44) ROCKY SHORES
The rocky shore is one of the most fascinating of all ecosystems.
It is packed with a wide variety of marine life. The shore is
covered by the sea and pounded by the waves at high tide in the
intertidal zone. Low tide means exposure to drying air and heat.
Plants and animals living here must be able to live both in air
and water; they must be able to survive the loss of almost 70 %
of their body water during dry periods, and, in addition, be able
to cope with freshwater rain when exposed and salty seawater when
submerged.
FIXED AND MOBILE ANIMALS
* Mobile animals: are very active and move up and down with the
tides. They can retreat into gulleys and under rocks when
conditions become stressful. They include the crabs, small
fishes, sea lice, star fishes and many of the sea snails. Mobile
animals are often well camouflaged which helps them escape
predation or to surprise prey. Others like the sea slugs are
brightly coloured to advertise that they are poisonous and so
warn off hungry predators.
* Fixed or sedentary animals: such as barnacles or mussels
survive best in a narrow range of conditions to which they are
adapted. Some are thus confined to the high shore while others
are found only at low tide level. They spend their lives firmly
attached to the rocks so that they cannot be washed away by the
waves. They are usually protected by shells which can be sealed
shut during the dry period to reduce water loss. As they cannot
move in search of food they rely on the filtration of small
particles of food from the water that washes over them at high
tide.
Mussels circulate water through their shells, barnacles use their
feathery limbs to comb particles from the water and many tube
worms have tentacles to collect food particles. The beautiful sea
anemones capture small creatures that stray within range of their
tentacles. Sedentary animals are easy prey for birds, fish, crabs
and octopuses and this is why they have developed various methods
of defence such as thick protective shells.
LIMPETS
South African shores are famous for their limpets. There are 13
species, each occurring in a particular zone on the shore. The
limpets roam the rocks scraping off algal spores and young
seaweeds with their rasp-like tongues. Each limpet has a home
scar on the rock to which its shell has grown for an exact fit.
It returns to this spot at low tide and clamps firmly to the
rock, providing itself with protection from wave action and water
loss as well as making it difficult for capture by predators.
Some species of limpets do not move far from their home scars and
cultivate and protect a private garden of seaweeds around them.
By nibbling only the tips of the seaweeds, like cutting a lawn,
they maintain a constant supply of food. The pear limpet is so
successful that it can survive at densities of 2 600 per square
metre. On the south and west coasts, it forms a dense band at the
low tide level.
THREATS TO ROCKY SHORES
Rocky shores and the life associated with them are harmed by
pollution, much of which is brought to the coast by rivers.
Sewage in river water can carry cholera which is then taken up
by shellfish and, when this is eaten by people, the latter can
become infected. Many rocky shores are subject to indiscriminate
collecting of bait, shells and rock pool life.
WHAT YOU CAN DO
* Visit the rocky shore at low spring tide. Walk from the high
water mark on a rock platform down the shore, noting how the
animals and seaweeds occur in particular zones.
* Do not buy sea shore curios unless you are sure they are
legally collected and are not a threatened species.
* Do not remove sea shore life unnecessarily or without a
licence if one is required.
FURTHER READING
THE LIVING SHORES OF SOUTHERN AFRICA.
G. and M. Branch. Struik, 1981.
EXPLORE THE SEASHORE.
M. Branch. Struik, 1987.
HANDS ON THE EAST COAST ROCKY SHORES: A FIELD GUIDE.
T. Stewart. 1991. Share-Net, P O Box 394, Howick, 3290.
A FIELD GUIDE TO THE EASTERN CAPE COAST.
R. Lubke, F. Gess and M. Bruton (eds). Wildlife Society,
Grahamstown, 1988.
EAST COAST IDENTIFICATION GUIDE.
Posters, available from Treasure Beach Project, PO Box 16126,
Brighton Beach, 4009. Tel. 031-478507.
SEASHORE LIFE.
C. and R. Griffiths. Struik, Cape Town, 1988.
All books available from Russel Friedman Books, PO Box 73,
Halfway House, 1685. Tel. 011-7022300/1.
USEFUL CONTACTS
Oceanographic Research Institute.
P O Box 10712, Marine Parade, 4056. Tel. 031-373536.
Treasure Beach Project.
PO Box 16126, Brighton Beach, 4009. Tel. 031-478507.
East London Museum.
Upper Oxford Street, East London, 5200. Tel. 0431-22623.
Albany Museum.
Somerset Street, Grahamstown, 6140. Tel. 0461-22243
Port Elizabeth Museum.
Beach Road, Humewood, Port Elizabeth, 6001. Tel. 0461-561050.
The South African Museum.
Marine Biology Department, Queen Victoria Street, Cape Town,
8001. Tel. 021-243330.
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