Giant Clam, Gigas Clam

The mantle of the Giant Clam is usually golden brown, yellow or green. The mantle will have many iridescent blue or green spots on its surface, especially around the edges. Larger T. gigas may have so many of these spots that the whole mantle appears blue or purple.
T. gigas also usually have numerous pale or clear spots, mostly located near the center of the mantle. These clear spots, referred to as "windows", may serve to focus or allow more light through according to The Reef Aquarium Volume One.
T. gigas have a maximum length of 4 feet (1.5m).

source : http://animal-world.com/Aquarium-Coral-Reefs/Gigas-Clam-Giant-Clam