Radianthus doreensis (Quoy & Gaimard, 1883)

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 : 19-20

Radianthus doreensis (Quoy & Gaimard, 1883) ( Figure 8 ; Figure S4 )

Radianthus doreensis , formerly in the genus Macrodactyla , is highly variable in terms of tentacle and oral disc color and pattern ( Figure 8 ; Figure S4 ). It is commonly referred to as the “corkscrew anemone” or “long-tentacled anemone” in reference to the long (> 150 mm), tapering tentacles that often curl or form spiral patterns ( Figure 8A, B ; Figure S4 ). The body and tentacle color of this species is often various shades of brown, yellow/white, or purple. Tentacles can be uniformly colored but are often longitudinally striped ( Figure 8D, E ). Tentacles are often sparse, but not always, leaving the oral disc highly visible ( Figure 8E ). The broad oral disc, up to 500 mm diameter in the largest individuals, typically lies flat on the surrounding sand or rubble and takes on a similar pattern to the tentacles. Individuals with longitudinally striped tentacles will also have highly striped oral discs, superficially resembling radially arranged zebra stripes ( Figure 8 ; Figure S4C, E ). Individuals without longitudinally striped tentacles will not display heavily striped oral discs ( Figure 8A ). Prominent, light-colored verrucae form longitudinal rows on the upper portion of the column, and contrast in color from the surrounding column, which is typically a purplish-gray or brown ( Figure 8C ; Figure S4 ). The light-colored verrucae are non-adhesive and are the most distinct feature in life that can be used for field identification. Verrucae are absent on the lower portion of the column which is typically bright orange and obscured from view by being burrowed deeply in the sediment.

Radianthus doreensis has a geographic range that is poorly resolved, likely due to this species being understudied ( Figure 9 ). It is documented from Australia, through the Coral Triangle, and North to Japan, as far east as the Marshall Islands in the Central Pacific. Dunn (1981) lists the Red Sea as part of its native range, but this has not been confirmed through recent major surveys in the region ( Bennett-Smith et al. 2021), and it has not been observed in the Indian Ocean outside of Western Australia.

Radianthus doreensis is a sand/rubble dwelling species that occupies sand pockets in coral reef habitats or occupies sand flats adjacent to reefs. It is only known to reproduce sexually and does not form large aggregations. The geographic range, habitat, tentacle shape and color, orange base, and non-adhesive verrucae that contrast highly with surrounding column are also features partially shared by other clownfish hosting species such as R. crispa , R. malu , and Heteractis aurora . Thus, in the field this species can be easy to misidentify. Radianthus crispa has s long tentacles that can also curl or spiral but have adhesive verrucae that are the same color as the surrounding column. Radianthus malu also has orange bases/pedal discs, but does not have long curling tentacles. Heteractis aurora has white verrucae that contrast with a grayish column, but has beaded or semi-beaded tentacles that do not curl.

Figures

FIGURE 4. Representative images of the beaded sea anemone Heteractis aurora encompassing a broad range of geographic and phenotypic variation. A) Multi-colored, whole individual in typical rubble microhabitat with characteristic beaded tentacle morphology (Maldives) B) Macro photograph of stereotypical beaded tentacle morphology (Kwajalein Atoll, Marshall Islands). C) Partially retracted/folded oral disc revealing highly contrasting white verrucae on gray column (arrow). Also note zebrastriped oral disc pattern (Fares-Maathodaa, Maldives). D) White phenotype hosting Amphiprion clarki (Fares-Maathodaa, Maldives). E) Individual with partially beaded tentacles. White peripheral tentacles with lightly beaded morphology with inner brown tentacles without beads and horizontal stripes (Anilao, Philippines). Photographs by Morgan Bennett-Smith, Benjamin M. Titus, and Scott Johnson.

FIGURE 8. Representative images of the long-tentacled sea anemone Radianthus doreensis encompassing a broad range of geographic and phenotypic variation.A) Whole individual in typical sandy microhabitat (Bali, Indonesia). B) Macro photograph of characteristic curling/spiraling tentacle pattern (Kwajalein Atoll, Marshall Islands). C) Partially retracted individual revealing highly contrasting white verrucae on brown column (arrow; Anilao, Philippines). D) Small purple individual with longitudinally striped tentacles and oral disc (Anilao, Philippines). E) Large individual with longitudinal “zebra-stripe” tentacle and oral disc pattern (Anilao, Philippines). All photographs by Scott and Jeanette Johnson.

FIGURE 9. Confirmed geographic range of Radianthus doreensis 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).