Rocky Shore

Rocky shore ecosystems are coastal shores made from solid rock. They are a tough habitat to live on yet they are home for a number of different animals and algae.

Rocky shore ecosystems are governed by the tidal movement of water. The tides create a gradient of environmental conditions moving from a terrestrial (land) to a marine ecosystem.

Ecosystems on rocky shores have bands of different species across the intertidal zone. The distribution of different species across the rocky shore is influenced by biotic and abiotic factors from above high tide to the sub-tidal zone.

Different species are adapted to different environmental conditions. Some organisms can withstand being exposed to the sun for most of the day and live in the upper parts of the rocky shore. Other organisms need to be covered by the tide for most of the day and are only found lower on the rocky shore.

 The different sections of the rocky shore are exposed to varying amounts of stress. Stress may be caused by an increased time out of water and in the Sun, more pressure from predation or grazing, or other factors that change across the intertidal zone. As the conditions are changing over the rocky shore ecosystem, various bands of different organisms are found along the intertidal zone.

Rocky shore at Hahei, New ZealandAbove the intertidal zone

The littoral/intertidal zone can be divided into different zones across the rocky shore that each have similar environments. Above the intertidal zone sits the supralittoral zone which is virtually a terrestrial environment. Species found within the supralittoral zone must be tolerant to high levels of salt caused from wave spray. Here it is common to find periwinkles (little snails with elongated shells) and lichens on the rocks but the diversity of species in the supralittoral zone is fairly limited.

Intertidal fringe

Between both the supralittoral and intertidal zones is the littoral fringe or the intertidal fringe. This is the area where the high tide may or may not be cover the rocks depending on the height of the tide. Tide heights vary depending on the placement of the Moon, the Sun and atmospheric pressure. The diversity and abundance of animals and plants is still low in the littoral fringe and periwinkles are still the animals you are most likely to find.

Intertidal zone

Below the littoral fringe is the most diverse and interesting area of the rocky shore, the intertidal zone. The regular covering and uncovering of the shore by the waves provides a regular income of food and nutrients for plants and animals.

The density of plant and animal communities in the intertidal zone are often very high. Due to the high levels of nutrients, the distribution of many species is limited by competition with other species for space.

Rocky shore invertebrates

Snakeskin chiton

The animal populations on the rocky shore are dominated by invertebrates such as barnacles, mussels, oysters, tubeworms, limpets, chitons, snails, crabs and starfish.

Barnacles, mussels and oysters are all stationary filter-feeders. They obtain their food by filtering for phytoplankton and edible particles straight from the water above them while they are submerged beneath the tide. They are often the dominant animals species on the rocky shore but their abundance is influenced by competition with each other, the environment of the shore and pressure predation from predators like starfish and oyster borers.

Limpets, chitons and various gastropods (snails) belong to another very important group of animals that live on the rocky shore – the mobile grazers. These grazers move around the intertidal zone and scrape the rocky shore free of any algae and settled juvenile shellfish. By scraping off algae they maintain the structure and diversity of rocky shore ecosystems by preventing algae from dominating the intertidal zone.

The large abundance of food on the rocky shore attracts a number of predators species that prey on the slow-moving grazers and stationary filter-feeders. Starfish and gastropods are the most effective predators on the rocky shore and have significant abilities to control grazer and filter-feeder abundance.

Red rock crabTheir control on these populations makes these predators important regulators of the structure of the rocky shore ecosystem. Feeding when the tide is in, these rocky shore predators are able to break through the hard exterior shell of many slow many species that are common on rocky shores.

Other predators include crabs which are the fastest moving animals on the rocky shore. Their superior mobility comes at a sacrifice. Crabs have a reduced ability to stick to the rocky substrate and subsequently must behave in a way to reduce the risk of being knocked around by incoming waves.

Algae

Seaweed

It is also common to find a number of algae on rocky shores and your chances increase as you move lower down the rocky shore. Seaweeds are most abundant in the sub-tidal zone where they often form dense kelp forests and provide habitats for many fish, worms, crustaceans, gastropods and many more marine animals. Certain species can be found in the intertidal zone and more commonly these are green species of algae such as sea lettuce.

NZ fur seal on the rocky shoreBirds and Mammals

The rocky shore is also visited by many birds and marine mammals as a place to rest, warm up and to breed. Many seabird species, such as penguins, shags, gannets and albatross, use the rocky shore as a nesting area and to dry off and warm up after a cold swim. If you are lucky enough to live or visit an area with seals or sea lions, chances are you will find them basking in the sun on the rocky shore, warming up after a cold swim.

Last edited: 5 April 2016