Marine animal forest formed by gorgonians Subergorgia on near-shore mesophotic ecosystems in Reunion Island

Abstract

Mesophotic coral ecosystems (MCEs), occurring between 30 and 150 m depth, are increasingly recognized for their ecological importance, yet they remain underexplored, particularly in the southwestern Indian Ocean. During benthic surveys conducted at depths ranging from 15 to 75 m off northwestern Reunion Island, we documented a dense and extensive marine animal forest (MAF) dominated by large arborescent octocorals Subergorgia cf. suberosa. This monospecific community formed a complex three-dimensional habitat spanning a substantial area at mesophotic depths over the northwestern abrupt slopes of Reunion Island. Colonies reached over 1.5 m in height, providing structural habitat for diverse fish and macroinvertebrate assemblages. A total of 53 fish species from 22 families were recorded in association with this MAF, alongside numerous epibionts and understory anthozoans. Despite its ecological value, this habitat faces significant anthropogenic pressures, particularly from fishing activities. Nearly 25% of photographed S. cf. suberosa colonies were entangled in fishing lines, with visible damage including tissue necrosis and polyp loss affecting up to 47% of colonies. These impacts, along with sedimentation from runoff, raise concerns for the long-term persistence of this mesophotic habitat-forming community, as long-lived, slow-growing organisms like gorgonians may be more impacted by these types of disturbances and take longer to recover than fast-growing organisms. Our findings represent the first quantitative assessment of Subergorgia dominance and associated biodiversity in this region and highlight its role as a potential refuge for shallow reef taxa. The ecological significance, spatial extent, and fragility of this MAF underscore the urgent need for spatially replicated surveys, targeted conservation strategies, and regulation of damaging activities such as fishing. We advocate for increased research efforts focused on mesophotic habitats, with particular attention to the population dynamics and ecological functions of large gorgonians, to better understand the role of MCEs in reef resilience under accelerating environmental changes.