Tridacha gigas is permanently sessile as adult, positioned hinge-down in reef, exposing open edge and mantle to sunlight. Valves remain widely gaping unless disturbed. Reaction to stimuli is rapid, although closure is slow due to great amount of water requiring discharge from mantle cavity. Larger specimens cannot completely close shells. Although little documented evidence exists, folklore claims that divers have drowned after catching a foot or hand in the closing "maws" of giant "killer" clams. Closing speed is slow and the bivalves certainly unaggressive, but the strength of closure and the sharpness of the valve edges can inflict serious injuries on unwary divers.
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