Tides and Trails

Between the high tide mark and the low tide mark lies a narrow band of shore—the intertidal zone. In this dynamic space, land and sea merge, flood and drought cycle, and strange lifeforms thrive. Animals submerged in saltwater are also exposed to air; they are battered by waves and scorched by sun. Spectacular adaptations are required of the creatures that live in this crowded, chaotic environment, and astonishing interactions abound.

In the intertidal zone, sea slugs as beautiful as butterflies steal stinging cells from their poisonous prey. Armor-plated mollusks graze algae that glows vivid pink. Giant green anemones use flowery tentacles to grab crabs; the anemones devour the crustaceans and belch empty shells from their emerald mouths.

Aggregating anemones cover great swathes of territory by cloning themselves. When colonies collide, warrior anemones at the borders of the colonies deploy battle armaments and exchange barrages of poisoned missiles.

Fish species that evolved at the ocean's edge breathe air when stranded on land by receding tides.

Sea stars, popular for their pretty colors, perform the role of carnivorous predator. The ochre sea star uses suctioned feet to pry a mussel shell open. Then the sea star thrusts its stomach out of its body and into a shell to slurp a mussel’s soft flesh.  

Should scientists someday find life on other worlds in far reaches of space, the alien beings they encounter will probably be no more bizarre than the creatures that populate the intertidal zone on planet Earth.

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Thanks to careful stewardship, the cliffs of Haystack Rock continue to support vast numbers of seabirds, and the boulders at its base harbor a rich world of marine life. A staggering assemblage of  creatures use the cold currents around Cannon Beach, from gray whales shepherding their calves close to shore during their annual migration, to sea slugs walking upside down across the surface tension of tidepools. Each species along the coast, however mundane it might appear, tells an extraordinary story: slugs, snails, and seaweeds speak volumes.

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A journey into the lowest reaches of the intertidal zone provides a glimpse of otherworldly creatures. From the alabaster sea slug to the bizarrely bristled sea urchin, strange beasts tell of life’s startling diversity. But in the overlooked and underappreciated upper reaches of the intertidal zone, the periwinkle snail spins a tale as compelling as Darwin’s finches in the Galapagos. This drab mollusk can survive for weeks away from the sea, for it is evolving to leave the watery world that birthed all life on this planet. The periwinkle is on a quest toward a new world beyond the tide. Clinging to rocks above the surf, sporadically wetted by splash and spray, a snail of unremarkable color and size reveals how an animal born of the ocean is transformed into a creature comfortable on land—which, of course, is our story.

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Ours is the story of an upright ape that carries the sea within us, in our blood and in our tears. We wander the ocean’s edge, peering into pools to glimpse our origins, and to gather clues about the world to come.

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Jellyfish pulsed in primeval seas. Now jellyfish are proliferating as the world’s oceans turn hotter and more acidic. Life on this planet has endured five major extinction events; life will survive the massive amount of carbon being released into the atmosphere by a clever ape. But the sixth wave of widespread extinction—which we have initiated and are now watching play out—will radically transform the oceans of the only planet in the universe known to support life.

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We couldn’t exterminate all life on Earth if we wanted to. Extremophile microbes can survive, and even thrive, in environments so demanding, just a few decades ago scientists were forced to expand their understanding of the outer limits that life can endure. Salt, heat, cold, acid, radiation: challenges that annihilate reptiles, birds, and mammals are but minor obstacles surmounted in the microbial march of life. This relentless march would continue if we were to sterilize the planet’s surface in a nuclear apocalypse. Much would be lost, of course. But life would continue.

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The more subtle apocalypse of ocean acidification is already stressing shelled species and coral reefs in the world’s changing seas. As we pump carbon into the atmosphere, diversity in the oceans diminishes. Microbes will evolve to survive the ocean chemistry we are altering, and many miniscule organisms will profit from the changes. Whether animals such as sea stars and humans can survive the Sixth Extinction is an open question.

Sound stewardship of the world’s oceans is imperative for the survival of our species. Every other breath we take is born of the oceans. The very air that sustains us comes from the sea. 

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Near the dawn of the world, after Earth coalesced from bits of cosmic debris, skies rained molten rock into seas of hellish fire. Icy comets bombarded the cooling planet, and water brimmed across a bluing sphere. Microscopic beings swarmed in primordial seas. Early forms of life thrived as methane skies smothered the planet. Oxygen was all but absent from the atmosphere. This early world was, from the point of view of our species, poisoned with unbreathable toxins.

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When life took an evolutionary turn, some organisms developed photosynthesis, freeing oxygen from its bondage to carbon. These liberated oxygen molecules made possible the evolution of complex life on Earth.

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Oxygen fueled a staggeringly efficient form of respiration. But the newly oxygenated atmosphere was a catastrophe for microbes powered by methane—they perished in uncountable numbers. Some methane-fueled species survived this mass extinction by retreating to spaces safe from oxygen: the muck of ocean floors, the deep recesses of our digestive system.

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The phytoplankton species that now thrive in the world’s seas provide half the oxygen we breathe. If we kill the oceans, not only will we go hungry. We’ll also be holding our breath.

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Diatoms, a form of phytoplankton, have filled the world’s oceans through countless eons. These tiny organisms use silica in seawater to fashion glass cases of exquisite beauty. When a microscope brings into focus the intricate etchings of a diatom’s glass case, our eyes widen at nature’s design. When we understand the crucial role phytoplankton played in our own evolution, and the vital role it plays in sustaining our species, we look at the ocean through a new lens.

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Becoming conscious of our connections to the ocean is a first step toward sound stewardship of the marine systems that support us.