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Intertidal Rocky shores of southern Africa
Zonation
The intertidal rocky shore is that area between the spring low and spring high tides.
Organisms are found in different zones in this area. This zonation differs slightly
for animals found on the west, south and east coast of southern Africa. Generally
the intertidal zone can be divided into the following zones: firstly the Littorina
zone on the upper shore. This region is named after the small snails that inhabit
this zone. Littorina spp. has the ability to survive the severe conditions
of heat stress and water loss of the upper shore.
Below this zone is a region dominated by barnacles (including Balanus spp.)
and called the Upper Balanoid zone. The barnacles are modified crustaceans, the
adults are sessile and encased in a volcano-shaped shell. They remove food particles
from the water by means of filtration.
The Lower Balanoid zone is found seaward and here muscles, seaweed or both play
an important role. In addition to this, oysters form a zone high up the shore on
the East coast while the South coast is characterized by a region represented mainly
by the limpet Patella cochlear and aptly named the Cochlear zone. These limpets
are pear-shaped and often covered by encrusting coralline algae. On the North-west
coast there is a similar lower shore zone but it is dominated by the limpet Patella
argenvillei and the region is called the Argenvillei zone. These limpets are
large, tall and almost oval in appearance.
When exploring the rocky shores - remember!
The rocky shore habitat is complex and many of the natural processes are not fully
understood. Many of the animals we know very little about and a large number of
species still await description. When exploring the rocky shore please remember
to replace all rocks as you found them in order to minimise habitat disturbance
and to ensure that the animals survive your visit.
Algae
Algae or seaweed form the first level of the food chain on the rocky shore. All
animals living here depend directly or indirectly on this food source. A variety
of algae are found on the rocky shores and reefs. These include thin sheets of Sea
lettuce (Ulva spp.), green Golf-ball codium (Codium megalophysum)
which is frequently found washed up on the beach as well as colourful Coral Plocamium
(Plocamium corallorhiza) and Different-leafed sargassum (Sargassum
heterophyllum). The Saw-edged jelly weed or algae (Gelidium pristoides) is
found in untidy lumps on the back of limpets and is harvested for agar (about 200
tons dry weight a year). Agar has a solidifying agent and is used in bacteriological
culture. It is also used in fish, poultry and meat canning industries as well as
in the manufacturing of medicines, cosmetics, wine and ice cream. A variety of upright
coralline and encrusting algae are also to be found all along the coast.
Animals
A common occupant of rock pools is the orange Crumb-of-bread sponge (Hymeniacedon
perlevis) found on the sides of small intertidal pools. Others are small clumps
of cream Tube sponge (Leucosolenia sp.) These sponges, like most of the sponges
found along the South African coast, differ from the ones you buy for cosmetic use.
These animals have two types of building components that form the structure of the
animal or colony, namely spongin, and numerous toothpick-like spicules. These spicules
make the South African sponges unsuitable for use as bath sponges, as thousands
of tiny spicules will embed themselves in your skin when you rub yourself with the
sponge. These spicules are visible with the naked eye in the small Golf-ball sponges
(Tethya sp.) found under rocks in the lower intertidal area. In these sponges
the larger spicules radiate out from the centre of the colony and the ends may be
visible on the surface of the sponge.
Not all animals on the reef are sessile. There is the fast moving Cape rock crab
(Plagusia chabrus), which is a common resident of intertidal pools as well
as the slower Yellow-banded hermit crab (Clibanarius virescens). Crabs are
scavengers and eat a variety of animal and plant matter. They belong to the group
known as Crustaceans represented by shrimps, barnacles, isopods, amphipods, lobsters,
prawns, crabs and hermit crabs. They have a hard exoskeleton and can only grow by
shedding the old outer skeleton and getting a new but bigger one. Hermit crabs have
the added difficulty that as they grow they need to find a bigger shell to live
in, which can result in aggressive competition between these animals.
Other peculiar inhabitants of the rocky shore are the Tentacled flatworm (Planocera
gilchristi), which is almost invisible when moving over rock as it is perfectly
camouflaged, and the Peanut worm (Golfingia capensis), which lives under
rocks and resembles a shelled peanut when contracted. If one looks carefully between
the seaweed you may be lucky to find a Pycnogonad or a sea spider. These animals
are not related to land spiders although they have four pairs of legs, as is the
case in true spiders. The males carry the egg masses with slender legs under the
head. The animals are extremely delicate and should not be handled.
Molluscs are well represented and one of the largest is the Spotted sea hare (Aplesia
oculifera), which can reach 25cm in length. We also find chitons with their
8-shelled backs. The Giant chiton or armadillo (Dinoplax gigas) is commonly
found half covered by rock and sediment and seems never to move.
The smaller spiny chiton (Acanthochiton garnoti) is more commonly found on
the edges of rocky pools and one should keep an eye out for them as they are well
camouflaged but covered in hard hair-like spines that easily break off in your skin.
These spines are not poisonous and are an irritation at worst. Another mollusc that
is one of the rare finds these days are small juvenile abalone (Haliotis midae).
The occurrence of these animals is decreasing rapidly due to increased poaching
on our rocky shores. Abalones are long-lived animals that reproduce only after 8
to 10 years and may reach a size of 190 mm in diameter. Another mollusc sought for
its tasty flesh is the Giant periwinkle (Turbo sarmaticus) and although these
animals are still common in the intertidal zone note should be taken that the majority
of these are small. Limpets are more visible along our rocky shores and a number
of species are represented. There is the Cape false limpet (Siphonaria capensis)
and the very beautiful Rayed limpet (Helcion pruinosus) with iridescent green
spotted rays on the shell. Other interesting animals are the Blotched nerite (Nerita
albicilla) with its characteristic white and black colouration as well as
the predatory Whelk (Burnupena sp). For those interested in the feeding habits
of mollusc it may be worth remembering that shells belonging to herbivorous snails
differ from that of predatory snails in that the shell aperture is round and smooth
while that of the carnivorous snail is oval with a slit on the side to accommodate
the proboscis.
Echinoderms are well represented on rocky shores. This group of animals has a radial
symmetry and include starfish, sea urchins, brittle stars, feather stars and sea
cucumbers. The Cape sea urchin (Parechinus angulosus) is common along the
southern African coast.
These animals are found in a variety of colours and protect themselves from predators
by their calcium carbonate shell in the pumpkin-shape is covered with spines (the
green skeleton is commonly washed up on the beach and denuded of spines is commonly
referred to as pumpkin-shells). Five rows of tube feet is visible when the animal
is submerged and is used to for movement and attachment.
Inter-tidal ascidians are not as visible as their sub-tidal counterparts and are
to be found deep under ledges or in crevices. The common solitary ascidian Pyura
stolonifera is used by local fishermen as bait and better known as Red bait.
The thick, tough and leather-like test of these large individuals are overgrown
by algae and a host of other invertebrates such as soft corals, sponges and other
ascidians. The intertidal ascidian fauna is mainly made up of small encrusting species
and include especially those belonging to the family Didemnidae. These animals are
colonial and although the colony start with a single larvae, consecutive vegetative
growth though asexual reproduction result in relatively extensive thin encrusting
colonies. The test is firm as a result of small embedded spicules.
Next: Sandy beaches
References and additional reading
BENEDDETTI-CECCHI, L. (2001). Variability in abundance of algae and invertebrates
at different spatial scales on rocky shores. Marine Ecology Progress Series
215, 79-92.
BRANCH, G.M. & BRANCH, M. (1981). The living shores of South Africa.
Cape Town: C. Struik.
BRANCH, G.M. & GRIFFITHS, C. (1988). The Benguela ecosystems. Part V. The coastal
zone. Oceanography and Marine Biology Annual Review 26, 396-486.
BRANCH, G.M., GRIFFITHS, C.L., BRANCH, M.L. & BECKLEY, L. (1994). Two Oceans.
A guide to the marine life of southern Africa . Cape Town and Johannesburg:
David Philip.
BRANCH, G.M. & MORENO, C.A. (1994). Intertidal and subtidal grazers. In Rocky
shores: Exploitation in Chile and South Africa , ed. SIEGFIELD, W.R., pp.
75-100. Berlin: Springer.
BROEKHUYSEN, G.J. (1940). A preliminary investigation of the importance of desiccation,
temperature and salinity as factors controlling the vertical distribution of certain
intertidal marine gastropods in False Bay, South Africa. Transaction of the Royal
Society of South Africa 28, 255-295.
GIBBONS, M.J. (1988). The impact of wave exposure on the meiofauna of Gelidium
pristoides (Turner) Keutzing (Gelidiales: Rhodophyta). Estuarine, Coastal and
Shelf Science 27, 581-593.
GIBBONS, M.J. & GRIFFITHS, C.L. (2000). A comparison of macrofaunal and meiofaunal
distribution and standing stock across a rocky shore, with an estimate of their
productivities. Marine Biology 93, 181-188.
GRIFFITHS, R. & THORPE, D. 1988. Die Kuswêrel. Struik-sakboekgids vir Suider-Afrika.
Struik Uitgewers, Kaapstad.
GRIFFITHS, C.L., MCQUAID, C.D., HARRIS, J.M. & DAY, A.H. (2000). Functional
Ecosystems: Rocky Shores. In Summary Marine Biodiversity Status Report for South
Africa., eds. DURHAM, B.D. & PAUW, J.C., National Research Foundation.
JACKSON,L.F. 1976. Aspects of the intertidal ecology of the east coast of South
Africa. 46, 1-72. Durban, South Africa, Oceanographic Research Institute. South
African Association for Marine Biological Research.
LITTLE, C. & KITCHING, J.A. (1996). The biology of rocky shores. Oxford:
Oxford University Press.
LUBKE, R.A., MARKER, M.E., STONE, A.W., ROS, G.J.B., SEAGRIEF, S.C., BECKLEY, L.,
BUTLER, G.S., SAMLE, M., BRANCH, W.R., VAN WAY, Y., BOK, A., GESS, F.W., GESS, S.K.,
CRAIG, A.J.F.K., PERRIN, M.R., HALL, S.L., BELL-CROSS, G., HUMMEL, H.C., COETZEE,
P.W., PALMER, A.R. & BRUTON, M.N. (1988). A field guide to the Eastern Cape
Coast. Grahamstown: Centre of the Wildlife Society of Southern Africa.
MCQUAID, C.D. & BRANCH, G.M. (1984). The influence of sea temperature, substratum
and wave exposure on rocky shore intertidal communities: an analysis of faunal and
flora biomass. Marine Ecology Progress Series 19, 145-151.
POVEY, A. & KEOUGH, M.J. (1991). Effect of trampling on plant and animals populations
on rocky shores. Oikos 61, 355-368.
ROBINSON, G.A. (1969). The Tsitsikamma Coastal National Park, South Africa. Biological
Conservation 2, 72-73.
SOUTHWARD, A.J. (1958). The zonation of plants and animals on rocky sea shores.
Biological Reviews 33, 137-177.
STEPHENSON, T.A. & STEPHENSON, A. (1972). Life between tidemarks on rocky shores.
San Francisco,Ca.: Freeman.
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