Stichodactyla mertensii, commonly known as Mertens' carpet sea anemone, is a species of sea anemones in the family Stichodactylidae. It is regarded as the largest sea anemone with a diameter of over 1 m (3.3 ft), the next largest being Heteractis magnifica, which has longer tentacles. This species has an oral disc that can be described as more ovoid than circular that contours to the surrounding substrate and is attached to the substrate by adhesive verrucae, which are wart-like projections. Its blunt or pointed tentacles are uniformly shaped, and are only about 1–2 centimetres (0.39–0.79 in) long. It contains obligate symbiotic zooxanthellae, and is a host to around half the species of anemonefish and one damselfish, Dascyllus trimaculatus.

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