Defining a Rocky Shore

What Is the Rocky Shore?

Low TideThe rocky seashore is an ideal place to investigate the mysteries of the sea. The regular rise and fall of sea level we call the tides has created one of the richest, most variable environments in the ocean. The narrow fringe of land and sea between the lowest and highest tidemarks is called the intertidal. During low tide, you can explore the seafloor; no other marine habitat is as accessible. Because of this, the rocky intertidal is the most thoroughly studied, best-known ocean area.

Wave force
Waves batter the rocky intertidal. During storms, a wave can hit the shore with the force of a car going 90 miles per hour. To protect themselves from being smashed by waves or torn from rocks, plants and animals here hold on, lie flat, bend with the waves or hide.

High TideMany intertidal animals hold on tight to avoid being swept away. Snails and chitons have a strong, muscular foot. Sea stars have thousands of tiny tube feet with suction-cup ends. Mussels anchor themselves by gluing threads to the rocks; seaweeds have strong, root like holdfasts that cling to the rocks.

Body shape and structure help plants and animals survive crashing waves. The Chinese-hat shape of limpets and barnacles and the flat shape of chitons and abalone offer little resistance to the water rushing past. Snails, crabs, barnacles and mussels have strong shells to protect them. Flexible anemones bend rather than break; seaweeds, too, are smooth, strong and flexible.

Many animals escape the waves by hiding under plants, among other animals or between and under rocks. Crabs crawl into rock crevices and small, delicate animals like brittle stars hide under rocks or in mussel beds and kelp holdfasts.

Air exposure
Air exposure also creates problems for intertidal creatures. Falling tides expose them to highly variable air temperatures: sometimes hot, sometimes bitter cold.

Plants and animals left out of water must find ways to keep from drying out. To cope, some snails draw into their shells and seal them with door like operculums; some also secrete a mucous seal. Mussels close their shells tightly to retain water, and anemones gather in masses so that less body surface is exposed to the air. Many animals hide under rocks or seaweeds to avoid drying out.

Seaweed are layered on rocks with upper layers shielding the lower layers so only a few plants are exposed. Some seaweeds can dry out completely, rehydrating when the tide returns.

At low tide, creatures submerged in tide pools may face low oxygen levels and widely fluctuating salinity. On warm days, evaporation raises salt concentrations, on rainy days, salt concentrations are lowered.

Sea Star

Competition and defense
To survive in the crowded intertidal, plants and animals must compete for s pace. Animals also need strategies to avid being eaten. While the armor like shells of crabs, barnacles and snails help protect them from predators, sea urchins and some intertidal fishes have spines. Other animals here are camouflaged like rocks and seaweeds; they’re practically invisible. The tidepool sculpin and octopus can change color and pattern to match their surroundings. And the decorator crab plants a garden of seaweeds, sponges and other sessile (attached) creatures on its back to escape detection. The same refuges that help protect an animal from the waves also protect it from predators.Gooseneck Barnacles

People and the rocky shore
Intertidal creatures can survive harsh conditions, but not human carelessness. In the past, people collected animals by the bucketful. Now, strict laws govern the collecting of plants and animals in the intertidal. If you visit, do your part to preserve the community: turn each rock back, and leave everything as you find it.

Text from the Seasearcher’s Handbook Monterey Bay Aquarium

Last revised 17-Aug-13