Giant Clams: Snorkelling over the Reef Drop-off one is immediately struck by the beauty of the 'underwater garden' comprising table corals – often hundreds of years old - waving soft corals with brightly coloured fish darting in and out and above all the varied colours of the giant clams. Clams are bivalves which have turned themselves over so that the hinge that joins the two shells is pointed downwards. The mantle between the two shells is upwards and spills out of the shell. This mantle can vary in colour from bright blue, through green and yellow to brown depending on the influence of microscopic algae called zooxantellae cultivated by the clam which then feeds on part of them. Its upward position enables the clam to expose its body to maximum sunlight in order to grow the zooxantellae as abundantly as possible.