Brimming Pools

A cast of thousands clings to rocky real estate in a narrow strip of shore called the intertidal zone.

This story appears in the June 2011 issue of National Geographic magazine.

The sea star—often called a starfish, though it's no more a fish than it is a sheepdog—ranks with the most spectacular creatures of the diverse menagerie inhabiting the shores near Bodega Bay. Big (sometimes a foot across) and obstinately colorful (some are orange, some are purple; no one knows why), the sea star is usually found in a rock fissure sprawled like a discarded toy. But despite its apparent lethargy,Pisaster ochraceus serves as a top predator of the intertidal zone—tiger of the tide pool—though it lacks anything like a brain.

Sarah Ann Thompson, a marine biologist from the Farallon Institute in Petaluma, California, is guiding me over the rugged rocks and through the tide pools of Bodega Head's Mussel Point, 65 miles north of San Francisco. (I, in rain gear, rubber boots, and knee-pads, am trying not to slip on the shiny, slick kelp and end up as fish food.) Thompson stoops to pick up an orange star.

In a bizarre adaptation right out of a superhero movie, Pisaster can, in the span of a heartbeat—or what would be a heartbeat, if it had a heart—rigidify the "mutable tissue" in its normally limp body to transform itself into a structure as solid as bone. It then employs an internal hydraulic system and hundreds of suckerlike feet to grab the shells of a mussel and summon enough force to pull them apart.

"This Pisaster has already killed the mussel," Thompson says, holding the sea star and the deceased in one hand and separating the mussel's shells a bit with the other. "Pisaster has everted its stomach out through its mouth, and it's digesting the mussel externally."

So that creamy goo inside the mussel … ?

"Yes, that's Pisaster's stomach. When it's finished eating, it pulls its stomach back inside itself and goes on its way."

Tide pools form in zones of rocky shoreline where ocean and land meet—strips of shore, sometimes only a few yards wide, where everything is covered and uncovered by tides each day. John Steinbeck famously described this zone as "ferocious with life." The observation applies spatially—lots of things are happening in a relatively small area—but also temporally: Things happen fast between tides.

Biologists value the intertidal zone as an easily observable model of ecological processes that happen on much larger scales. Those who study life zones—the way flora and fauna change from the desert up to alpine peaks—must traverse many miles of landscape to experience a wide range of habitats. The intertidal strip displays zonation—from the sea grass at the bottom up through strata of sea anemones and mussels and barnacles to the limpets at the top—all within a few steps.

Tidal Time Lapse

Watch the tide roll in at the Pacific coast's Fitzgerald Marine Reserve.

When a tornado rips through a mature forest, and growth begins anew, centuries will pass as grasses give way to shrubs and pioneer trees eventually yield to the species of an old-growth forest. When a wave-tossed log scrapes away a patch of intertidal life down to the bare rock, biologists can watch mature life return practically before their eyes, the cycle of succession lasting just a few years.

A coincidence of geology and climate makes the northwestern coast of North America one of the world's most diverse and productive intertidal regions. Near-shore upwelling of cold Pacific Ocean currents provides nutrient-rich water, winter freezes and rock-scraping ice are rare, and abundant fog softens the drying effect of sunlight on marine animals that must spend half their lives or more out of the sea.

The rocks and pools here create an abundance of opportunities and host a diversity of life to rival any rain forest. Pisaster is just one of scores of species that have adapted to innumerable micro-habitats with a seemingly endless variety of physical shapes and lifestyles. One little worm can shoot a harpoon out of its head to stab its prey. A limpet tends and guards its own farm plot. A seaweed releases acid for defense when it's injured. A nudibranch (which looks like a gussied-up slug) eats poisonous creatures and implants stinging cells under its own skin to repel predators.

Why all the aggression? It's simply the result of lots of plants and animals competing for resources in a highly productive but limited space. In nature, as in real estate, location is everything, and the intertidal zone is Park Avenue.

Eric Sanford likes to perform a kind of magic trick for his students at Bodega Marine Laboratory, giving his introductory patter in the classroom and heading down to the rocky coast for the payoff.

First, the audience must understand the concept of phylum (plural, phyla): the organizing principle for classifying the entire animal kingdom. The German word for phylum, bauplan (body plan), is helpful here in that all animals are grouped according to their physical structure. For instance, everything with a notochord (which would be a spine for sharks, pythons, pelicans, you) belongs in the phylum Chordata. Butterflies and shrimps and other animals with jointed legs belong to Arthropoda. Depending on who's doing the classifying, biologists list around 33 phyla.

Next, Sanford leads his audience—in this case, me—on the short walk to Horseshoe Cove, another indentation in the rugged shoreline of Bodega Head. After a little exploration through the thick growth of kelp and sea grass, he picks up a promising-looking rock about the size and shape of a slightly melted bowling ball.

"Let's see what we can find," he says. "This yellow crust is a sponge, in the phylum Porifera. This sea anemone is in Cnidaria, and this tiny sea star is in Echinodermata. This snail and this nudibranch are both in Mollusca, and here are several tube worms, in Annelida.

"We've got a couple of things here that look something like sponges, but they're in Bryozoa. Here's a tunicate, also called a sea squirt, which is in Chordata, and a crab, in Arthropoda." It takes some searching, but he comes up with one more: "And this is a flatworm, in Platyhelminthes."

So there's the magic. Eric Sanford is holding, in one hand, representatives of more than one-fourth of all the animal life on Earth: nine phyla on one rock. In comparison, the entire land surface of the planet, from Poles to Equator, is home to only about a dozen phyla.

Sanford is actually a little crestfallen because he can't find a peanut worm, an odd thing in the phylum Sipuncula that would give us an even ten. The thrill would have been strictly numerical, though. I've already seen a peanut worm, and it has all the aesthetic appeal of used chewing gum. (I must admit, however, that the one thing it does, it does very well: extending a hydraulically powered, tentacle-tipped proboscis several times the length of its body to grab tiny bits of drifting dead stuff. Sanford calls it "this crazy sort of Dr. Seuss-like thing.")

"A lot of this diversity occurs because life first evolved in the sea," Sanford says. "And intertidal systems are really a microcosm of the ocean in general. In a tide pool, for the few hours that the tide is out, you're able to access it all."

A couple of days later, I'm rock-hopping above Horseshoe Cove with Jackie Sones, research coordinator at the Bodega Marine Reserve. "This," she says, holding up a pale orange creature about the size of her fingernail, "is a pycnogonid, commonly called a sea spider."

Through a hand lens, it does in fact resemble a spider, although one with a touch of the Michelin Man, body and legs ribbed and puffy. I can see this thing, blown up a couple of thousand times, doing battle with some outer space monster in a low-budget fifties sci-fi flick.

"It uses its proboscis to puncture sea anemones and suck out fluids," Sones says. But this minute predator has a nurturing side. Sones upends the pycnogonid to reveal a cluster of tiny spheres like whitish caviar. "The males care for the developing young," she says. "They gather the eggs from different females and hold them with specially modified legs." Pycnogonids fascinate biologists because they're one of very few types of animals in which only males care for the young.

Juvenile development of intertidal creatures varies nearly as much as their physical forms. Many go through a free-swimming larval stage lasting weeks or months, venturing into the immense ocean before settling down as adults on a patch of rock.

We kneel to examine one of those larval roamers—or rather the resulting adult. The giant green sea anemone, Anthopleura xanthogrammica, is a formidable predator, though it waits for unsuspecting prey to wander within reach rather than actively hunting. Resembling a fist-size blob of lime Jell-O out of the water, Anthopleura blooms when submerged, extending delicate tentacles around a sucking maw that swallows prey whole. Sea anemones show their kinship to jellyfish in their use of stinging structures called nematocysts, which they fire like microscopic darts to stun prey. I extend a finger as an anemone taste test and feel only a faint sticky sensation. If I were a crab, I'd be lunch.

Once they've set up housekeeping, Anthopleura and many of its neighbors in the intertidal zone show exceptional longevity, both individually and as species. Sea anemones have lived decades in laboratories without showing any discernible signs of aging, and some in the wild are believed to be 150 years old or more. "Short of predation or other fatal accidents," one reference book states, "anemones are potentially immortal."

Biologists question, though, how even highly adaptable intertidal plants and animals will respond to threats they've never known before. Local issues range from pollution and siltation due to coastal runoff to an increase in the harvest of some seaweeds, fueled by the demand for natural food.

Of far greater significance is ocean acidification caused by higher levels of atmospheric carbon dioxide. Mollusks, crustaceans, and even coralline algae are among the living things that use calcium in their structures, a process that could be impeded by higher acidity levels in seawater. Rising ocean temperatures are also a threat: Warm water holds less oxygen than cold water. National and state marine reserves can help protect against overharvest of ocean resources, but they're just as vulnerable as the rest of the seas in the face of global climate change.

In a meditation on the interconnectedness of life, Steinbeck wrote, "It is advisable to look from the tide pool to the stars and then back to the tide pool again." As a microcosm of the ocean—the nursery of all life including our own—the intertidal zone serves as another galaxy in the universe, one easily within our grasp.