Radianthus magnifica (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 : 20

Radianthus magnifica (Quoy & Gaimard, 1883) ( Figure 10 ; Figure S5 )

The magnificent sea anemone Radianthus magnifica , formerly in the genus Heteractis , is one of the most iconic and heavily photographed clownfish-hosting sea anemones due to its prominent microhabitat, colorful and fully visible column, and a body size which can reach up to 1 m in oral disc diameter ( Figure 10 ; Figure S5 ). In older literature and especially in the ornamental aquarium trade, this species is often colloquially referred to as the “Ritteri” sea anemone. The tentacles of this species are distinctive, all alike, densely packed, digitiform but some might be split at the tip (Y-shaped), elongate (up to 100 mm), or bluntly rounded ( Figure 10C ). Often a small dot is present in the center of the blunt tentacle tip ( Figure 10C ). Tentacles are typically tan, brown, or shade of green, but can also take on various shades of purple and have brightly colored tips ( Figure 10 ). Tentacles contrast sharply with a brightly colored column, which is typically visible and vibrant shades of purple/magenta, red, and orange ( Figure 10A, B ), but can also be tan or brown and the same color as the tentacles ( Figure S5B ). Verrucae are present in longitudinal rows on the upper portion of the column only, are inconspicuous, and of the same color as the surrounding column. Verrucae typically do not hold debris.

This species occupies prominent positions on coral reef habitats, attaching its pedal disc directly to hard substrate. This is the only species that regularly has both its column and pedal disc fully exposed and visible ( Figure 10A, B ; Figure 5B ). Due to its conspicuous placement on fore reef habitats, this species of anemone is the most encountered host anemone by SCUBA divers, and thus, the most heavily photographed. When disturbed, or possibly after capturing prey, R. magnifica will curl up into a distinctive ball, leaving only a small tuft of tentacles exposed ( Figure 10B ). In high wave energy environments R. magnifica often has a fully flared oral disc, which can obscure both the column and pedal disc ( Figure 10D, E ). Although this species occupies prominent positions, requires hard stable substrate, and is often common on fore reefs, R. magnifica is something of a habitat generalist in that it will also be common on calm patch reefs and even occurs on extremely shallow reef flats at less than 1m depth ( Figure S5B, D, E ). The shallowest individuals do not always host clownfishes ( Figure S5B, D, E ). Radianthus magnifica reproduces both sexually and asexually. It can thus be found as a solitary individual or in extensive clonal aggregations of dozens to hundreds of individuals ( Figure 10 ; Figure S5 ).

Geographically, R. magnifica is widespread, ranging from the Northern Red Sea (but not in the Gulf of Aqaba), throughout the Indian Ocean, Coral Triangle, and the Central Pacific where it occurs at least as far east as French Polynesia (Moorea, Tahiti, and Tuomotu Archipelago) and the Line Islands ( Figure 11 ). This species appears to require fully tropical waters and does not extend into high-latitude subtropical waters in the Japanese Archipelago or Australia. Interestingly, this species also appears to be absent from the Gulf of Aden, Gulf of Oman, and Persian/ Arabian Gulf, and much of the Arabian Sea within the Northern stretches of the Indian Ocean ( Figure 11 ).

Figures

FIGURE 5. Confirmed geographic range of Heteractis aurora 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 10. Representative images of the magnificent sea anemone Radianthus magnifica encompassing a broad range of geographic and phenotypic variation. A) Wide-angle photograph of stereotypical R. magnifica perched prominently on exposed rocky outcrop with fully visible purple column and pedal disc (Maldives). B) Retracted individual with column curled into a ball leaving only a small tuft of tentacles visible (Kimbe Bay, Papua New Guinea). C) Macro photograph of tentacles. Note the blunt-rounded ends that are all alike and the small “dot” present in the center of the tentacle tip (Fares-Maathodaa, Maldives). D) Cluster of at least four anemones on rocky outcrop (Saudi Arabia, Red Sea). E) Large solitary individual with pink tentacle tips. Note that the oral disc is fully expanded over the substrate obscuring the column and pedal disc from view (Kimbe Bay, Papua New Guinea. Photographs by Morgan Bennett-Smith and Benjamin Titus.

FIGURE 11. Confirmed geographic range of Radianthus magnifica 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).