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8th July 2020 - The Hidden World of the Intertidal Zone

The Solent coastline is made up of an incredible range of habitats from sandy beaches, saltmarsh, mudflats and estuaries to rocky ledges and even seagrass meadows. These form part of the intertidal zone and provide a home for all kinds of marine life. Thousands of types of seaweed and creatures such as crustaceans, molluscs, anemones and worms live under the rocks and in the surrounding pools, sand and mud. Young fish can also be seen using these areas as a nursery thanks to the shelter they provide.

The intertidal zone is the area of the shore that is exposed at low tide (and subsequently covered back up again at high tide). It can be a harsh environment because it is constantly changing, so anything that lives there needs to be tough enough to survive in the extreme conditions. Not only does the water keep rising and falling with the incoming and outgoing tides, but temperatures and salinity levels can also fluctuate significantly. Then you have to deal with the powerful waves crashing down when the tide comes in or during stormy weather!

Each species found on the intertidal zone has a tolerance level of how long they can survive out of the water, and this will determine where on the shore you might find them. This is called zonation and affects seaweeds as well as all the different creatures. Other factors such as the presence of predators, and also how many species are competing for food and space in the same area can impact where things might be found.

Some species are well adapted to living higher up on the shore (where they are exposed for the longest time) such as beadlet anemones, who can retract their tentacles to store water which prevents them from drying out. The base of their body also acts as a strong sucker, helping to secure them in place on the rocks.

Crabs who are more mobile than the anemones can actually hold a small amount of water in their gills, which enables them to keep breathing when they are out of the water, meaning they can travel up and down the shore for longer.

The lowest part of the intertidal zone (exposed for the shortest amount of time) usually will have the greatest diversity of species, due to the conditions being more favourable.

So why are we talking about the intertidal zone? Well this area on the shore is key to helping our overwintering birds survive, by providing a valuable feeding resource. This becomes a lifeline for them over the cold winter months, as they need to feed as much as possible and build up their energy reserves.

Dark-bellied brent geese use their serrated beaks to feast on seaweeds such as sea lettuce and the only flowering plant in the ocean - seagrass. They wait until the tide starts to go down and then will begin grazing, following the tide out across the shore. As their diet contains high levels of salt, they have a large and well developed salt gland, which helps to remove the excess salt from their bodies and even allows them to drink some sea water!

Curlews on the other hand, have a completely different diet to the geese and they use their long beaks to dig down into the mud and sand during low tides to find juicy items of prey such as lugworms.

Then we have some of the smaller birds such as dunlin and sanderling. They only have short beaks, so they use them to pick off small creatures on the surface such as shrimps, crabs and insects.

As you can see these birds are all adapted in different ways and have various feeding techniques. This means that one small area of the intertidal zone can support a huge number and diversity of birds as they are not all competing for the same food source.

The intertidal zone is a hugely important part of our shores providing homes and food for species both above and below the water. Next time you are at the coast, take a moment to appreciate these habitats and see just how many different species you can spot in one place!

Ranger Charlotte