In brief
The rocky intertidal zone is the strip of rock between the highest and lowest tides. Its animals and seaweeds live in horizontal bands. Heat and drying set the upper limit of each band; competition and predators often set the lower limit.
What it is
The intertidal zone is the part of the shore that is underwater at high tide and in the air at low tide. On rock, water collects in dips and cracks as the tide falls. These are tide pools. A tide pool is a small, separate world that is cut off from the sea for hours at a time.
| Zone | Underwater | Typical residents |
|---|---|---|
| Splash zone | Almost never; wetted by spray | Small periwinkle snails, lichens |
| High shore | Only at high tides | Barnacles, limpets |
| Mid shore | Roughly half the day | Mussels, dog whelks, rockweed |
| Low shore | Most of the time | Sea anemones, sea stars, kelp |
Exact species depend on the ocean and the exposure to waves, but the pattern of bands is found on rocky shores around the world.
Why the bands form
High up the shore an animal is out of the water for hours. It can dry out, overheat in sun or freeze in winter. Further down it is wet for longer, but then other animals can eat it or crowd it out. The famous experiments of Joseph Connell (1961) on two barnacle species showed this clearly: one species could live higher up because it tolerates drying better, but where both could survive the other species crowded it out.
| End of the band | Main limit | Example |
|---|---|---|
| Upper edge | Physical stress: drying, heat, cold, fresh rain | A high-shore barnacle that survives drying better than its rivals do |
| Lower edge | Biological pressure: predators and competition | Mussels are eaten by sea stars if they live too low on the shore |
In 1966, Robert Paine removed sea stars from a stretch of rocky shore. The mussels they had been eating spread, crowded out other species and the number of species dropped. The sea star was called a keystone species, one with a much larger effect on its community than its numbers suggest.
How animals cope
- Barnacles close their plates and keep a drop of water inside.
- Limpets clamp to one spot on the rock. Some species return to the same “home scar” after feeding, so that the shell edge seals tightly against the rock and keeps moisture in.
- Mussels shut their shells and hold on with strong threads. Dense groups hold moisture and break up the force of waves.
- Sea anemones pull in their tentacles and look like small blobs of jelly until the tide returns.
- Snails withdraw into the shell and close the opening with a hard lid.
- Crabs and fish hide under wet seaweed and stones, or retreat with the water.
A pool left behind by the tide changes quickly. In sunshine, evaporation makes the water saltier and warmer; in rain it becomes fresher. During the day, plants and algae in the pool make oxygen by photosynthesis. At night every living thing in the pool uses oxygen, so oxygen can fall, and the water becomes more acidic as carbon dioxide builds up. The animals in the highest pools have to tolerate the biggest swings.
Pressures and effects
- Trampling and collecting crush barnacles and remove animals that can take years to regrow.
- Heat waves at low tide can kill many animals on a shore at once.
- Oil and other pollution coats rock and animals when it reaches the shore.
- Ocean acidification makes it harder for shell-builders such as mussels and snails to build shells.
- Loss of a keystone predator can change an entire community, as Paine showed.
Because a rocky shore is easy to reach and easy to observe, it has become one of the best-studied habitats in the world.
Visiting kindly
- Check a tide table first and go on a falling tide. Leave well before the tide turns.
- Walk on bare rock rather than on seaweed or animals; wet seaweed is slippery.
- If you turn a stone over, put it back the way it was.
- Look, sketch and photograph. Leave every animal where you found it.
Field exercise
Choose a pool and sit beside it for ten minutes. Write down every animal that moves, and note how its behaviour changes when your shadow falls across the water. Then draw a strip map from the top of the shore to the bottom and label each band.
Check yourself
Read the question, think of your answer, then open it with a tap or the Enter key.
What is the intertidal zone?
The part of the shore that is underwater at high tide and exposed to air at low tide.
Which animals typically live highest on a rocky shore?
Periwinkle snails and lichens in the splash zone, then barnacles and limpets on the high shore.
What usually limits an animal at the top of its band?
Physical stress such as drying out, heat or cold.
What did Connell’s barnacle experiments show?
That two species can divide the shore: one tolerates drying and lives higher; the other is a better competitor lower down.
What is a keystone species?
A species whose effect on its community is much larger than its numbers would suggest, such as the sea star in Paine’s experiment.
Why can oxygen in a tide pool fall at night?
Photosynthesis stops without light, but every animal and plant in the pool keeps using oxygen.
Basis of the facts
Where these facts come from. The pattern of zones is standard in marine ecology textbooks (for example Raffaelli and Hawkins, 1996; Nybakken and Bertness). The barnacle experiments are Connell (1961) in the journal Ecology; the sea star removal experiment is in Paine (1966) in The American Naturalist, and Paine introduced the term keystone species a few years later, in 1969. Diurnal swings in oxygen and acidity in pools are a standard topic in the same textbooks. Species named are examples; ranges differ by coast. Full details are on the sources list.