Rock pools



A rock pool is a habitat for a variety of marine plants and animals. The plants belong to a group that includes the seaweeds. The animals belong to many different groups including worms, mussel-like animals, crab-like animals, jellyfish and starfish, and swimming fish.

How do seaweeds hold onto rocks?
They grow a structure which looks like a root, but it is not a real root. It is called a holdfast. A holdfast grows into the surface of the rock and fills in the spaces between the rock grains to make a tight grip. The purpose of the holdfast is to simply hold the seaweed to the rock. It does not take in water or minerals like the roots of flowering plants. The holdfast forms a microhabitat for many small crustaceans.

Is there just one kind of seaweed on the rocky shore?
No. There are many kinds of seaweeds and they are divided into three groups - brown, green and red. The brown seaweeds are large and are the first ones you notice when you visit the shore. The green seaweeds are smaller than the brown seaweeds and may be seen growing among them. The red seaweeds are usually the smallest on the shore. They may be found growing on other seaweeds or in rockpools.

How are the seaweeds arranged on the shore?
They are arranged in bands. In the lowest band of seaweeds are the oarweeds or kelp. They may only be seen at low tide because they are not adapted to being exposed to the air for a long time.
Knotted wrack forms the next band of seaweed. It has swellings or bladders full of air arranged in a line along each frond. They help the frond to float in the sunlit water when the tide comes in. The floating fronds can then use the light and the water around them to make food. Saw wrack forms the next band of seaweed. It has saw-like teeth along the edges of its fronds. Above the saw wrack is a band of bladder wrack. These sea weeds have fronds with air bladders arranged in pairs. They have the same purpose as the bladders in knotted wrack.

Are animals arranged in bands on a rocky shore?
Yes. On the lowest part of the shore you would find sea anemones and sea urchins. Above them you would find mussels. Further up the shore are barnacles and at the top of the shore are winkles.

Why are living things found in bands on a rocky shore?
The conditions on the shore vary. The lowest part of the shore is only uncovered by the sea for a short time so it remains wet there and the temperature does not change very much. Further up the shore the rocks and pools are exposed for a longer time. The temperature may change greatly and conditions are drier. At the very top of the shore the rocks and pools may only receive water from the splashes of the waves.
Most living things are adapted to a particular set of environmental conditions and will only be found where these conditions are found on the shore. For example, sea anemones cannot survive exposure to the air and high temperatures, so are found on the lower shore or in deep rock pools. Barnacles can stand higher temperatures and long periods out of water so are found higher up the shore.


What kinds of crabs may be found on the shore?
The most common is the shore crab. It is dark green. The velvet crab is a similar looking crab but browner and with very flat back legs. When this crab is discovered it will rear up and strike the air with its claws. The edible crab is pink and has a shell which looks like a pie crust. The hermit crab lives in the shells of winkles and whelks.

How is the shore crab adapted to its way of life?
The shore crab has a flattened body to help it move through gaps in the rocks and below seaweed to search for food. It is a predator and a scavenger. The crab's large claws are used for attacking any animal that it finds or for breaking up the bodies of dead fish. The crab's shell is covered with blotches which give it camouflage because it can also fall prey to other animals such as gulls.

What is the food of the starfish?
Starfish feed on mussels. When a starfish chooses a mussel to eat it puts its arms over the mussel's body. The mussel has two shells which it clamps together for protection. The starfish has hundreds of feet on its arms. Each foot has a sucker on its end. The starfish grips the shells with its feet and steadily pulls the shells apart so it can get inside and feed on the mussel.

What fish may be found in rock pools?
The most common fish to be found in rock pools are the blenny, rock goby and butterfish. The blenny may be 3-13cm long with a large head and a blotchy, dark green body. The rock goby is about 10cm long with a blotchy, brown body and a pale line on the fin behind its head. The butterfish is between 7cm and 28cm long. It has an eel-shaped body which is brown with pairs of black spots along its back.

How are fish adapted to life on the shore?
The blenny digs a number of shelters under rocks and moves between them as it looks for food. This helps to protect it from predators. The blotches on its body give it camouflage. The rock goby has fins under its body which act like a sucker. The fish uses the sucker to stick to rocks when strong currents in the tide rush up the shore. The butterfish got its name because its skin is very slippery, like butter. This makes it difficult for predators to hold on to so when a butterfish is attacked it may be able to simply slip away.