The Intertidal Zone: a reef platform

The intertidal zone, also known as the littoral zone, is that area between high tide and low tide. It can be divided the following subzones – spray zone, upper intertidal zone, mid intertidal zone and the lower intertidal zone. It is an area that is constantly changing as the water moves in and out with the tides. Thus organisms inhabiting this area have a variety of adaptations that allow them to survive.

The spray zone, or supra-littoral zone is the highest zone of true marine life. It is usually only kept damp through wave splash. Organisms surviving this environment include barnacles, limpets and periwinkles (pictured).

The upper intertidal zone is only covered by water at high tide. Any algal growth is green, and the zone is characterised by barnacles, limpets, chitons, crabs, mussels, sea stars and periwinkles.

The middle intertidal zone is regularly covered by water. Seaweed is more prominent. Organisms in this zone include anemones (pictured), barnacles, crabs, mussels, sea stars, gastropods and sponges.

The lower intertidal zone is usually submerged, only being exposed at very low tides. The zone is characterised by brown algae (funnel weed pictured – top), encrusting sponges, abalone, sea stars, crabs, sea cucumbers, gastropods and sea urchins (pictured). Small fish may also inhabit the area.

Organisms exposed to air must be able to prevent (or tolerate) desiccation (drying out). Many organisms have a protective covering such as a shell.

© Department of Fisheries, Michael Burgess

Intertidal zones can be high energy environments, as waves pound the area with force. Algae (seaweed) possess a root-like structure called a holdfast that they use to anchor themselves to the substrate.

Barnacles (pictured) attach themselves to the substrate with a strong ‘glue’ and mussels with their byssal threads. Mobile animals such as limpets and chitons use their muscular foot to cling on to the reef during heavy wave action.

Animals in more exposed locations tend to have thicker shells (e.g. turban snails – pictured) than those in sheltered locations (e.g. pipis). Likewise, many intertidal organisms, such as barnacles, limpets and chitons have low profiles, close to the rocks.

Salinity in the intertidal zone can be quite variable depending on the amount of rainfall, and the rate of evaporation of the water. Organisms inhabiting this area, particularly rock pools, can usually withstand changes in salinity, and also temperature. Some animals in rock pools will take shelter from direct sunlight under algae.

Animals inhabiting the intertidal zone may be restricted as to when they can feed. Many sessile animals are filter feeders (feed on planktonic material in the water) and thus are unable to feed when the tide is out. Those animals that are not filter feeders may also be restricted as they seek shelter from the elements and predators at low tide.

In addition to the environmental challenges that organisms inhabiting the intertidal zone face, one of their greatest threats is humans. We cause damage to this environment as we step on organisms and their habitats, and remove organisms altogether.

© Department of Fisheries, Alicia Edwards