*2.1. Site Description*

Sample collection took place near the Great Reef of Toliara (GRT), in SW Madagascar, at a barrier reef almost 20 km long and 2 km wide, bordered by freshwater rivers at its northern (Fiherenana River) and southern (Onilahy River) extremities (Figure 1a). Coral communities, including scleractinians and antipatharians, were first documented

in shallow (<30 m depth) and mesophotic reef (at depths between 35 m and 55 m) areas in the 1970s [39]. More recent studies have documented severe reef degradation due to fisheries, pollution and heavy sedimentation derived from the adjacent rivers [40–42], as well as coral bleaching episodes [43]. Antipatharian samples were collected in November– December 2018 on shallow (20 m depth, 23◦20.978 S, 43◦36.885 E—Site 1) and mesophotic (40 m depth, 23◦21.345 S, 43◦36.348 E—Site 2) reefs (Figure 1a) during a non-bleaching episode. Figure 1b shows the annual median for 2018 of the diffuse attenuation coefficient for downwelling irradiance at 490 nm in m−<sup>1</sup> (Kd 490) over the wider region as obtained from satellite data (MODIS-Aqua), a proxy for water turbidity [44].

**Figure 1.** (**a**) Map showing the location of study sites 1 and 2 (white circles) near the Great Reef of Toliara (GRT) in SW Madagascar. (**b**) Annual median of the diffuse attenuation coefficient for downwelling irradiance at 490 nm in m−<sup>1</sup> (Kd 490) for the year 2018 over the wider region (black circle indicated the location of the GRT). Satellite data derived from MODIS-Aqua accessed through OceanColor (https://oceancolor.gsfc.nasa.gov, accessed on 2 April 2019). Increasing coefficient values indicate higher water turbidity.
