Along California's central coast, an unusual underwater scene is unfolding. Where sea urchins previously hid in cracks and crevices, they now carpet the seafloor. Yet a couple hundred miles away, in southern California, urchins are losing their spines and dying.

This emergence of urchins, as well as the mass mortalities to the south, are newly discovered phenomena that appear to be connected to a die-off of sea stars that scientists have called the largest marine disease outbreak ever recorded.

Effects of the sea star die-off seem to be reverberating along the California coast, altering the prey and predator relationships of urchins, sea otters, kelp, and even human anglers.

The ecological spin-off begins with the lowly sea urchin, a pincushion-like creature that appears to be responding to sea star deaths in very different ways, depending on where it lives.

In scattered southern seashore pockets from Santa Barbara to Baja California, urchins' spines are falling out, leaving a circular patch that loses more spines and enlarges with time, marine scientists say. No one is sure what is causing it, although the symptoms are hallmarks of a disease.

"We think that there's a wasting event going on with urchins," says Peter Raimondi, an ecology professor at the University of California, Santa Cruz. "It is by no means as rapid as the sea star wasting was at the beginning. But we do see it in multiple sites. And when we do see it, we see it in a lot of animals."

Yet to the north, in waters off Santa Cruz and Monterey, the spiky sea creatures scrabble around in plain view. They apparently have come out of hiding in the absence of sea stars, their predators.

View Images Purple urchins aggregate near Soberanes Point, along the iconic Big Sur coastline and within the Monterey Bay National Marine Sanctuary. Photograph by Dr. Steve Lonhart, NOAA’s Monterey Bay National Marine Sanctuary

Ecologists say the urchins, uncurbed by sea stars and otters, soon may march forth and mow down central California's kelp forests, which support a variety of fish, crabs, and other creatures. So far, a fragile population of sea otters in Monterey Bay has yet to notice these easy pickings of urchins, one of their favorite foods.

It Began With Sea Stars

In Washington State in June 2013, scientists observed sea stars dying just as William Butler Yeats described in a poem: "Things fall apart; the centre cannot hold." Within days, the animals' arms walked off in different directions, and their entire bodies melted into goo.

Similar events in the 1970s, 1980s, and 1990s also killed sea stars along southern California and in Mexico's Gulf of California. But scientists say that this time, the die-off is extensive and shockingly rapid.

Since 2013, massive numbers of sea stars have died along the Pacific coast from Mexico to Alaska, affecting 19 species. Three species have nearly disappeared from some locations. At first, scientists observed a high level of wasting at 39 percent of sites surveyed in central and northern California. By the summer of 2014, it had soared to 87 percent.

View Images The sunflower star, one of the world's largest sea stars, preys upon red and purple sea urchins. With widespread sea star die-offs, urchins are coming out of hiding along the central California coast. Photograph by Norbert Wu, Science Faction/Corbis

A year of intense detective work by scientists revealed a virus active in marine ecosystems for seven decades had suddenly flourished, although no one at this point understands why. Scientists think that the ocean's warming, its increasing acidity, or some other factor has weakened sea stars and made them vulnerable to the virus. Evolutionary biologists have suggested that the virus may have evolved, becoming an extra-potent slayer of sea stars.

Raimondi is continuing to hunt for the malignant combination that is causing sea stars—and now perhaps sea urchins—to waste away.

Urchins Under Attack

So far, urchin die-offs have been observed and documented at four sites along the 200 miles between Point Conception and Santa Catalina Island, and at a fifth site off Baja California. Most are purple urchins, Raimondi says, and there are reports of mass mortalities. Some scientists think another species, the green sea urchin, which lives along the north coast, could be next.

"There are particular signs that point to this being a wasting event, the way the animals are dying," Raimondi says. The most common kinds of California urchins—purple, red, and white—are losing their spines, and the leading edge of spine loss is often discolored pink or white.

For sea stars and sea urchins, the symptoms appear in response to stress. But a bona fide "wasting event" is defined by several markers, such as the identification of a virus. With sea stars, scientists have observed it in many locations, with a high percentage of individuals affected. These same markers indicate that sea urchins in southern California are now in a wasting event, Raimondi says.

Sea urchins thrive and die along California's coast Disease is killing off sea star populations, reducing their numbers by more than 50 percent. Sea urchins are moving in to fill the void, creating different scenes to the north and the south of Point Conception. North Urchins are coming out of their rock crevices and congregating in the open. The urchins can overgraze kelp beds, leaving behind a barren landscape. South Unlike urchins in the north, some south coast urchins are doing poorly. In several locations (shown in red), they are losing spines and going bald. Researchers fear that sea star wasting disease has spread to urchins. North Urchins are coming out of their rock crevices and congregating in the open. The urchins can overgraze kelp beds, leaving behind a barren landscape. South Unlike urchins in the north, some south coast urchins are doing poorly. In several locations (shown in red), they are losing spines and going bald. Researchers fear that sea star wasting disease has spread to urchins. Emily M. Eng, Manuel Canales, Lauren C. Tierney, NG Staff.SOURCE: Pete Raimondi

Raimondi knows of no diagnosis. "But these are very classic symptoms," he says. "In the past they have been associated with bacteriological infections."

In previous big wasting events, in 1983-84 and 1997-98, sea stars and sea urchins died at the same time or in close sequence, mostly associated with El Niño ocean warming. But no one can say for sure whether the newly discovered urchin wasting is linked to sea star wasting. "We're just in the exploratory stage at this point," Raimondi says.

If urchin numbers are reduced in southern California, their predators—the California spiny lobster, the California sheephead fish, and human fishermen—stand to lose. And there may be winners, too, says biologist Jeremy Claisse of Occidental College in Los Angeles. Off the Palos Verdes Peninsula, purple urchins eat the kelp, creating "urchin barrens." When urchins are removed, kelp regrows, providing more habitat for young kelp bass.

The urchin deaths could also have economic effects.

In 2013 anglers gathered 6,500 tons (5,900 metric tons) of red urchins, which grow to seven inches (178 millimeters) across, in California. Restaurateurs feature "uni," the pricey, bright-orange urchin gonads, on sushi menus in America and Japan. In southern California alone, their value as a fishery was estimated at six million dollars in 2013, nearly ten million dollars statewide.

In the south, lobsters and sheephead keep urchin populations in check. Along the central coast, it's otters and sunflower sea stars.

That is, until sea star die-off transformed the seascape.

Absent the Sea Star, 'Bazillions' of Urchins

Sea urchins now traverse the ocean bottom with impunity in and around Monterey Bay, a national marine sanctuary. Some sections of seafloor appear littered with tufts of fur, like an array of hedgehogs.

"In areas where the sea stars have gone away, we've seen aggregations of sea urchins," Raimondi says. "There are just bazillions of them!"

Sea urchins may have come out of the crevices because sea star die-off removed a predator, the sunflower sea star. A yard in diameter, a sunflower star sports 24 arms and easily engulfs an entire sea urchin, digests it, and then spits out its spines and shell.

"Sea urchins have these really strong behavioral responses" to the death of other sea urchins, says ecologist Mark Carr of UC Santa Cruz. "If you grab a purple urchin and crush it, then the other purple urchins or the red urchins will run. Either they'll run away or, most importantly, they'll hide in cracks or crevices."

It wouldn't be the first time that profound changes have been tied to removal of an ocean predator.

Ecologist Robert Paine, now retired from the University of Washington, ran landmark experiments in which he removed the ochre sea star from an area, and then saw its prey—mussels—take over.

"That proved that these starfish are major players [keystone species] in the organization of these ecosystems," Paine says.

Otters Can Be Picky Eaters

Sunflower stars aren't the only animals in Monterey Bay that eat urchins. Sea otters eat them too. The sea star die-off may indirectly help otters by increasing the availability of high-fat, high-protein sea urchins.

Fur traders hunted sea otters to the edge of extinction in California. In later years, under federal protection, they expanded their range and their population grew. Now it appears they are hitting the maximum population size that the central coast environment can hold—what's known as carrying capacity. The nutrient-rich sea urchins may help them thrive.

Otters' taste for urchins is legendary. UC Santa Cruz ecology professor James Estes, who has studied sites in Alaska's Aleutian Islands for decades, showed that when otters increase or vanish, it dramatically affects populations of sea urchins and kelp forests that are important to fish and crabs.

Kelp forest food chain Sunflower sea stars prey upon sea urchins, which consume kelp. When sea stars die, urchins come out of hiding and overgraze on kelp, creating a shortage of food and habitat for otters, fish, and other marine life. Sunflower sea star Purple sea urchin Giant kelp Emily M. Eng, Manuel Canales, NG Staff.SOURCE: Pete Raimondi

So why haven't Monterey Bay's otters gobbled up the sea urchins that have come out of hiding?

Tim Tinker, a sea otter specialist with the U.S. Geological Survey, offers an explanation: Individual otters can be picky eaters. Each otter around the Monterey Peninsula specializes in just two or three foods—say, cancer crabs and urchins, or kelp crabs and snails.

Only 10 percent of Monterey's otters specialize in urchins.

View Images A sea otter eats an urchin along California's central coast. Each individual otter around the Monterey Peninsula specializes in only two or three foods. Photograph by Norbert Wu, Minden/Corbis

"It takes time to develop these skills—you can't just instantaneously turn a switch and become a new type of specialist, even when a new prey becomes available," Tinker says. However, the other otters may eventually exploit the newly available urchins because they tend to sample foods outside their specialties.

On one hand, urchins as food may help otters. On the other, urchins as kelp-eaters may keep the fuzzy-faced mammals awake at night. Otters wrap themselves in kelp fronds, which keep them from washing out to sea as they sleep.

"Once sea urchins start moving around, which is what they commonly do when they aggregate, they can become destructive grazers," Estes says. "It can happen quickly." If the otters start eating more urchins, they may stop the urchins in their tracks, before the invertebrates mow down their bedrolls.

The region's nearshore ecosystem continues to adjust to sea star wasting. Free-ranging urchins may dominate at the expense of kelp. Otters may dominate at the expense of urchins. Sea stars may recover.

And there's another possibility that could have its own cascading effects, Carr says: "The urchins could get abundant enough that they are more prone to disease."