatures shift, the narrow thermal window in which their muscles work well enough to swim could shrink further.
Robots Inspired by Clam Movement
Engineers have taken an interest in how clams move, particularly the razor clam’s burrowing trick. The principle that you can dramatically reduce digging force by locally fluidizing soil has obvious applications for anchoring systems, underwater cables, and seafloor sensors. A robot called RoboClam was designed to mimic the kinematics of Ensis directus, and using a genetic algorithm to optimize its digging motions, it achieved burrowing performance comparable to the real animal in both idealized glass beads and actual mudflat sediment.16Bioinspiration & Biomimetics. Razor clam to RoboClam: burrowing drag reduction mechanisms and their robotic adaptation The key insight was that the energy required to dig scaled linearly with depth in fluidized soil, rather than exponentially as it would in static soil, making deep burrowing feasible with small, low-power devices.
Other researchers have gone in the opposite direction, building a soft robot inspired by razor clams that can burrow itself out of sediment on command. The self-burrowing-out robot, or SBOR, uses a single inflatable silicone tube to reverse the burrowing process, a potential tool for retrievable anchors or sensors that need to resurface after deployment.17PubMed. SBOR: a minimalistic soft self-burrowing-out robot inspired by razor clams The simplicity is part of the appeal. Razor clams accomplish complex locomotion with very simple body parts, and that maps well onto soft robotics, where you want capable motion from minimal mechanical complexity.