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Published: June 2011

Tide Pools

Violet coraline algae and sea stars

Brimming Pools

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

By Mel White
Photograph by David Liittschwager

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.

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.

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