PADI Aware Community Day 12 September 2026
We are excited to announce that Life Below Water will be attending the PADI AWARE Community Day in Zeeland on 12 September!
On rocky shores, different organisms live on open rock and boulders, and hidden away in cracks, crevices and rock pools.
Many of the factors that matter most to rocky shore organisms come down to one thing: being out of the water.
For marine organisms in the intertidal zone, desiccation is one of the greatest problems. Drying out, in other words. These animals evolved permanently surrounded by seawater, so they never needed packaging as water-tight as human skin. The longer an organism sits out of the water, the greater the risk. To survive on a shore, living things therefore have to avoid desiccation, or find a way to cope with it.
Animals on rocky shores deal with desiccation in two main ways. The first is to seal their soft body parts up. They shut out any exchange with the drying air and sit tight until the tide returns. That way they hold in moisture throughout emersion. Barnacles and mussels, for example, close their shells completely and seal themselves off from the outside world.
Limpets cannot do that. Their shells have a large opening that will not close. Instead, they clamp down tightly onto the rock with a specialised mucus.
The second way of avoiding desiccation is behavioural. Animals simply move to a spot that stays damp enough through low tide. Biologists call this micro-habitat selection, because organisms seek out small, specialised habitats within the larger shore. Mobile animals such as the common shore crab sense the pressure change as the water level drops. They then head for a refuge: a rock pool, the shade under seaweed, a crevice or an overhang.
Seaweeds have neither option. They cannot hide in a shell, and they cannot move away. Many species therefore grow only in permanently wet or moist places, such as crevices and rock pools.
Sometimes that is because their spores settle or survive only there. Other seaweeds tolerate drying on open rock to varying degrees. Several of the larger brown species can lose up to 90 per cent of their water during emersion and become extremely dry. When the tide returns, they take the water straight back up.
Salinity is one of the defining characteristics of seawater.
Across the oceans, salinity stays remarkably constant at around 34 to 36. It shifts substantially only in some enclosed seas and around estuaries, where a lot of freshwater comes in. Most marine species therefore meet a very narrow range of salinities. Intertidal species are not that fortunate. Animals and plants on rocky shores face dramatic swings. Rainfall, for instance, can push salinity down.
That is called hyposaline conditions. The opposite happens in rock pools on warm summer days. Cut off from the tide, the water evaporates and leaves the salt behind. Over a low tide period, a pool can therefore turn increasingly salty, or hypersaline.
Marine vertebrate predators of rocky shore invertebrates include the variety of intertidal fish species.
Next to intertidal species, fish from the shallow subtidal can access the abundant intertidal food supply when the tide is in. Some fish are consuming barnacles, crabs, shrimp, urchins, worms, snails and large clumps of mussels.
When you are walking on rocky shores, take the time to look closely and you’ll discover all sorts of fascinating marine life.
Life Below Water organises beach and river clean-ups, guest lectures and ocean events across the Netherlands. See what we do, find out how to get involved, or support the work through our eco shop.
Want the bigger picture? The United Nations explains what is at stake in Sustainable Development Goal 14: Life Below Water.
We are excited to announce that Life Below Water will be attending the PADI AWARE Community Day in Zeeland on 12 September!
Despite the rain, our Walk & SUP Clean-Up in Huizen was a great success!
