Life and Death in The Intertidal Zone

As the mysterious sea star wasting syndrome shows, life ebbs and flows in the tide pools around Santa Cruz.

by Samantha Chavez

Sept. 25, 2015—Like a curtain at the start of a play, the ocean pulls away from the rocky outcroppings that line the beach. The intertidal stage is set and hidden marine life is revealed. But the cast is different from what it was just three years ago.

Caught between land and sea, the intertidal zone in Santa Cruz teems with barnacles, limpets, snails, squishy anemones, scuttling rock crabs, shy hermit crabs, and plenty of mussels. A lone sea star, pale orange, clings to the rocks as the tides wane. The spring tides are lowest in the early morning, so that’s when UC Santa Cruz graduate student Monica Moritsch sets out to work. She grips a length of rope tightly as she shimmies down a cliff face near Long Marine Lab. Her intern and I follow ungracefully, stumbling on clunky rain boots and hesitating before every step. Down at the beach, we start setting up transect lines. The tides are just beginning to wane and a large swell butts my legs, filling my boots with water. “The king tides are almost here,” Moritsch says. “Those are the lowest tides of the year.”

The tide pools at Terrace Point are relatively undisturbed; they’re difficult for tourists to reach. Moritsch has spent the past two years surveying the species here and taking mussel bed measurements along the coast near Santa Cruz and Monterey. The work is wet and cold, but such data collection has become newly important. A major resident, the starfish, has nearly vanished.

Sea stars have almost entirely disappeared along the California coast from a massive scourge called sea star wasting syndrome. Their absence is felt strongly by their intertidal neighbors. It’s a lamentable loss, but it sets up a large-scale ecological experiment that scientists couldn’t recreate in a lab. Removing this keystone predator will change the intertidal community profoundly. Moritsch is aiming to find out just will happen next.

Sea star wasting syndrome has swept Pacific shores before. After waves of death in 1983 and 1997, the populations bounced back. When the illness struck again in 2013, many scientists hesitated to call it another outbreak of wasting syndrome. The syndrome has the same effect on sea stars as other stress factors, such as salinity and desiccation. Those same skeptics noted that warm water historically accompanied wasting syndrome near the shore. At the time, there was no warm water along the coast. Yet what started as troubling white lesions on the bumpy orange arms of sea stars deteriorated into ruined limbs. The syndrome even exposed internal organs. Sea stars in both the intertidal and deeper subtidal zones were melting away, sometimes disintegrating within three days.

In 2013, as sea stars disappeared, Moritsch was working on a project involving mussel harvesting. “I was trying to convince myself that I liked the project, which is never a good thing,” she recalls. Worried about the impact sea star wasting syndrome would have on her mussel beds, she hit upon a sudden idea: perhaps that was the very question her graduate research should address.

Davenport Landing Survey by Leslie Willoughby / 2015 from SciCom Slugs on Vimeo.

Mussels don’t understand the concept of personal space. Each mussel packs tightly against its neighbors, often including leaf barnacles. Normally, mussels are kept from expanding their range too high along the shoreline because they may not be covered during high tide. If they wander too deeply, they’re eaten up by hungry starfish. Mussel beds live comfortably in the middle of the intertidal, where it’s just right.

Mussel surveyors measure the size of each mussel, the extents of their range, and how deeply their beds reach. Since starfish died off, the deeper parts of the intertidal are free of predators—and mussels are expanding their ranges. It’s not a complete mussel takeover in Santa Cruz, but in some places outside of Moritsch’s study sites, young shiny black mussels are popping up beyond the previous lower limit of the beds. “It’s not necessarily bad, it’s just different,” she says. But mussels aren’t the only marine life affected by the sudden loss of starfish.

Community composition surveys look beyond the mussel beds. Moritsch receives help from volunteers to count species along transect lines laid out on the rocks. Mobile species, even slow-moving ones like limpets and turban snails, are too tricky to count. But the stationary mussels, barnacles, and algae have nowhere else to be—and they don’t complain when soaked volunteers prod them with pencils and measuring tape. The mussels are worn, black, and often covered by tiny acorn barnacles. Ivory leaf barnacles, which compete with mussels for space, bleed red if someone handles them a little too roughly. Algae add patches of color to the rocks. Pink crustose algae could be mistaken for leftover bubblegum; black tar spot algae lives up to its name; the rubbery red seaweed Mazzaella splendens shines with an oily iridescence.

They’re all competing for the same territory: a newly larger space, now that sea stars are gone. Whether each species will take advantage of the new realty or get pushed out by mussel manifest destiny remains to be seen. Moritsch has not yet compared the first two years of her data, and she still must finish another year of community composition surveys.

The tide pools may be changing, but UCSC marine ecologist Pete Raimondi points out that they always do. It’s difficult for the public to appreciate nuances in the intertidal zone, Raimondi says. Most people don’t explore the low tides every day for years and track the changes. They visit every once in a while, mostly in the summer, and those visits set their frame of reference. This influences people into believing that any change is catastrophic. But the intertidal zone is resilient. This ecosystem lives under the abuse of crashing waves, strong winds, and blistering sun—and it still remains intact. “We see these communities change,” Raimondi says. “It’s not like this steady state all the time.”

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Fortunately, scientists are seeing lots of juvenile starfish. They hope sea stars will recover from this epidemic faster than previously expected. It’s a preliminary hope. Young sea stars are often too small to show evidence of wasting syndrome, and they still need to survive the summer. But Raimondi is confident: “We fully expect sea stars to come back. The community will recover.”

Moritsch realizes the uniqueness of her research. Scientists can’t just remove a top predator from an entire ecosystem on their own. Larger mussel beds may grow, but a complete domination of the intertidal by mussel forces is unlikely. After all, mussels have more predators to curb their numbers than just sea stars, such as birds and people. So even if fewer sea stars mean more mussels, Moritsch notes, “Mussels are tasty, too!”

I help Moritsch and her intern do a quick sea star count at Terrace Point. A small purple starfish peeks out from within a crevice surrounded by anemones and covered by a layer of detritus. We see six more small stars and two larger ones. The sea stars aren’t gone forever, but we can’t predict whether these will survive to the end of summer. In a decade, or maybe sooner, the tide pools should have their main attraction back in droves.

Across several more rocky outcroppings, tide pool visitors are picking their way along the slick rocks. Young children crouch and point at the small pools teeming with life. They’ll carry with them a memory of exploring the intertidal with their parents and seeing a single sea star. Perhaps in their teen years they will come back on a class field trip, frown at the five-armed invaders, and worry that they’re destroying the lush mussel beds of their youth.


Samantha Chavez is studying ecology and evolutionary biology at UC Santa Cruz. She wrote this story for SCIC 160: Introduction to Science Writing.


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