Entacmaea quadricolor (Leuckart in Ruppell & Leuckart, 1828)

Titus, Benjamin M., Bennett-Smith, Morgan F., Chiodo, Tommaso & Rodríguez, Estefanía, 2024, The clownfish-hosting sea anemones (Anthozoa: Actiniaria): updated nomenclature, biogeography, and practical field guide., Zootaxa 5506 (1), pp. 1-34 : 11-13

Entacmaea quadricolor (Leuckart in Ruppell & Leuckart, 1828) ( Figure 2 ; Figure S1 )

The bubble-tip sea anemone Entacmaea quadricolor is among the most iconic and phenotypically variable clownfish-hosting sea anemone species in terms of color, pattern, tentacle morphology, and growth patterns ( Figure 2 ; Figure S1 ). This species gets its common name from the characteristic bulbous swellings that commonly form at, or near, the tentacle tips. When bulbs are present, this species is easy to identify underwater. However, tentacle shape is highly variable both intra-specifically and even intra-individually. It is not uncommon to encounter individuals whose tentacles are simultaneously bulbed and digitiform (smooth, not-bulbed, uniformly shaped; Figure 2A ), completely bulbous ( Figure 2B, C, D ), or completely digitiform and long (up to 100 mm; Figure 2E ). Tentacle tips, as a rule, are blunt ended. Tentacle color is typically brown/tan or green ( Figure 2 A-D) but can also be bright red or orange ( Figure 2E ). Tentacle tips are frequently purple/magenta. Tentacle patterns are variable. Some tentacles take on a dull matte appearance ( Figure 2A ; Figure S1D ), but can also be striated ( Figure 2C ), translucent ( Figure S1A, C ), speckled ( Figure S1A, C ), or some combination thereof. In many individuals the equator of the bulb is a mottled white pattern ( Figure 2 ; Figure S1 ). Tentacles are densely packed, typically obscuring the oral disc and mouth. Verrucae are absent on the column E. quadricolor ( Figure S1C ). Column color is usually bright red or magenta. The column and pedal disc are not typically visible and burrowed deep in a crevice or hole in the reef. The body wall is thin and tentacles tear easily.

Entacmaea quadricolor exhibits two primary growth patterns and can be found as either large (up to 400 mm in oral disc diameter) solitary individuals ( Figure 2A ) or clonal aggregations of smaller individuals. Depending on habitat and geography, clonal aggregations can be comprised of small clusters of medium-sized individuals (typically 2-6 anemones per cluster), or dense aggregations of small individuals that can form extensive fields of anemones (dozens to hundreds of individuals; Figure 2B ). This latter growth form is typically found in the Coral Triangle to Central Pacific Ocean in shallow habitats where anemones grow on tops of reefs or among the branches of shallow stony corals ( Figure 2A ). Both shallow aggregations and large solitary individuals can be found on the same reefs and typically segregate by depth, with the large solitary individuals occurring in deeper water. Regardless of growth form, E. quadricolor requires hard stable substrate and is never found with its pedal disc and column burrowed in the sand. This species generally requires calm habitats with low wave exposure. When disturbed, E. quadricolor can disappear fully from view by withdrawing into the reef structure.

The geographic range of this species is broad, ranging from the very Northern Red Sea, throughout the Indian Ocean, Coral Triangle, and into the Central Pacific reaching the Marshall Islands but not East to French Polynesia ( Figure 3 ). High-latitude populations of this species are also common, nearly reaching temperate habitats in the Japanese Archipelago to the North, as well as marginal reef habitats in Australia (Solitary Islands) and South Africa. This is among the most common host anemones encountered throughout its range and almost always is found in association with clownfishes. However, in marginal reef habitats in high latitudes it is not uncommon for this species to be found without fish symbionts, particularly the small clonal populations.

Figures

FIGURE 1. Labeled illustration of Merten’s carpet anemone Stichodactyla mertensii.

FIGURE 2. Representative images of the bubble-tip sea anemone Entacmaea quadricolor encompassing a broad range of geographic and phenotypic variation. A) Wide angle photograph of large solitary individual highlighting typical microhabitat requirements for the species. Note the tentacles are both bulbous and digitiform within this individual (Saudi Arabia, Red Sea). B) Dense aggregation of hundreds of small clonal sea anemones growing among a shallow branching stony coral serving as hosts to a colony of Amphiprion melanopus (Kwajalein Atoll, Marshall Islands). C) Macro photograph of characteristic bubble tips. Note the striated pattern of the tentacle (Fares-Maathodaa, Maldives). D) Individual anemone with all tentacles exhibiting characteristic bulbous tentacle morphology (Fares-Maathodaa, Maldives). E) Red solitary individual with long digitiform tentacles hosting (Saudi Arabia, Red Sea). Photographs by Morgan Bennett-Smith, Benjamin M. Titus, and Scott Johnson.

FIGURE 3. Confirmed geographic range of Entacmaea quadricolor in the Indo-West Pacific. Red dots represent species observations from the Global Biodiversity Information Facility (GBIF). Blue shaded area represents shallow water habitat (60 m bathymetry).