Description. In Marine and Estuarine Systems, Rocky Shores are generally high-energy habitats which lie exposed as a result of continuous erosion by wind-driven waves or strong currents. The substrate is stable enough to permit the attachment and growth of sessile or sedentary invertebrates and attached algae or lichens. Rocky Shores usually display a vertical zonation that is a function of tidal range, wave action, and degree of exposure to the sun. In the Lacustrine and Riverine Systems, Rocky Shores support sparse plant and animal communities.
Subclasses and Dominance Types.
The uppermost zone (here termed the littorine-lichen zone) is dominated by periwinkles (Littorina and Nerita) and lichens. This zone frequently takes on a dark, or even black appearance, although abundant lichens may lend a colorful tone. These organisms are rarely submerged, but are kept moist by sea spray. Frequently this habitat is invaded from the landward side by semimarine genera such as the slater Ligia.
The next lower zone (the balanoid zone) is commonly dominated by mollusks, green algae, and barnacles of the balanoid group. The zone appears white. Dominance Types such as the barnacles Balanus, Chthamalus, and Tetraclita may form an almost pure sheet, or these animals may be interspersed with mollusks, tube worms, and algae such as Pelvetia, Enteromorpha, and Ulva.
The transition between the littorine-lichen and balanoid zones is frequently marked by the replacement of the periwinkles with limpets such as Acmaea and Siphonaria. The limpet band approximates the upper limit of the regularly flooded intertidal zone.
In the middle and lower intertidal areas, which are flooded and exposed by tides at least once daily, lie a number of other communities which can be characterized by dominant genera. Mytilus and gooseneck barnacles (Pollicipes) form communities exposed to strong wave action. Aquatic Beds dominated by Fucus and Laminaria lie slightly lower, just above those dominated by coralline algae (Lithothamnion). The Laminaria Dominance Type approximates the lower end of the Intertidal Subsystem; it is generally exposed at least once daily. The Lithothamnion Dominance Type forms the transition to the Subtidal Subsystem and is exposed only irregularly.
In the Palustrine, Riverine, and Lacustrine various species of lichens such as Verrucaria spp. and Dermatocarpon fluviatile, as well as blue-green algae, frequently form characteristic zones on Rocky Shores. The distribution of these species depends on the duration of flooding or wetting by spray and is similar to the zonation of species in the Marine and Estuarine Systems (Hutchinson 1975). Though less abundant than lichens, aquatic liverworts such as Marsupella emarginata var. aquatica or mosses such as Fissidens julianus are found on the Rocky Shores of lakes and rivers. If aquatic liverworts or mosses cover 30% or more of the substrate, they should be placed in the Class Aquatic Bed. Other examples of Rocky Shore Dominance Types are the caddisfly Hydropsyche and the fingernail clam Pisidium.