Giant clam

A colourful shell weighing 200 kilograms

This is the biggest shell in the world: 1.2 metres long and 200 kg — three times heavier than an adult! Its 'lips' shimmer blue, green and purple because tiny algae live inside and cook food from sunlight. A giant clam sits in one spot for 100 years. Legends say it 'bites' people, but really it closes very slowly.

The giant clam (Tridacna gigas) is the largest bivalve (1.2 m, 200 kg), living on Indo-Pacific coral reefs down to 20 m. It gets 70% of its food from symbiotic zooxanthellae algae in its mantle — which is why it lives in shallow, sunlit water; the mantle colours (iridophore cells) spread light to the algae. The rest comes from filtering plankton. It closes slowly — the 'man-eating clam' legend is baseless. It lives over 100 years, with growth rings on the shell. A hermaphrodite, it releases both eggs and sperm (500 million eggs). Overharvested for meat and shells; farms in Fiji and the Philippines breed it for reef restocking. IUCN: Vulnerable.

Fun facts

• Algae inside the clam 'cook' 70% of its food from sunlight.
• It sits in one place for over 100 years.
• One clam releases 500 million eggs.