A Visit to the Intertidal Zone

by Lauren Klappenbach

A small, rocky island in the Gulf of Maine is the perfect place to explore the delicate, dynamic habitats that thrive where land meets sea. These brine-soaked borderlands known as intertidal zones are home to a rugged assortment of organisms, all armed to deal with battering waves, parching sun, and salty inundation.

On Appledore Island off the coast of Maine, a group of students from Shoals Marine Laboratory walked along a narrow path that wound its way to the island's western edge. They were accompanied by a trio of biologists, Lauren Quevillon of Cornell University, Kipp Quinby, a Lab Preparator from Shoals Marine Laboratory, and Helen Hess, a Professor from College of the Atlantic. The three women, all experts in marine biology, led the group on a modest expedition of discovery and learning. Their destination was the intertidal zone.

The path wove through a succession of grassy clearings before stepping down onto a wide beach of rocks and pebbles. Smooth stones formed a jumbled mix with organic debris that had been churned up by surf and wind--decaying seaweed, wave-warn bits of wood, broken snail shells.

At the edge of the rocky beach, about a fifty yards from the sea, the terrain turned rough and gave way to hefty slabs of granite that bulged from the surrounding land at precarious angles. The students progress slowed as they struggled to find their footing. Finally, they arrived at the uppermost edge of the intertidal, the furthest reach of the high tide's waves.

Helen Hess moved to the front of the group. She stood facing the students who had gathered around her in a semi-circle. The sea served as backdrop, it rolled in gentle waves that slapped and gurgled as they hit the rocks. "How many of you have explored the intertidal before?" she asked. Several students nodded, others shook their head.

Professor Hess told the students to look at the landscape that surrounded them. She described things through the eyes of a biologist. She spoke of natural gradients in the intertidal, of habitat pockets shaped by the animals that were adapted to live in them. People tend to look for patterns and edges in everything and try to draw lines between one habitat and the next. In reality, nature is a continuous blend of one patch to the next. Gradients, not grids, are the stuff of nature.

Each creature has its own set of "physiological tricks" that enables it to cope with the harsh conditions in a particular plot of the intertidal. "Barnacles are the classic example of tough customers," Hess said, pointing to the clusters of cup-like shells that stuck stubbornly to the rocks.

The intertidal zone is a habitat driven by the rhythms of the tides. The creatures that inhabit the intertidal zone--sea urchins, snails, mussels--spread out along its depth to occupy only the patches to which they are ideally suited. As you move from one location to the next in the intertidal zone, creatures face different physical demands. Different demands at each level mean different inhabitants at each level.

Periwinkles graze on seaweed at the high end of the intertidal. Filter feeders like barnacles and mussels nestle amongst the rocks at the middle of the intertidal, wetter areas that remain soaked for longer periods of time than the high reaches of the intertidal. At the lowest layers of the intertidal, closest to the water, an assortment of strange sea creatures such as tunicates, hydroids and sponges take up residence.

Rocky seashores like those on Appledore are most common along the coast of New England north of Cape Cod. During the last ice age, glaciers scrubbed and scoured the shorelines in this region, leaving them clean of sediments. All that remained was exposed bedrock. The habitats that sprouted up on the barren surface are unlike any other. Algae and diatoms form the base of the food chain.

Helen Hess, Kip Quinby, and Lauren Quevillon guided the students closer to the water, deeper into the intertidal zone. There, the rocks were draped with thick mats of the leathery, brown algae Ascopyllum, also known as “knotted wrack”. Other seaweeds such as Fucus or “rockweed” adhered to small patches where clumps of Ascophyllum had been torn away by the crashing waves.

Lauren Quevillon stepped to the front of the group. From a dry vantage at the edge of the high tide mark, she pointed across a continuous mat of seaweed. Four small cages, each six inches tall and fifteen inches square, sat atop olive brown ribbons of seaweed, "These cages take a beating. We set up several others but a storm came through and tore them away, even though they were bolted into the granite." she says. Life in these rocky tidal waters is wrought with challenge--both for the creatures that live there and for the scientific equipment deployed to study them.

Quevillon studies an invisible creature in the intertidal--a parasite. The cages each hold a single healthy crab, one of the parasite's host species. Closer to the water where deep pools of saltwater are left behind at low tide, there are more cages housing healthy crabs. Quevillon hopes to find out where in the intertidal zone crabs are most likely to become infected with the parasites.

A dark storm that had been gathering on the horizon snapped a bright strand of lightening across the sky to the south of the island. "We better get going," a student said, looking at the slippery rocks that they had to cross on their return. The wind picked up and the light began to fade as the thick clouds rolled in off the sea. A misty rain started to fall. Even the rocks far from the intertidal start to glisten and grow slick with moisture.

The group decided to end their intertidal visit and return later in the evening. In the meantime they headed back to the lecture hall to talk more about the intertidal zone.

The rain was falling in thick sheets by the time the students made their way back to the center of Appledore Island. The journey ended at Leighton Hall, a small shingle-sided building that sits tall on a bed of granite. Inside, they shed their wet raincoats and soggy backpacks as they made their way into the lecture room. There, Quevillon explained the things they've just seen, and more importantly, the things they didn't see.

Quevillon's research focuses on a tiny parasite that lives in the intertidal zone. It's a species of flatworm with a peculiar life history. To get from one stage of its life to the next, it relies not on a single host but on a greedy succession of three: a bird, a snail, and a crab.

The parasite begins its life cycle as a tiny worm deep in the small intestine of a host bird. There it produces eggs in vast quantities.

"It's really a game of numbers with parasites," Lauren said. It is crucial for the worm to manufacture a surplus of eggs to sustain its numbers. The bird, not harmed by the parasite, excretes the eggs in its droppings that litter the seaweed. "Many of the eggs get washed away, but some adhere to the seaweed."

The eggs, left to chance, only survive if ingested by a snail that grazes on egg-laden seaweed. Despite slim odds, some eggs do succeed and are eaten by the snails. Once inside the snail, the parasite makes its way deep inside the snail's shell. The parasite feeds on the snail's internal organs. As it does, it reduces the feeding efficiency of the snail and the snail eats less seaweed. This in turn affects the balance of algae in the intertidal zone.

"These parasites, things you cannot see with the naked eye, are structuring communities--that's huge," Lauren said.

That evening, the students return to the intertidal zone again with Quevillon.

Quevillon picked a snail from beneath a rubbery frond of knotted wrack. It retreated into its shell. Infected snails are easy to identify. The parasite damages the snail's liver, causing discoloration of the snail's foot. "The vast majority of snails out here have parasites, you turn them over and they have bright carrot-orange feet," Lauren explained. "There's an eighty percent chance this one's infected," she said as she watched it, waiting for it to expand its foot from its shell. The snail remained stubbornly entrenched in its shell. "This one's not going to come out for a while," Lauren said. She placed the back in the bed of seaweed.

The sky began to lose its luminosity as the sun sank beneath the horizon. The tidal pools grew grey and colorless in the dimming light. The students gathered and began their journey back, stumbling across slippery rocks and knotted strands of seaweed. The animals and algae of these rocky shores are not the only creatures that know where they belong when facing the harsh intertidal environment--humans know too.