Almost everyone has a favorite chair. It’s where you sit to read a book, or watch movies, or play video games, or stare blindly into space.
Imagine that you’ve decided to live in your favorite chair forever, while food and drink just float by for you to snatch when you’re hungry or thirsty (some of you may have friends like this already!). After awhile, your backside begins to look like the seat of your chair, melding so the two of you become one. If you also feel yourself growing multiple eyes and orange lips, you might have just become a giant rock scallop.
Like clams, giant rock scallops have two rough, ridged oval shells that are hinged at one side. Although the giant rock scallop lifestyle – settling into a favorite rocky “chair” and waiting for food to drift by – sounds like a safe and mellow way to live, these scallops are among California’s most hunted shellfish.
Divers, snorkelers, tidepool hunters, and foodies everywhere prize the giant rock scallop for its large, tasty adductor muscle, the part we love to sauté in butter and garlic. It’s the adductor muscle that clamps the shells tightly together for protection when the scallop senses a threat nearby.
Hunting the Wily Giant Rock Scallop
Giant rock scallops are found all along the California coast. The current state record giant rock scallop, which measures over 11 inches in diameter, was taken from Santa Catalina Island in 1972. These scallops are most frequently found in water depths between 10 and 150 ft, but lucky intertidal hunters can sometimes find them in cold water shallows during low tides.
photo by K. Joe
Rock scallops eat microscopic phytoplankton, especially dinoflagellates, and bits of dead organic matter carried to them on strong ocean currents. Because they depend on moving water to provide them with food, giant rock scallops thrive where water movement is strong. These places can include:
- Oil rig platforms
- Rocks that are frequently inundated by breaking waves (called “wash rocks”)
- Rock piles, pinnacles, and reefs with large walls or rocks that funnel strong water currents
- Rocky holes, especially long cracks in rocks
- Undercut rock ledges
The best conditions for taking giant rock scallops are those rare days when the ocean becomes calm and lake-like with minimal wind, which makes it a little safer and easier to venture into high-current locations. Divers generally manage to take limits of scallops even when conditions are less than optimal, though, and often collect them during spear fishing dives or when hunting California spiny lobster (in Southern California) and sea urchins.
The stationary nature of giant rock scallops attracts colorful sponges, anemones, limpets, barnacles, and algae that colonize the upper shell of the scallop, creating miniature ecosystems that serve as natural camouflage. Divers and tidepool hunters can see through this disguise by looking for the tell-tale orange, green, or purple mantle of the scallops, which can look like a brightly painted pair of lips surrounded by whiskery tentacles.
Found One! Now What?
Techniques and Tips for Harvesting Giant Rock Scallops
To preserve the little ecosystems living atop the scallops, divers can insert a butter knife or similar tool between the shells of an unwary scallop to keep it from closing. Quickly move the knife towards the underside of the top shell, slicing the adductor muscle as close as possible to the shell, and scraping the shell to maximize the amount of meat recovery. With the muscle severed from the shell, the scallop will not be able to close.
With another slice between the bottom shell and the muscle, you can remove the insides. Often a collection of colorful fish hovering nearby will gobble up the free meal of scallop guts (all but the adductor muscle, of course!). Avoid cutting the muscle in half accidentally, because a wildlife officer might believe the two pieces came from two separate animals. It is possible to extract the meat without fully removing the top shell; or you can break the hinge and remove the top shell by hand. With this “harvest the muscle only” method, you can leave the top shell with all the colonizing creatures alive and (mostly) intact on the sea floor.
“When hunting for giant rock scallops, try to proceed slowly with minimal movement,” said Derek Stein, a California Department of Fish and Wildlife (CDFW) Environmental Scientist who has collected scallops for over 25 years. “This way, you can avoid alarming the scallop so that it doesn’t clamp down to protect itself.” With multiple eyes rimming the scallop’s shell, “they can definitely tell you’re coming, and close up really quickly,” said Stein.
Another method for gathering giant rock scallops is to pry the entire scallop from its attachment surface with a knife or a metal bar known as an abalone iron. If you’re looking for shell soap dishes or backyard decorations in addition to delicious scallops, this is the way to go. Take into consideration, though, how much heavier your dive bag will be if it contains a limit of large scallops in the shell, compared to a limit of just the adductor muscles.
Giant Rock Scallop Biology
Giant rock scallops, Crassadoma gigantea, also known as purple-hinged scallops, can be found from Sitka, Alaska, to Magdalena Bay, Baja California, Mexico. They generally don’t live in ocean bays or estuaries diluted by freshwater rivers or streams, as they require the saltiness of full-strength ocean water.
Five- to six-inch scallops are common in California, but occasionally individuals exceeding eight inches can be found. A giant rock scallop measuring 8 inches or larger is considered trophy sized.
Giant rock scallops grow slowly and can live for an estimated 25 years or more. “I’ve been photographing one group of scallops for over 15 years, and they’ve only grown from about two or three inches to now six inches [in diameter],” said Kevin Joe, a retired CDFW Environmental Scientist.
photo by K. Joe
While giant rock scallops in Southern California have two peak spawning times per year, some northern California populations spawn only once a year. Newly hatched rock scallops drift with the currents and then, while they are still quite young, swim about by clapping their shells together and expelling water, like ordinary scallops. At about five weeks old, they settle – sometimes temporarily – on hard surfaces.
By the time giant rock scallops reach six months old and one-half to one inch in diameter, they have attached themselves permanently to suitable rocks or other stationary objects. They do this by depositing cement-like shell material into the crevices and irregularities of their rocky seats, so that the lower shell often takes on the appearance of the rock itself.
Giant rock scallops are most vulnerable to predators such as sea stars and crabs when they are young and free swimming, or newly cemented to a rocky seat. Larger, adult scallops are much tougher targets for predators, although sea otters have been seen eating large scallops using cobblestones as hammers to break them open.
CDFW photo by D. Stein
Like other mollusks including clams and mussels, giant rock scallops filter their food from seawater and sometimes consume natural biotoxins that they store in their guts and other non-muscle tissues. The natural biotoxins come from harmful algae blooms and can cause paralytic shellfish poisoning or amnesic shellfish poisoning in humans. Because people most often consume just the scallop’s adductor muscle, which does not store these toxins, the risk of falling ill is minimal.
Consider checking the California Department of Public Health’s Shellfish Advisories Map if you like to eat more than just the scallop’s muscle, and be aware that consuming a sport-harvested mollusk’s internal organs will always carry some risk.
Commercial and Sport Harvest of Giant Rock Scallops
There is no commercial fishery for giant rock scallops in California. CDFW determined decades ago that while rock scallops were common in some locations, especially on offshore reefs, overall they are not that numerous. With only patchy distribution coastwide, commercial harvest of giant rock scallops would most likely endanger the mollusk’s survival.
Studies indicate that giant rock scallops could be reared from microscopic eggs to marketable size (4 to 5 inches in diameter) in about two and a half years. When kept in strong ocean-water currents, nearly all juvenile giant rock scallops survived to adulthood. In recent years, researchers have made progress toward successfully raising them in aquaculture facilities.
Large numbers of giant rock scallops are taken statewide by divers near shore, and aboard sport diving vessels in Southern California, as well as by collectors at low tide. Large rock scallops are becoming rare in locations frequented by sport diving vessels, and scallops as small as two inches are being taken in large numbers. In the future, giant rock scallop populations could benefit from size limits and closing the season for part of the year, most likely during spawning. More studies would need to be done to better understand the status of the population before considering any changes to the existing regulations.
For 2024, the recreational giant rock scallop season is open year-round, and the daily bag and possession limit is 10 rock scallops. There is no size limit, however scallops smaller than the palm of your hand typically do not yield much meat, and conservation-minded divers and tidepool hunters pass them over. Be sure to check the latest sport fishing regulations before diving or tidepooling for giant rock scallops.
Preparing Giant Rock Scallops
If you’ve opted to take whole scallops in the shell, your first step will be extracting the adductor muscles from the whole scallops. Leaving whole scallops in the refrigerator overnight usually causes them to open their shells wide enough to slip a knife in and sever the adductor muscles. If this doesn’t happen, most scallops have a small gap in the shell close to the hinge on one side where you can insert a small wire or zip-tie; when you wiggle it, sometimes the scallop will momentarily open its shell. Insert a knife into this opening to sever the muscle. If all else fails, a hammer or abalone iron can be used to break off an edge of the shell so a knife can be inserted.
Giant rock scallops are delicious prepared in any number of ways and in many different dishes. They can become rubbery if you overcook them, so don’t keep them in a hot pan for too long. The following are some recommended ways to prepare giant rock scallops:
- Fresh, raw scallops, sliced or whole (can’t get much easier than that!)
- Seared quickly in a very hot pan with a little bit of peanut oil just until brown on both flat sides, and served with some wasabi and soy sauce
- Sauteed quickly in a hot pan with butter and thinly sliced garlic, just until lightly brown on both flat sides, adding salt and pepper to taste
- Wrapped in sushi rolls with spicy mayo and avocado
- Add to a seafood paella. When California mussels are not quarantined (happens every year from May through October, and possibly at other times) gather some mussels too, and add them along with the scallops.
These fascinating mollusks with their many eyes, orange lips and living camouflage are a challenge for divers to find and a delicious addition to any meal. Selecting only the largest scallops will help to ensure their populations thrive. Happy diving!
Further Information
Hubble in a bubble: Scallop eyes act like tiny telescopes Science Magazine video
Rock Scallops Like It Rough California Sea Grant article
Rock Scallop from California’s Living Marine Resources – A Status Report (2001)
Giant Rock Scallop Sport Fishing Regulations: § 29.60. Rock Scallops.
post by Mary Patyten, CDFW Research Writer
Special thanks to CDFW Environmental Scientist Derek Stein and retired CDFW Environmental Scientist Kevin Joe for their guidance and contributions to this article.
Marine Management News 