Density distribution and photo-physiological responses of the sea anemone Radianthus magnifica around the east of Rodrigues Island, Western Indian Ocean

Abstract

This study assessed the density and distribution of the sea anemone Radianthus magnifica and examined its bleaching responses around the east coast of Rodrigues Island, Republic of Mauritius, Western Indian Ocean. The abundance of anemone-associated fish and anemone body size along with the relationship between depth and R. magnifica bleaching were investigated. Surveys were conducted at depths from 2 to 23 m via belt transects of 30×1 m, in triplicates during April 2024. Radianthus magnifica bleaching was observed and a comparison of chlorophyll-a fluorometry between non-bleached and bleached anemones was done using a Junior Pulse-Amplitude-Modulated (J-PAM) fluorometer. Radianthus magnifica was reported at only two out of seven surveyed sites, and its density did not differ significantly (p>0.05, One-Way ANOVA). Significant negative correlations were obtained between R. magnifica bleached individuals and depth, possibly due to lower irradiance and thermal regimes at deeper areas. A damselfish species, Dascyllus trimaculatus, was more abundant than Amphiprion chrysogaster and the presence of D. trimaculatus was also negatively correlated with degree of R. magnifica bleaching. Bleached R. magnifica exhibited a significantly lower photosystem II (PSII) efficiency and slightly lower relative maximum electron transport rate (rETR_max), and a higher non-photochemical quenching (NPQ_max) than non-bleached individuals, indicating an affected photo-physiological state of the remaining Symbiodiniaceae symbionts. These findings suggest that the distribution of R. magnifica is limited and to a certain extent, vulnerable to thermally-induced bleaching around Rodrigues Island, and thus may require further attention in terms of adaptive management and conservation strategies.