Different organisms live on open rock and boulders, and hidden away in cracks, crevices and rock pools.
Many of the factors important to rocky shore organisms are related to being out of the water.
One of the greatest problems for marine organisms that live in an intertidal area is desiccation – drying up. Having evolved in an environment permanently surrounded by seawater, there was not a need for marine organisms to develop packaging as water-tight as that of human skin!
The longer an organism is out of the water, the more at risk it is of drying out. To survive on a shore, living things therefore need to either avoid desiccation altogether or find a way to cope with it happening.
Animals living on rocky shores have two main ways of dealing with desiccation. The first is to effectively seal their soft body parts up, excluding any exchange with the drying atmosphere, and ‘sit tight’ until the return of the tide. By doing this, organisms hold in moisture throughout emersion. Organisms such as barnacles and mussels completely shut their shells to seal themselves in, and are protected from the outside world.
Limpets have shells with a large opening that is incapable of closing.
Instead, they clamp down tightly to the surface of the rock using a specialised mucus.
The second major way of avoiding desiccation that animals use is a behavioural adaptation, to move into an area that will remain sufficiently damp throughout the low tide period. This is called a micro-habitat selection, as organisms are finding small, specialised habitats within the larger shore environment. Mobile animals such as the common shore crab, sense pressure changes as the water level above them alters with the receding tide, and locate refuges such as rock pools, under seaweed, in crevices, and under overhangs in the rocks.
Macroalgae (or seaweeds) on the shore do not have the option to hide in a shell or move away from areas where desiccation will be a problem at low tide. Many species are found only in permanently wet or moist areas, such as crevices or rock pools.
This can be because their spores only settle, or only survive, in these areas. Certain species of seaweed can survive desiccation on open rocks to varying degrees. A number of the larger brown species can lose up to 90% of their water during emersion, becoming incredibly dry. When the tide returns, they quickly take up water again.
Salinity
One of the defining characteristics of seawater is its saltiness, or salinity.
The salinity of seawater is generally very constant across the oceans of the world, at around 34 -36 S. It only changes substantially in certain enclosed seas, and in areas surrounding estuaries, where there is a high input of freshwater. This means that the majority of marine species are exposed to a very narrow range of salinities. Intertidal species, however, are not that fortunate. The animals and plants that inhabit rocky shores are exposed to dramatically fluctuating salinity conditions. Rainfall can cause low salinity, or
hyposaline conditions. The opposite condition can arise in rock pools on warm summer days: isolated from the tides, water evaporates, leaving the salt behind. Over the course of a low tide period, therefore, rock pools can become 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.
