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.