This the second most disgusting thing I’ve ever done,” Simon declares while struggling to traverse shoe-sucking mud during low tide. Wearing a T-shirt sporting a vibrant crab that proclaims “I’m Crabby,” the red-headed third-grader has joined two dozen classmates from Oakland’s Glenview School on a winter afternoon at Crab Cove in Alameda to explore the exposed mudflat and see what lives here.
After clambering over rocks coated with slick sea lettuce, the students soon discover that the silt, clay, and sand of the mudflat also make for some slippery footing. Complaints ripple through the group about cold water seeping into shoes and feet sinking deeper in the muck. But before long—thanks to the enthusiasm for the creatures underfoot expressed by East Bay Regional Park District naturalist Susan Ramos—the young explorers ignore their discomfort and begin peeking beneath rocks, shouting out their discoveries. “I’m your friend,” Simon tells a mud-green crab he’s uncovered. Out on the mudflat, it can be a short distance from disgust to delight.
Such is the conundrum of a mudflat. It takes patient attention—and some imagination—to realize all that’s going on in the seemingly placid habitat. The mudflat at Crab Cove is crammed full with life, but much of the action takes place out of sight. At low tide, animals seek shelter beneath the surface of the mud. High tide brings sharks, rays, and other fish closer to shore in search of food. One clue to the density of life within the mud is visible in the scores of shorebirds and waterbirds—coots, cormorants, curlews, egrets, sandpipers, and pelicans—congregating on the mudflat and adjacent breakwater.
When the Bay water recedes, a visitor needs to step and scan carefully to see evidence of other residents. What’s more, Crab Cove’s designation as a marine reserve in 1980—the first reserve for a California estuary—prohibits humans from probing down into the mud. There’s no collecting, either, although careful handling is encouraged.
On the state’s behalf, the East Bay Regional Park District operates Crab Cove Visitor Center and adjacent Robert W. Crown Memorial State Beach, part of the longest beach on San Francisco Bay. Some 1.5 million people use the park annually, mostly to picnic or stroll along the beach or, in the warmer months, play in the gentle surf. But with all the nature happening here, park naturalists present nearly 450 programs a year about the Bay environment for students and families, including birding walks, aquarium feedings, and low-tide excursions onto the mudflat. In 2015, more than 7,200 students from 269 classrooms participated in field trips.
From a show of hands, half of Miss Anderson’s Glenview third-graders have visited Crab Cove before. Naturalist Ramos orients the students and their chaperones with an enlarged satellite photograph of San Francisco Bay. She asks someone to name the water body. “Pacific Ocean,” calls out a student, which Ramos replies is half right. She adds that they are standing on the shore of an estuary, or a place where salt water and freshwater converge. That meeting and mixing of waters enables more biodiversity to occur offshore here.
For younger visitors, a Crab Cove field trip begins with an imaginary dive. It has to be a shallow one, given the Bay’s average depth of 12 feet. Ramos gets everyone to mime pulling on a wet suit, mask, and flippers, then hoisting an air tank onto their shoulders and a weight belt around the waist. The students file into the Old Wharf Classroom, decorated with barrels and crates like a dock from a bygone era. They mimic the sounds of a chugging boat engine and a splashing anchor, then close their eyes and count to ten while Ramos unveils a 3-D diorama depicting examples of life above and below the Bay’s surface. A chorus of “Whoa!” rises from the tween crowd—an encouraging response to a static, silent museum display.
In the diorama a marbled godwit and western sandpiper patrol the shoreline. An airborne northern pintail prepares to land beside a bufflehead duck. Perched on a wharf piling, a double-crested cormorant peers for fish. Underwater, a leopard shark cruises toward the viewer, and a striped bass pursues a school of jacksmelt. A bat ray descends toward a cluster of bivalves. The prominent pincers of a lined shore crab protrude from a crevice.
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Before swapping this indoor classroom for the outdoor one, Ramos wants to talk about food chains, which she defines as “a whole progression of who eats who and what eats what.” To demonstrate, she calls up kids from the crowd to don funny hats and play parts in the Bay’s food chain. A girl in a floppy fishing hat represents the base of the chain: nutrients, sunlight, and water. Two boys sport green and blue hard hats to signify phytoplankton and zooplankton. A black baseball cap adorned with a trio of plush, sparkly fish represents smelt, and another hard hat topped with a large plush fish is the predatory striped bass. The final link is us—people who catch fish from the Bay. Ramos starts the students thinking about the consequences for the entire food chain when one of the links breaks from some disturbance. What if a chemical spill, for instance, clogs the gills of jacksmelt until they suffocate? Their prey the zooplankton could multiply so much that they overeat phytoplankton. One change can cascade throughout the ecosystem, she emphasizes.
Lesson imparted, it’s time to hit the mud. Ramos reminds her charges, “Every time you see a hole, that’s an animal’s front door.” The mudflat is pocked everywhere, it seems, with holes of all sizes—clues that critters underfoot are hiding, eating, maybe mating. Most abundant are tiny circular holes left after a clam retracts its siphon—often a fringed white tube that resembles an anemone. It’s used to draw in or expel water that the clam filters for detritus to eat.
Another common feature on the otherwise flat mudscape, a conical, miniature mound with a hole in the middle, signals a branching burrow belonging to the bay ghost shrimp (Neotrypaea californiensis). Dug with the aid of an oversize claw (called a cheliped, which can make up a quarter of a shrimp’s total weight) pushing sand out into piles around the entrance, the burrow shelters the shrimp’s otherwise defenseless pale pink body. Ghost shrimp subsist as detritivores, sifting through sandy mud for bits of organic matter and plankton. An undisturbed ghost shrimp may live up to ten years, but the gracefully curved beak of a long-billed curlew, a large shorebird that forages at Crab Cove, seems precisely designed to pluck concealed crustaceans from the mud.
Slender strands of mud that look an awful lot like a forkful of spaghetti (especially if you’re in third grade) pile up outside the home of a lugworm (Arenicola brasiliensis), a relative of earthworms and leeches. About the length and width of an index finger, the lugworm dwells head-down in a J- or L-shaped burrow. It sucks up sand and mud using a sticky proboscis to extract organic nutrients and deposits processed sand outside the burrow. Those coiled strands: lugworm poop (or, more delicately, mud castings). Like earthworms in terrestrial soil, lugworms move large amounts of organic matter throughout the mud by bioturbation, the mixing of sediment by burrowing or other animal activity.
Several students squat around another sign of the lugworm’s presence: a translucent egg sac the size of a flattened tennis ball. Tethered to the burrow below by a gelatinous thread, the orb jiggles like Jell-O. A female lugworm releases eggs within the burrow, possibly triggered by traces of lugworm sperm in the water. Once fertilized, the eggs become enshrouded by the sac and extruded to the mud surface. Larvae hatch a few days later and begin a brief free-swimming phase before excavating new burrows.
The largest animal moving mud around may be the bat ray (Myliobatis californica). With wing-like pectoral fins that can stretch more than five feet across, the ray swims shoreward at high tide. Sensing the presence of clams or other invertebrates buried in the mud, it descends to hover just above the substrate. Then the ray rapidly flaps its fins up and down, moving the upper layers of mud until the prey is exposed. The first half of the bat ray’s genus name means “grinder” in Greek, and this fish possesses powerful plate-like teeth on the underside of its body for crushing clams and other shelled creatures. The bat ray’s effective excavation technique leaves a shallow divot in the mud several feet in diameter that remains when the tide ebbs.
An array of mollusks small and large also awaits a careful observer. Lentil-size flecks of white scattered atop the mud’s surface are the shells of gem clams (Gemma gemma), an exotic species devoured by busy sandpipers foraging the bayward edge of exposed mud. At the other end of the size spectrum is an Atlantic immigrant, the channeled whelk (Busycon canaliculatum), the Bay’s biggest snail. These whelks have long been East Coast delicacies in Chinese and Italian cuisine and may have been brought here in the 1940s for human consumption. Still, Crab Cove is one of few spots in the Bay where channeled whelks can be found with some luck. (Both a conch-like mature whelk the size of a human hand and a twisted whelk egg case of many strung-together discs turned up during a low tide evening mudflat outing last fall.)
Much more likely to be spotted is another molluscan import from the Atlantic, the oyster drill (Urosalpinx cinerea), with its elegant whorled shell. Probably introduced to the Bay when native oyster fisheries thrived here in the late 19th century, the oyster drill deploys a corrosive secretion and its strong, sandpaper-like tongue to penetrate the shell of an oyster, mussel, or barnacle before pushing its proboscis through the hole it’s created and consuming the shell’s contents. No need for shucking. Abandoned oyster drill shells get reused regularly as hermit crab homes.
Notice a recurring theme? Much of the Bay’s mudflat fauna has arrived from elsewhere—often after hitching a ride aboard ships. In a corner of Crab Cove’s visitor center stands a glass case beneath a neon sign declaring “Aliens.” Spookily lit specimen jars filling several shelves sample some of the rogue’s gallery of foreign fish and invertebrates that have been inadvertently introduced from the Atlantic Ocean, Asia, or the South Pacific.
San Francisco Bay and the Delta have earned the dubious distinction of being the world’s most invaded estuary. At least 234 species of exotic plants and animals now reside in the estuary, and the total number is likely higher. “We find new invaders all the time,” says Andrew Cohen, director of the Center for Research on Aquatic Bioinvasions (aka CRAB) in Richmond. Recently, marine species from Southern California have followed warmer El Niño waters northward, including a kind of blenny fish that Cohen found under a rock while he and his son explored the shoreline at Crown Beach. No one knows how much impact invasive species have on native ones throughout the Bay, but their abundant presence has undeniably altered the aquatic ecology.
Recognizing that her audience’s attention span has receded like the tide, Ramos guides the group toward the rocky shore and better opportunities for finding Crab Cove’s eponymous crustaceans. She’s instructed the students to carefully overturn rocks, examine beneath them, and then replace them gently. Encouraging everyone to try touching a crab, she describes proper handling technique: Keep the crab nestled within cupped palms and close to the ground in case it escapes, in order to avoid damage to the carapace from a fall.
Students jostle shoulder-to-shoulder around Ramos to see a native green shore crab (Hemigrapsus oregonensis), about two inches across, lying belly-up in her hands. “It’s a female, and she has eggs,” explains the naturalist, pointing out the sand-size specks along the curving edge of the crab’s abdomen. Even without being clued in by eggs, it’s easy to determine a crab’s sex from its abdomen and to become an expert on crab sexing before leaving the mudflat: Males have a narrow triangular abdomen, like a fan that’s been snapped closed, whereas females have a broader abdomen resembling a fully extended fan.
Crabs have kept the same basic design for millions of years. Head and thorax are fused and encased in a protective keratin carapace. A crab molts several times per year; a split forms between the carapace and abdomen, and the crab wriggles backward out of the old shell, or exoskeleton, and waits for its new armor to harden. Molting may also allow a crab to regrow a missing limb. Four pairs of segmented legs—a fifth pair has been modified into intimidating pincers—suspended from the carapace enable a crab’s distinctive sideways gait, with the leading legs pulling and the opposite side pushing the body. (Young students who participate in Crab Cove’s “Creatures of the Bay” school program get to watch a classmate metamorphose into a crab, with the help of a naturalist dispensing the body parts of an elaborate costume: a red vest with gills along the sides, a maroon tunic for the exoskeleton, arm-length sleeves ending in padded pincers, legs Velcroed to each side of the carapace, plus an abdomen and cap with extendable eyestalks.)
Multiple species of crab coexist at Crab Cove. By far the most common are the green shore crabs that blend in remarkably well against the mud. Mostly herbivorous, they consume the abundant alga called sea lettuce (Ulva lactuca) that drapes shoreline rocks in vibrant green but also filter feed for plankton and detritus or set out to scavenge for whatever they can find. Other native species found in the intertidal rocky shore habitat include seaweed-eating lined shore crabs (Pachygrapsus crassipes) and larger mollusk-munching red rock crabs (Cancer productus).
Because it’s protected from the wind and waves, Crab Cove also serves as a nursery for our delectable Dungeness crab (Cancer magister). Each winter, a female Dungeness releases up to 2 million eggs in nearshore water. Hatchlings go through several molts and metamorphoses during their planktonic first year before moving out as juvenile crabs into deeper, saltier San Francisco Bay. There they attempt to evade all the fish species that would consume them by remaining buried in mud until mature enough to head out to sea for mating.
The Glenview School group retreats from the rising tide and concludes its field trip with a whirlwind tour of the aquariums and interactive exhibits at the visitor center. More crabs can be seen there, including live European green crabs (Carcinus maenas), introduced to San Francisco Bay around 1989, and preserved Chinese mitten crabs (Eirocheir sinensis), another recent intruder that begins life in brackish water and moves upstream into the Delta.
The exhibits are informative but still no substitute for seizing the slippery opportunity of a firsthand look at the muddy domain next door. Crab Cove is a rare place, for the easy access to one of the Bay Area’s least-known habitats and for protecting its mud-bound multitudes. Be sure to consult a tide table before visiting, and remember to bring rubber boots or a change of shoes and socks.