OUR COASTAL TREASURE
A territory between land and sea
The rocky sea shore lies between low-tide and high-tide levels, and is alternately covered by sea water and then exposed to air. This means that animals living there have to be able to absorb oxygen both when submerged under the water and when sitting in the open air, sometimes on a hot and windy day.
On top of this, the waves crash over the area with great force and living creatures have to cling onto the rocks tightly or they will be washed away. Despite these severe physical stresses, the rocky shores around St Francis are inhabited by an astonishing variety of animal and seaweed life.
Life arranged in zones
On rocky shores, animals and seaweeds are arranged into horizontal bands or zones, depending on the ability of the organisms to withstand exposure to air and wave action, as the tides rise and fall. Around St Francis, the zonation is typical of what is found on rocky shores all along the south coast of southern Africa.
Littorina Zone: At the top of the shore, the time of exposure to the air and heat is greatest. Because it is so inhospitable, very few species are found in this zone. Tiny periwinkle snails occur in their hundreds in this zone; they are Nodilittorina and the zone is named after them. They are herbivores that graze minute algae and only need to be covered by seawater occasionally, during spring high tides when they release their eggs. They can close off their shells with an operculum (cover) to reduce evaporation from their bodies.
Upper Barnacle Zone: Further down the shore and closer to the sea lies the upper barnacle zone. As the name suggests, barnacles are very common in this zone. These animals are actually crustaceans and so are related to crabs and crayfish. They are filter feeders and need to be immersed in seawater regularly in order to feed.
When a barnacle settles on a rock, it cements its head to the surface using its cement glands – it essentially fixes itself upside down by its forehead for the rest of its life. This leaves its modified legs free to waft in the water and catch microscopic food particles. Barnacles are enclosed by sturdy shells that can be completely shut during times of exposure to air.
Being permanently stuck to the rock (sessile) is a good idea because it prevents barnacles from being washed off. However, it is difficult for male and female barnacles to get together during reproduction. To solve this problem, barnacles have a very long penis, the longest relative to body size of any animal in the world. This is how they reach the barnacle next door.
Limpets are also found in this zone. They are grazing herbivores that have a muscular foot that they smear with a mucus secretion to help them adhere to the rock. Their dorsal surfaces are covered by a sturdy shell with a streamlined shape that enables them to withstand significant wave action without being dislodged.
During times of exposure to air, limpets can absorb oxygen from water that is trapped in a ring just inside the shell. If you stand on a limpet’s shell, or tap it when it’s exposed to the air, it will quickly stick itself down more firmly to the rock and, in so doing, squeeze out and lose the vital water stored under its shell. This could stress the limpet, or even lead to its death.
The granular limpet, Patella granularis
Lower Barnacle Zone: This zone is even closer to the sea and is covered in water for most of the time. Because conditions here are more favourable, many different species make this zone their home. Apart from the barnacles, there are also different species of limpets along with mussels, worms, whelks, chitons and many types of seaweeds.
Mussels are bivalve molluscs that attach themselves to the rocks with strong threads and a powerful underwater adhesive. They thrive where there is constant wave action because they are filter feeders; the crashing waves bring oxygen and limitless microscopic plankton that nourishes them. In turn, mussels form food for other invertebrates, birds and humans. The mussel beds give shelter to numerous other invertebrates, including predatory mussel worms, a favourite bait for coastal fishermen.
There are three main species of mussels found in the intertidal mussel beds around St Francis. Brown mussels (Perna perna) and black mussels (Choromytilus meridionalis) are indigenous species, but the Mediterranean mussel (Mytilus galloprovincialis) is an invader from Europe. In fact, it is listed as one of the World’s Worst 100 Invasive Alien Species.
It was first recorded on South Africa’s west coast in 1979, but by 1988 had spread all the way to Port Elizabeth. It probably originally arrived in a ship’s ballast water or as hull fouling. It grows faster and reproduces more successfully than the indigenous species, factors that have allowed it to outcompete the locals. But this alien species has brought some benefits: populations of African black oystercatchers now have a better food supply and the numbers of this once threatened bird species have increased in areas where the alien mussel thrives.
Cochlear Zone: The lowest intertidal zone is unmistakeable. It is a pinkish colour due to the paint-like coralline algae that coat the rocks in this zone. Its predominant residents are pear limpets, Patella cochlear, that occur in very large densities. These limpets graze the coralline algae and also cultivate gardens of fast-growing red seaweed around themselves, which they graze in the same way as one would mow a lawn. In fact, the red seaweeds grow faster when grazed by limpets than they do when limpets are absent. Each pear limpet defends its garden from its neighbours and will aggressively push out any other limpet that intrudes into its garden.
You will only be able to see this remarkable cochlear zone and its fascinating inhabitants during spring low tides.
Pear limpet (Patella cochlear) surrounded by its own garden of red seaweed. Young limpets settle on the backs of adults because it is the only place that is not grazed; the young limpets are safe here. They cause no harm to the adults and happily graze the algae that settle on the adult’s shell.
The rocky shore ecosystem
Rocky shore communities are made up of animals and seaweeds (algae) that interact with each other and with the physical factors prevalent in the narrow band of shore between the sea and the land. At first glance these organisms appear very different from those to which we are accustomed. However, intertidal ecosystems work in much the same way as terrestrial ones.
- Producers make food using the energy of the sunlight. On rocky shores, the producers include seaweeds that grow on the surface, as well as phytoplankton (tiny drifting algal organisms) that is carried in the waves.
Seaweeds are types of algae and they may be red, brown or green. They all produce their own food from water, carbon dioxide and the energy of the sun.
Even though algae are producers, they are not classified as plants. Plants have well-defined roots, stems and leaves – which algae do not have.
Limpets graze the algae on the rock surface around them and manage to keep patches of rock open. This is where the tiny algal sporelings settle and germinate each time the tide comes in, only to be grazed by the resident limpet.
The only safe place for algal sporelings to settle is on top of a limpet’s shell – a limpet cannot possibly graze its own back – although young limpets that settle on an adult’s shell will use the settling algae for food.
- Herbivores eat the seaweeds and phytoplankton. Some herbivores, such as the limpets, chitons, winkles, and turban shells graze the tiny seaweed sporelings just like antelope graze the grass. Others, such as barnacles and mussels, filter the phytoplankton out of the water using their gills, legs, tentacles or bristles.
Spiny chitons (left) and winkles (right) are common herbivorous molluscs that graze algae in the intertidal zone.
Sea urchins are important grazers that control the abundance of seaweeds on rocky shores.
They sometimes use their tube feet to hold empty shells against their bodies in order to protect themselves from the sun.
Cushion stars are also grazers; a cushion star feeds by extruding (pushing) its stomach out through its own mouth, sticking it to the rock surface and digesting algal sporelings using its digestive juices.
Cushion stars and sea urchins belong to the group of animals known as Echinoderms. They all have a hard, spiny skin and an unusual type of symmetry. Instead of having two symmetrical halves, like most animals, they have five rays of symmetry – they have five similar parts all arranged around a central disc.
- Herbivores are preyed upon by carnivores. Predators in intertidal communities are every bit as scary as they are on the plains of the Serengeti.
Whelks and cone shells drill holes through the shells of unsuspecting mussels and limpets and suck out their flesh.
A whelk creeps up on a limpet that may well be dinner.
Predatory starfish use their sucker feet to prize apart the shells of a mussel and then pour the acidic contents of their own stomachs in between the two shells. This soon causes a mussel to relax and the starfish can then access the flesh inside.
Birds such as African black oystercatchers use their bills to stab at the edge of a limpet’s shell, or at a mussel, with such force that the mollusc becomes dislodged.
Scientists have even found a type of intertidal whelk that injects sulphuric acid into a worm’s burrow and waits for the worm to die.
And the camouflage, deception and dexterity of a hunting octopus are fearful weapons against small fish or crabs.
African Black Oystercatchers
(Photo C. Smith)
- Scavengers are also present in intertidal communities. Some sea urchins and sea cucumbers feed on dead seaweeds while certain whelks track down dead animals from the slightest ‘whiff’ of their body fluids
Black sea cucumbers are common in cervices and pools in the lower shore regions.
As in other natural ecosystems, there are many more producers than herbivores, and more herbivores than carnivores.
Intertidal animals have to compete with each other for space and food, avoid predators and adapt to environmental stresses such as heat and evaporation. On top of this, they have to withstand the relentless pounding of the waves. They are indeed magnificently adapted and worthy of our admiration.
If you wish to find out any more information about the living creatures that are found on rocky shores, you could refer to the following publications.
Branch, M. & Branch, G. 1981. The living shores of southern Africa. Struik Publishers.
Branch, G. M., Griffiths, C. L., Branch, M. L. & Beckley, L. E. 1994. Two oceans: A guide to the marine life of southern Africa. David Philip Publishers.
Lubke, R. & De Moor, I. (Eds.) 1998. Field Guide to the Easter and Southern Cape Coasts. University of Cape Town Press.
Tietz, R. M. & Robinson, G. A. 1974. The Tsitsikama Shore: A guide to the marine invertebrate fauna of the Tsitsikama Coastal National Park. National Parks Board.
Black sea cucumber https://commons.wikimedia.org/wiki/File:Black_long_sea_cucumber_(Holothuria_leucospilota).jpg