by Jennifer Calkins

Sand colored egg cases litter the Puget Sound Beaches dotting the city of Seattle at low tide.  These cases are produced by Lewis’ moon snails (Polinices lewisii). The mucus emitted to form the egg cases is shaped by the snail’s large foot. If you see the egg cases, you will also see empty shells of other animals with neat little holes drilled in them.  A hungry moon snail grabs a clam or mussel or some other shellfish, drills a hole with its radula (a drill-like structure found in mollusks), and sucks out the inside of its prey.

Moon snails live subtidally and spend much of their time partially buried in the sand. Often, it is only the signs of the snails that one sees when visiting a beach at low tide. But to me these concrete signs–the egg cases and shells with holes and the partially destroyed moon snail shells, their strong central helix still intact—stand for the hidden interdependence among the species living in the intertidal zone.

In many ways, the attraction of a beach at low tide is the revelation of interdependence: the crows and gulls picking their way through for food, the distribution of various sessile creatures determined by competition with one another, the hermit crabs housed within the discarded shells of small snails, the human collecting other species to consume around a bonfire. At low tide, these interconnections are obvious—exposed for a short moment in time and then covered up.

Because of the dynamic nature of the intertidal zone, we see a density of interdependence not obvious in other habitats.  But of course, the entire world is a tangled net of interconnections—organisms do not exist except through their relationship to other organisms.  The most dynamic forces in evolutionary biology—driving the diversity of life on earth—arise out of the relationships among organisms. An example:  Earth houses the richness of the living world because of the wealth of oxygen gas (O2) available in the atmosphere as well as the protective ozone layer (O3).  This oxygen was not present during the early days of the planet; rather, it was produced by early living creatures, the cyanobacteria, which appear to have emerged at least 2.7 billion years ago. By photosynthesizing, these bacteria produced oxygen gas that, by 2.4 billion years ago, had accumulated at appreciative levels in the atmosphere.  It was only after this buildup that life on earth diversified: First the eukaryotes emerged and then multicellular organisms—the plants and animals we see around us, including ourselves. This relationship among photosynthesizing species that release oxygen and most everything that lives, including humans, continues as a necessary foundation for life on earth.

Jennifer Calkins is a writer and biologist. She is the author of A Story of Witchery, Devil Card, and From a Letter on the Encouragement of Virgins.

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