Stichodactyla mertensii Brandt, 1835 (Figure 18; Figure S9)

Merten’s carpet anemone, Stichodactyla mertensii, is among the largest sea anemone species in the world (clownfish-host or otherwise), regularly reaching oral disc diameters of one meter (Figure 18; Figure S9). Stichodactyla mertensii has tentacles that are variable in length, with the inner tentacles around the mouth considerably longer (50-80 mm) than marginal ones (6-12 mm; Figure 18C). This difference in tentacle morphology is unique among the carpet anemones and serves as one diagnostic character that can be used to differentiate S. mertensii from S. haddoni or S. gigantea. Regardless of length, all tentacles are digitiform and otherwise alike (Figure 18B). Tentacles are less densely packed than S. haddoni, leaving the oral disc more exposed in some individuals. Tentacle color is almost exclusively browns and greens. Endocoelic tentacles can form radial striping patterns as in S. haddoni (Figure 18E). The column is typically pale in color and contains bright and highly contrasting verrucae in longitudinal rows that are orange, red, and purple. Spots that are similarly colored to the verrucae extend the entire length of the column; this is unique among the carpet anemone host species (Figure S9B).

This species is the only carpet anemone to be found draped prominently over hard substrate on fore reef environments (Figure 18; Figure S9). It attaches its column deeply into crevices in the reef substrate (Figure S9B). Weak column musculature prohibits the anemone from fully retracting. The combination of longer tentacles around the mouth, reddish non-adhesive verrucae, and hard reef microhabitat form three important characters for identifying this species in the field.

Geographically, this species is widespread throughout the Indo-West Pacific. It ranges from the Northern Red Sea, Indian Ocean, Coral Triangle, and into the Central Pacific at least to the Marshall Islands (Figure 19). It is found north into the Japanese Archipelago, south to Australia and Mozambique Channel. Its range in the South Pacific appears to stop at Tonga and does not extend to the Cook Island or French Polynesia.