Science

Tide Pooling

We gather for the American Society of Naturalists meetings at Asilomar State Beach to argue about life. We share our most specialized natural history discoveries and our broadest theories about the nature of evolution. We debate the scale and strength of adaptation, sketch out complex feedback loops connecting ecological and evolutionary processes, and weigh the behavioral and life history strategies which organisms use to deal with fluctuating environmental pressures. Are there observable, unambiguous patterns of convergence and divergence, stasis and change, or is most of what we see the product of historical contingency?

And when the day’s presentations are done, I go tide pooling.

In the fading January afternoon light, I trip my way along the Pacific coastline, pausing at every patch of water pooling amid the rugged topography of the rocks. I love tide pooling. Every pool is an experiment, a unique ecological community thrown together at random by one tidal flow and swept away just as violently by another. Some are disappointing, mainly sand and bits of shell, and a stray hermit crab scuttling among broken stems of kelp. Some are saturated with green mats of algae that choke out any life underneath. But a few are wonderfully diverse, microcosms of the ocean nestled in a fissure of rock.

Several pools harbor giant pastel Anthopleura sola anemones as big around as my hand. It’s astonishing that something so large could live in such a tiny patch of habitat, and such an unstable one at that. For every anemone submerged in a pool, there are many others left exposed. They curl their tentacles into the center of their bodies to hold in what little moisture they can and gather an armor of shell fragments, pebbles, and sand to protect their epithelium from the elements. They will weather long, brutal hours of scarcity until the water returns. Many may die waiting. I try not to squelch them with a careless boot fall as I skirt the lapping waves.

It seems that making a living in the tidal basin is a matter of pure serendipity. Some anemones sink in deep pools that persist for months undisturbed, others settle in shallow, ephemeral puddles that dry and refill with the daily cycle of the tides. In large pools fed by regular flows, anemones glut themselves on a replenishing bounty of zooplankton, copepods, mollusks, and wayward fish. In small secluded pools, they scrabble for detrital scraps washed in by the occasional wave. From rock to rock, crevice to crevice, chances of survival vary dramatically.

Can there be a winning strategy to prosper on such an uncertain terrain? During high tide, anemones may detach from the rock and swim between pools, but should they bother? Stray too far ashore and you may never reach the tides again, or you may drop in with a larger, rival anemone that will shred you ragged with its defensive acrorhagi tentacles. If your pool is plentiful and deep, it seems safest to stay, but for how long? The next tide could wash in a feast of nutritious krill, or a predatory Pisaster sea star with a taste for tentacles. How could an anemone possibly anticipate what the tides will bring tomorrow?

Yet somehow they persevere, because some individuals are fat, shiny, and vibrantly colorful. Are these hardy survivors better adapted to their harsh and capricious world? Perhaps some inherited a higher metabolic efficiency that allowed them to quickly outgrow their predators and competitors, or tissues that retain moisture like a sponge when the tide recedes. Perhaps they were merely fortunate to find a plentiful food source and a dependable habitat.

Above the dunes, we ask: how powerful and predictable of a force is selection, really? Is it like the gentle tides, subtly yet continuously molding populations to their environment? Or does successful adaptation require catastrophic environmental pressure, like a rogue riptide crashing over the rocky coastline? How much of survival is pure dumb luck?

Staring out at the expanse of dark, rippling water, I am overwhelmed by the enormity of our unknowns. Will we ever understand how nature really works? Or will we return over and over to these same unsolvable, improbable questions?

A playful wave laps over my boots, beckoning my gaze downward to the colors in the rock pool at my feet. I kneel to examine it more closely. At the bottom hides a small A. sola. This one has cleverly buried itself in white sand, leaving only its pink tentacles exposed to snag a passing meal.

As scientists, we develop questions as deep and vast as the ocean, yearning to decipher the true story of life’s wild diversity.

And then we go tide pooling.

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