by Gavriel Wolf, Cornell University
To most, the intertidal zone, or what amateurs like myself would call the shore line, is a rocky beach with little more danger than some slippery algae and a whole lot of falling bird poop Through the eyes of scientists who study the intertidal, however, it is a war zone. The algae are armored competitors, and the bird poop is filled with parasites that can puree the insides of a snail.
Lauren Quevillon, who is doing research on a parasite called Cryptocotyle lingua on Appledore Island, one of the Isles of Shoals, guided our group to the intertidal on to get a closer look at her work. As a storm was brewing in the distance, she pointed out a set of shoebox-sized cages holding crabs. As bolts of lighting began to connect the sky and ocean, Lauren hastily explained that the cages were set in the water at different heights, to determine the regions where the parasite was most likely to invade the crab.
The cages that Quevillon originally set are now gone. Lauren spent half an hour drilling two three-inch screws into limestone to hold each cage. She thought, "These things are going nowhere." But soon, a storm came and ripped the bolts out, pulling the cages out to sea. "Now think of what the storm can do to the organisms," Quevillon pointed out. At this point we were not only thinking about the strength of the storm, but we were scared of experiencing it, we ran back for shelter.
Slow and stationary organisms are completely exposed to the elements on the wave battered rocks of the Atlantic shores. On the boulder-strewn shores, there is no sand or loose rock for crabs and snails to find refuge. The tide moves in and out twice a day. While the tide is out, plants and crustaceans must be able to withstand the hot, dry rock, where birds dive-bomb crabs, and snails drill through mussel shells. As the water rushes back in, they must be prepared for an ocean filled with insatiable fish, and the dehydration of salt water.
In addition to the summer waves that tear snails and algae from the rocks, sliding winter ice sheets strip everything except the most robust creatures from surviving on these rocky shores.
The ice sheets are a small reminder of the historical glaciers that gouged out the intertidal shores, between 10,000 and 20,000 years ago. During the last ice age, the sand and pebble beaches were dragged away by glaciers, leaving large rocks in their wake. Only the most daring organisms were left to recolonize the uninhabited shores from New York to Canada.
While the exposed shoreline poses a laundry list of dangers, it also offers a buffet of perks. The constantly undulating ocean delivers food particles to barnacles and other organisms that feed by filtering water. The sweeping water helps algae inhale carbon dioxide and exhale oxygen. Finally, algae sunbathe in the large amounts of energy that shines down uninterrupted by trees or bushes, and reflects off the ocean water.
The variety of opportunities for both danger and growth divides the Appledore shore into zones. The very top of the intertidal only gets a spraying of water, while the lowest regions are almost always submerged. The area in between is a slowly changing continuum of different environments. "The way we tend to describe the intertidal is by the most dominant creatures ", explained Kipp Quinby, a lab preparator on the island.
On one outing Quinby painted a picture of precisely what is happening on the frontlines, where ocean meets land.
The brown line marking the very top of the intertidal, known as the splash zone, is tattooed into the rock by a plant like bacteria. With so little water reaching this region the bacteria are the only sea creatures that can survive
Just a foot or two below the bacteria lays a flattened forest of algae. The long linguini-like plant is about as tough as they come. A waxy surface provides protection against predators, while a death grip to the rock protects it from being swept away by the waves. As we walked along the algae during low tide we heard it crunching underfoot. I was sure it was dead. Quinby explained, however, that the algae does not die, but dries out almost completely as the water recedes, and comes back to life when the water returns.
Underneath the blanket of algae, slightly lower down on the intertidal, the rock is littered with small white mounds of limestone. Barnacles create these bone-like structures. "As a larvae they basically glue their head to the rock," Quinby explained, and build calcium up around themselves for protection.
Barnacles inhabit the middle region of the intertidal where they can out survive the mussel, their biggest competitors for food. As we moved into the depths of the intertidal we could see the mussels amongst a moss-like algae. Mussels stay lower on the intertidal where they will be submerged for a greater portion of the day, to protect themselves from drying out. In order to take control of the zone, mussels will literally sit on top of barnacles, essentially starving them to death.
The intertidal also teems with skittering and creeping crustaceans. Crabs and snails move between zones intensifying the ongoing battle. The snails set up camp in the upper and middle regions of the intertidal, while the crabs reign supreme in the lower and middle regions. It is in this overlap where the true carnage lies. Snails make mincemeat of barnacles and mussels, crabs turn snails into crunchy snacks, and giant Blackback seagulls swoop in and impale the crabs. A littering of snail shells and crab legs are all that is left.
On the battlefield lie Quevillon’s cages. The cages appear to protect the crabs from any ocean dwelling creatures, however the metal wire does little to keep out the microscopic parasites.
The parasite makes seagulls, snails, crabs, and fish the hosts for its lifecycle of growth and reproduction. The process begins and ends in the gulls. The parasite grows to its adult stage in the intestines of the gull and begins a constant production of tens of thousands of eggs. The eggs set up shop in the bird’s poop and wait for deployment. The original parasite dies in the gull’s stomach, while its progeny are launched indiscriminately into the world. With any luck, the parasite-filled feces will land on some seaweed. An unsuspecting snail, just trying to grab a bite to eat, will then ingest a few privileged parasites.
The parasite then moves into the snail’s internal organs where it begins to wreak havoc. After going through a series of life stages, and reproducing asexually several thousand times, the parasite replaces the tissue of the snail’s reproductive organs, effectively castrating the snail. The parasite attack also slows down the snail’s daily activity, in effect stunting its growth and thinning its shell. The snail essentially becomes a robot for the parasite’s reproduction factory.
After going through a complete metamorphosis inside the snail, the parasite pops out as a Cercaria, a two-eyed head with a nutrient filled tail. It must find a host quickly; the tail is like "a boxed lunch, and it only lasts so long", Quinby joked.
Cercaria, too small to see with the naked eye, must find a fish or a crab in an infinitely large ocean. Their odds of survival are next to none. The parasite epitomizes the volatile existence of living in the intertidal. In just one life cycle, they alternate between exponential growth and population crushing destruction a total of five times.
The cercaria that do find a host, form a cyst on a fish or inside of a crab, where it waits for the gull to take the bait. Once inside the gull’s gut, the process starts all over again.
The parasites life cycle may seem like it is only affecting the host organisms. With an aerial view, however, the parasite’s immense impact on the intertidal becomes clear. When parasite populations get out of control large numbers of snails are castrated and crippled, causing snail numbers to drop. The barnacle population, in turn, grows unchecked by one of their primary predators. The expanding barnacle population takes over rock space leaving less room for other organisms such as algae. The ripple effect goes on, possibly changing the framework of the intertidal.
Quevillon’s research focuses on one small part of the parasite’s involved life cycle. She sees her work as helping to paint a more accurate picture of the large implications that this parasite may have on the Atlantic intertidal. Each scientific discovery adds another piece to this complicated intertidal puzzle.
The intertidal is a complex system, where a creature’s daily life is a struggle for survival. This twenty foot wide strip of land is a place that scientists have been studying for hundreds of years, each generation getting a little closer to a complete understanding of the intertidal.
In the last 150 years, with Louis Pasteur’s discovery of life on a microscopic scale, new players have been added to the struggle. At times, scientist seem to have a good grasp of the who, why, and how of the intertidal. With a keen eye and maybe an island full of research equipment, however, the unknown seems to continually assert its dominance.