Distribution
''S. mertensii'' is found on rocky or coral substrate and is widespread throughout the tropical and subtropical waters of the Indo-Pacific area from Mauritius to Fiji and from the Ryukyu Islands of southern Japan to Australia. While this species can be confused with ''Stichodactyla gigantea'' and ''S. haddoni'', they are most easily distinguished by habitat since the former two prefer sand while ''S. mertensii'' prefers rocky or coral substrate.
Behavior
The relationship between anemonefish and their host sea anemones is highly nested in structure. With at least 13 species of hosted anemonefish, ''S. mertensii'' is highly generalist, and appears to be a preferred host. ''A. akallopisos'' is the only anemonefish in the Comoro Islands and is hosted by ''Heteractis magnifica'' and ''S. mertensii''. It appears that in the Seychelles that ''A. fuscocaudatus'' displaces ''A. akallopisos'' from ''S. mertensii'' while in the Maldives ''A. akallopisos'' is displaced from ''S. mertensii'' by ''A. clarkii'', as in both locations ''A. akallopisos'' is only hosted by ''H. magnifica''. There are 2 specialist anemonefish that are only found with ''S. mertensii'', ''A. fuscocaudatus'' and ''A. latifasciatus''.Recently, ''Amphiprion clarkii'' and ''Amphiprion sandaracinos'' were observed to coexist within one host anemone of ''Stichodactyla mertensii''. A. clarkii was not aggressive towards the A. sandaracinos but was aggressive towards all fish approaching the anemone. The anemonefish didn't divide the host into separate territories.
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