Mid-intertidal Zone

 Mid-Intertidal zone

Below the upper intertidal zone lies the mid-intertidal zone, which is the area between the high and low tide marks and also the largest of all the three intertidal zones. This area is occupied largely by barnacles, mussels, and seaweeds. Within this zone, barnacles are dominant in many regions of the world, with some distinct local variations in abundance. A variety of organisms may successfully compete for resources especially in the lower parts of this zone, as this area is covered with water most of the time. While in temperate waters, major animals include sedentary mussels, limpets, gastropods and polychaetes, in tropical regions barnacles dominate. Two common species of barnacles, the rock barnacle (Balanus balanoides) and the bay barnacle (B. improvisus), are found attached firmly on to the rocks by cementing or gluing so that the most powerful wave cannot dislodge them.

At high tide, barnacles are covered by water. During this time, they filter feed on plankton and organic debris by rhythmically whipping their feathery cirri to capture food. When the tide goes out, the barnacles are exposed to air for several hours, so they shut their shells tight to prevent them from drying. The barnacle’s sharp, overlapping shells also help to protect it from predators. However, a marine snail called the dog whelk (Nucella lapillius) can drill a hole through the shell and eat the barnacle. For feeding on barnacles, the whelk produces an acidic secretion from its glandular foot, which softens the barnacle shell before the whelk drills into it with its radula.

Below the layer of barnacles lie seaweeds and a densely packed bed of mussels (Mytilus edulis, Perna viridis, P.indica etc.). Mussels attach to the rocks by means of byssal threads. The threads prevent the mussels from being dislodged by waves. Like barnacles, mussels are filter feeders. The beating of the mussels’ tiny cilia (on the surface of their gill membranes) creates currents which carry food and oxygen into, and wastes out of, their shells. This bed of mussels attracts several predators. The dog whelk that eats barnacles also eats mussels. Sea stars creep over the mussel beds and consume large numbers of these bivalves. The blue mussel (Mytilus edulis) is found along the Atlantic and Pacific coasts and is harvested and eaten by humans. The brown seaweed called rockweed (Pelvetia, Fucus) also lives in the mid-intertidal zone. When the tide is low, thick mats of rockweed can be seen hanging or draped over the rocks. The seaweed clings to the rocks by means of its holdfast pad. Rockweed provides cover for a variety of marine animals that live in and among the rocks, such as snails, limpets, small crabs, and worms. When the tide is high, rockweed floats near the surface, buoyed up by its gas-filled bladders.

Last modified: Thursday, 28 June 2012, 5:18 AM