Fig. 1: Hypotheses (H1-4) for the origin of scleractinian bathymetric distribution.

H1: origin of corals in the upper bathyal zone (~200–1,000 m). Corals start to diversify and colonize shallower and deeper waters. H2a: the appearance of coloniality linked to faster colonization of shallower waters. H2b: the appearance of symbiosis related to faster colonization of shallower waters. H3: These processes would result in an accelerated long-term offshore-onshore trend. H4: thus, the rate of colonization should decrease with depth. Arrow colors represent different colonization evolutionary rates I, where r = 1 is the background colonization dynamic under a single rate evolutionary model expected if the phenotypic innovations do not matter for colonization. If r values are below one, colonization is slower, and if r values are over one, it is faster; these last two scenarios are consistent with the general evolutionary hypothesis that colonized different depth habitats depend on the acquisition of photosymbiosis and coloniality as key phenotypic innovations. These hypotheses are formulated based on the phylogenetically corrected median log₁₀ depth for each group of corals (plot to the right): AS (azooxanthellate solitary, n = 108), AC (azooxanthellate colonial, n = 31), ZS (zooxanthellate solitary, n = 24), and ZC (zooxanthellate colonial, n = 336), obtained from a variable rate phylogenetic regression model (Supplementary Fig. 3). Dashed lines point to the phylogenetic median of the posterior distribution for the median depth. Photographs: V. Häussermann (AS – Desmophyllum dianthus), A. N. Campoy (AC – Astroides calycularis), J. Veron (ZS – Cynarina lacrymalis, ZC – Goniopora minor).