Rocky Seashores

On your next visit to the seaside try to spend some time on a rocky seashore where you will find lots of interesting animals and plants. When you have had a good rummage around in the seaweed and found lots of different snails, crabs and other invertebrates you should sit down on a nice smooth rock and have a think about it all. Rocky seashores have some interesting features.

These rocky shore photos were all taken in Broadhaven and Littlehaven in Pembrokeshire.

Here is a bit of rocky seashore, can you see any zones ?

Do NOT expect to find all the answers on this page. The answers are on the seashore and in your brain: you just have to go to the shore, look carefully and use your brain. Lower on the rocks you can see brown and red algae (seaweeds); higher up there are barnacles.

Zonation: How high up or down the shore do you live?

Exposure: Does your seashore get exposed to heavy waves?

The seashore is a very interesting ecosystem because the kinds of animals and plants that live there are able to tolerate some very extreme conditions. It is lovely when the tide comes in, but when it goes out it could get very hot OR very cold and it could get very wet (no salt if it rains) OR very dry. Things living on the seashore must be able to cope with the nasty conditions which occur when the tide goes out. It might not be so lovely when the tide comes in if there is a storm. Just imagine what it is like for a tiny animal or a small seaweed when the waves come crashing down on top of you.

This is the splash zone, it is not covered by seawater when the tide comes up.

Take a closer look; here are some lichens, they get splashed by waves.

I find the most exposed seashore the most interesting ones to look at. Imagine standing on the rocks when the waves are big enough to wash a human being away. If you were a little snail you would creep into the cracks in the rocks to get shelter from the waves. If you were a seaweed you would be thrashed against the rocks and the tips of your fronds would be worn off you as fast as you could grow.

This is the top of the barnacle zone.

Barnacles cannot live any higher up the shore because they cannot tolerate the long “emersion” between spring tides.

Zonation

Have a look at the rocks in the distance: you will probably see a line between the rocks covered in seaweed and the bare rocks above that. Or are the rocks really bare? Above the seaweed there is a zone of rock which is completely covered in barnacles. This zone is a different colour from the rocks higher up the shore where there are no barnacles. It is easier to see these zones on the seashore from a distance that it is from close up. Now you can have a good think about what it is like for the animals and plants on the seashore as the tide comes in and goes out.


Lower down this shore barnacles and mussels compete for space.

We should say that the tide “goes up and down” rather than saying that it “comes in and goes out”.

When the tide goes up, it covers all the animals and plants in seawater. That is very obvious, but it is only when they are covered in seawater that they start to do things. The seaweeds carry out photosynthesis, and the animals start eating. When the tide goes down, they all stop. You could sit there all day with a stopwatch timing how long they are all covered in seawater. The higher up the seashore an animal lives, the shorter its feeding time becomes. So why live at the top of the seashore where you will be exposed to the air for long periods?

What is much worse than not being able to feed is getting hot and dried up when the tide has gone down. At the very top of the seashore the animals and plants are only covered by seawater when there is a spring tide (a very big tide). Spring tides happen every two weeks. The spring tides in the spring and autumn are very big indeed. In between the spring tides there are neap tides: living at the top of the seashore probably means being dried out for between one and two weeks depending upon how high up you are. Perhaps the most difficult place to live is up in the splash zone. This is above the high tide mark; animals and plants living here only get wet if it rains, or if the waves are big enough to splash up and over the high tide mark.

Here is a zone of Pelvetia canaliculata, it is very near the top of the shore.
So why don’t all the animals and plants live at the bottom of the shore? Perhaps there would be too much competition for light, food or space. Some organisms just cannot cope with all that competition. Living higher up the shore there might be less competition because many organisms cannot cope with the extreme conditions when the tides goes down.

Exposure

I don’t know what kind of rocky shore you are sitting on with your laptop plugged into the internet on your mobile phone. It might be a very sheltered shore. You would know that it is very sheltered if you saw the rocks and seaweed are covered in a fine film of mud. There are not likely to be any rock pools because they would get filled up with this mud. However you might be standing on a very exposed shore watching the waves and hoping that you are not washed out to sea. These shores can be very dangerous.

Different kinds of animals and plants live on shore with different exposures. If you are doing “A” Level biology you might like to do a project on “exposure”. You will find ideas for projects in the library at any of the Field Studies Centres. Their teaching staff, or your own teachers might have some ideas; but it is much the best if you have a good look at a rocky seashore first and try to come up with your own ideas. Keep it simple: choose an animal or plant and try to work out where it lives on the shore (higher or lower) and which kind of shore (sheltered or exposed) it lives on. Alternatively you could choose a group of animals or plants. and compare their zones on the shore.

If you have a look at the seaweeds on the rocky shore, you will probably find a seaweed called Fucus vesiculosus: its common name is bladder wrack. You might also find Fucus serratus, or Fucus spiralis. Bladder wrack has air bladders on the fronds: these help the wrack to float up in the water to get as much light as possible. The serrated wrack has fronds with teeth like a saw on its edges. The spiral wrack has its fronds slightly twisted into a spiral. You could get a book from the library on the fauna and flora of the seashore to help you identify the animals and plants that you find.

Here are brown and red algae competing for space at the bottom of the shore.

If you are going to the seaside with “mummy & daddy” instead of your biology teacher you might need to get yourself a book on the seashore. Try “Collins Guide to the Seashore” It has some good pictures to help you identify the organisms that you will find.

One Response to Rocky Seashores

  1. Chico Sanguinario says:

    nice article.

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