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Intertidal Zone Information Sheet

Last modified August 07, 2008

Starfishes (Echinoderm family)

Starfishes have a 'radial' structure with 5 arms reaching out from a central disk. The body shape is robust with stout arms, the mouth lying underneath, facing downwards. Turn the starfish over and have a look at the tube feet sticking out from the grooves in each arm. Starfish have the amazing ability to grow new arms if one or more get chopped off! They have no eyes, but can detect gradients of light. They feed by inserting their stomachs through the shells of bivalves (often the gap is tiny) and releasing enzymes that turn the tissues of the bivalve into a pulp which can then be digested.

Brittle stars (Echinoderm family)

Brittle stars look similar to starfish, although the arms are thinner and longer. The arms wave about in the water, while the body is often hidden under rocks or in crevices. They also have no eyes, but can detect light and dark.

Sea urchins (Echinoderm family)

The 5-star structure of an Echinoderm has been rolled up into a ball. The spines may be short or long, but all are poisonous and can be very painful if you stand on them. Sea urchins are important in coral reef ecology because they maintain the balance of the reef by preventing excessive growth of corals. However, if fish that feed on sea urchins (pufferfish and triggerfish) are removed by overfishing, the urchin numbers increase rapidly and they cause a great deal of damage by preventing young coral larvae from settling. Sea urchins move very little during the day, but at night they move together like an army in a group of bristling, waving spines, to feed on algae and seagrasses. They walk on tube feet between the spines.

Nudibranchs (Gastropods from the Phyllum Mollusca)

Nudibranchs are snails without shells. Some species hide from sunlight and feed at night on sponges, corals and anemones. ('Gastro' means stomach and 'pod' means foot.)

Snails (Gastropods)

There are many different species you might see on the tidal flats. Snails move on a single foot and have a domed shell on their backs. They feed on algae and have a tongue-like organ called a 'radula' for scraping the algae from surfaces. Sometimes the empty shells are taken over by Hermit crabs.

Hermit crabs

These are very common on tidal flats. They have soft vulnerable bodies and so live inside snail shells. If you pick up a shell, some legs and claws start to emerge then you've found a hermit crab! As they grow, they have to move into new shells to fit their new size.


They consist of a mass of tentacles surrounding a mouth on top of an oral disc. They eat many types of food from microscopic plankton to small fishes! Some species have symbolic algae (zooxanthelle) living inside them as corals do.


These can be seen covering many surfaces, forming mats over rocks or corals in a variety of different colors. Algae photosynthesizes, capturing energy from the sun and converting it into chemical energy and producing oxygen as a by-product.

Source: Nature Camp for High School Students - Facilitators' Handouts Book Two