Radianthus crispa (Hemprich & Ehrenberg, 1834 )

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 : 17-19

Radianthus crispa (Hemprich & Ehrenberg, 1834) ( Figure 6 ; Figure S3 )

The leathery sea anemone, Radianthus crispa ( Figure 6 ; Figure S3 ), was originally described from the Red Sea. This species gets its common name from its thick body wall, which gives the animal a tough leathery texture. The tentacles in this species are all alike, elongate (up to 200 mm), tapering gradually to a pointed tip ( Figure 6A ; Figure S3 ). Tentacles are numerous (up to 800 in the largest individuals) and can be densely packed in some individuals. Tentacles frequently curl, yet only slightly, and not to the same degree as in R. doreensis ( Figure 6C & E , Figure S3A, C ). The body and tentacle color of this species range from white and light purple to tan/brown or yellow and may take on a matte appearance ( Figure 6 ; Figure S3 ). Some individuals are bright pink/magenta, and it is not uncommon to encounter individuals with pink-tipped tentacles ( Figure 6B ; Figure S3C, E ). Tentacle patterns vary, but generally have a mottled ( Figure S3A, B, E ), lightly striped ( Figure 6E ), or speckled appearance ( Figure S3B ). The column of R. crispa is typically white/gray/tan in color and dotted with conspicuous raised verrucae which form longitudinal rows and are the same color as the column ( Figure 6D ). Verrucae are strongly adherent to the surrounding substrate and regularly hold debris, shells, and other sediment that can be seen when the oral disc is folded back.

A morphological and microhabitat shift seems to occur in this species between Indian and Pacific Ocean populations, and although this is not unilateral across all individuals, it is noticeable. Generally, R. crispa in the Indian Ocean occupy sand pocket microhabitats that are found adjacent to hard reef substrate and have the pedal disc and column burrowed deeply in the sand ( Figure 6C, E ; Figure S3A, B ). Indian Ocean specimens of R. crispa are typically found in calm patch reef or marginal reef habitats. In contrast, Pacific Ocean individuals more commonly occupy hard substrata directly in fore reef habitats and are more commonly found competing for space with stony corals ( Figure 6A, B ; Figure S3C, D ). Indian Ocean members of R. crispa have tentacles that are fewer in number and less densely packed than those from the Pacific, and are smaller in their body size and maximum oral disc diameter. Finally, Pacific Ocean members of R. crispa are more colorful in general, often have pink tentacle tips, and are more likely to take on body colors with pinks and yellows whereas Indian Ocean R. crispa are typically tan or brown ( Figure 6 ; Figure S3 ).

Geographically, R. crispa is the most widespread host anemone species, ranging from the Northern Red Sea throughout the entire Indian Ocean, Coral Triangle, and extending East all the way to French Polynesia ( Figure 7 ). Like E. quadricolor , R. crispa extends into high latitudes, nearly reaching temperate habitats in the Japanese Archipelago to the North, as well as marginal reef habitats in Australia (Solitary Islands) and South Africa.Although this anemone forms associations with nearly as many clownfish species as E. quadricolor , in our experience, it is found without clownfish symbionts more frequently than E. quadricolor . Solitary individuals that do not host fishes are common in the Red Sea and around the Arabian Peninsula. This species regularly hosts juvenile rather than adult clownfishes. It is not known to reproduce asexually and does not form extensive aggregations, but it is not uncommon to find this species adjacent to other clownfish-hosting anemones.

Figures

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).

FIGURE 6. Representative images of the leathery sea anemone Radianthus crispa encompassing a broad range of geographic and phenotypic variation. A) Yellow individual hosting juvenile Amphiprion chrysopterus on a typical fore reef environment in the Pacific Ocean. Note the densely packed tentacles with tapered tips coming to a point (Moorea, French Polynesia). B) White individual with pink tentacle tips (Moorea, French Polynesia). C) Tan/Brown individual with less densely packed tentacles that curl slightly (Saudi Arabia, Red Sea). D) Macro photograph underneath oral disc. i = well-defined raised verrucae that are similarly colored to the surrounding column (Okinawa, Japan). E) Small individual in stereotypical sandy microhabitat with slightly curled tentacled tips and horizontally striated tentacle pattern (Gulf of Oman, United Arab Emirates). Photographs by Benjamin M. Titus and James Reimer.

FIGURE 7. Confirmed geographic range of Radianthus crispa 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).