Barnacles, Mussels, Snails - Rocky Intertial Acadia Maine
Barnacles, periwinkles, and blue mussels crowded together on exposed granite — this is the intertidal zone of Acadia National Park, Maine, and few ecological communities on the Atlantic coast are as rugged, competitive, or quietly dramatic as this one. Acadia's Intertidal The rocky coastline of Acadia National Park along Mount Desert Island represents some of the most studied and celebrated intertidal habitat in North America. Shaped by some of the most extreme tidal ranges on the U.S. East Coast — regularly swinging six to twelve feet between low and high tide — these rocks are exposed to air, drenched in seawater, baked by summer sun, and locked in winter ice, sometimes within the same day. Life here is organized into distinct horizontal bands, each species staking out the precise elevation it can physiologically tolerate. The Barnacles The dominant barnacles in Acadia's intertidal are most likely acorn barnacles — primarily Semibalanus balanoides, the northern rock barnacle, which carpets the mid-to-high intertidal in dense, chalk-white colonies. What appears to be a crusty mineral encrustation is actually a community of animals. Each cone is an individual crustacean — closely related to crabs and shrimp — that has cemented its head permanently to the rock and built a volcano-shaped limestone fortress around itself. When submerged, the plates at the top open and feathery feeding appendages called cirri sweep the water for plankton in rhythmic, raking strokes. When the tide retreats, the plates clamp shut, sealing in moisture and protecting soft tissue from desiccation and predators. A barnacle can survive hours of aerial exposure that would kill most marine invertebrates. The Periwinkles The small snails grazing among the barnacles are almost certainly common periwinkles (Littorina littorea), an intertidal species so successful it has become one of the dominant grazers on the entire New England coast. Periwinkles rasp algae and biofilm from rock surfaces using a ribbon of tiny teeth called a radula, and their grazing pressure actually shapes which species of algae and organisms can establish themselves — making them quiet but powerful ecological engineers. They are extraordinarily tolerant of exposure, capable of sealing their shell opening with a tough plate called an operculum and entering a dormant state during low tide. Periwinkles are also, notably, a non-native species introduced from Europe in the 1800s that has thoroughly naturalized into the New England intertidal ecosystem. The Mussels The blue mussels (Mytilus edulis) nestled into the barnacle matrix represent the intertidal's master colonizer. Mussels attach to rock and to each other using byssus threads — strong, elastic protein fibers secreted from a gland near the foot — forming dense mats that can eventually outcompete barnacles for space if left unchecked. They are filter feeders, drawing water through their gills and straining out phytoplankton and suspended organic matter with extraordinary efficiency. A single mussel can filter more than a liter of seawater per hour. Their aggregations create habitat for dozens of other small invertebrates — worms, amphipods, small crabs — sheltering within the mussel matrix like a city within a city. The Competition What looks like a static rock surface is in ecological terms a slow-motion battlefield. Barnacles, mussels, algae, and periwinkles are in perpetual competition for the most precious intertidal resource — bare rock to settle on. Barnacles colonize open space quickly after disturbance, mussels can overgrow them, sea stars (Asterias forbesi and A. vulgaris) prey heavily on mussels from below at low tide, and winter ice scour periodically resets patches to bare rock and starts the succession over. The ecologist Robert Paine's foundational work on intertidal community structure — including the concept of the keystone species — grew directly out of research conducted in this kind of rocky intertidal habitat. Seasonality in Maine In Acadia, the intertidal community shifts with the seasons in ways invisible in warmer climates. Barnacles spawn in late winter and early spring, releasing nauplii larvae into frigid water. By late spring, newly settled barnacle spat — barely visible to the naked eye — dust the rock in a faint haze of white. Summer brings maximum biological activity, predation pressure, and algal growth. Winter strips some of the softer community back and, in hard freeze years, ice rafting physically tears mussel beds and barnacle patches from the rock entirely, creating the disturbance patches that the community then races to recolonize come spring.
Uploaded Oct 10, 2012 by Jason Buchheim username:odyssey copyright
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