Imagine that you are a marine animal fighting for survival, and you find yourself in a very difficult environment, one that is periodically exposed and then inundated with water. In addition to this, the substrate is constantly moving and frequently hammered by strong, breaking waves. Oxygen concentrations fluctuate. Salinity levels vary. And the area is constantly surveyed by hungry predators.
Such is the place called the intertidal zone, and it is one of the most hostile marine habitats for marine life. This area between the daily high and low tides is a dynamic system, one in which the sediments are constantly moving. If the beach has a steep profile, it may exhibit very coarse, large materials such as pebbles and rocks. This is often accompanied by strong waves which carry any remaining light sand, and making it even more difficult for animals to inhabit. The opposite of this is a quieter and sandier beach with a more uniform profile. Burrowing animals are more likely to be found here.
Of course these sections of a beach can change even more if there are jetties, groins, or seawalls present. Jetties, for example, tend to starve the beach of its sand. Instead of protecting the beach from erosion, jetties actually accelerate the erosion process. They do this by focusing wave energy on a smaller, more concentrated area. Many coastal communities have lost their beaches because of the numerous jetties built there. An extensive cobble beach exists in Sandwich next to the jetty at the entrance to the Cape Cod Canal.
The animals that are successful inhabitants of the intertidal zone have certain characteristics in common. One is that they are able to burrow deep into the substrate so that they arenít carried away with the next wave. They can also move quite quickly if necessary. One of my favorites is the mole crab (Emerita talpoida), a small arthropod with an egg-shaped body, and conspicuous antennae that are used to capture small bits of food. There are also several species of worms that have adapted to life in the intertidal zone. The lugworm (Arenicola marina) forms a two-part opening in the substrate, and anchors itself by expanding at its base.
Most of the plants and animals seen in this part of the beach washed up from deeper water. A rocky shoreline provides a more solid substrate for critters to cling or attach to. Such places of attachment and concealment are lacking on a sandy beach.
Copyright Gil Newton 2009 Thanks to Chris Dumas for logo image.