What is a giant clam?
The giant clam is the world's largest bivalve mollusc, living on coral reefs including the Great Barrier Reef. The biggest can grow to more than a metre across and weigh over 200 kilograms.
Giant clams sit on the reef with their shells open, showing a colourful fleshy mantle in shades of blue, green, purple and gold. The colours come partly from tiny algae living in their tissues.
Like corals, giant clams have a partnership with these algae, called zooxanthellae. The algae make food through photosynthesis and share it with the clam, so giant clams live in shallow, sunny water. They also filter plankton from the water.
Myths and threats
Old stories describe giant clams trapping divers' feet, but they close their shells slowly and are not dangerous. Their shells cannot even close fully in the largest individuals.
Giant clams have been overharvested in parts of the Pacific and Asia for food, shells and the aquarium trade. They are protected in Australian waters, and several species live on the Great Barrier Reef.
Like corals, giant clams can bleach when water gets too warm. Snorkellers often see them on reef flats around islands such as Lizard Island and Heron Island, where their bright mantles stand out.
Did You Know?
Giant clams can live for more than 100 years.