At 32°N of the equator, the reefs of Bermuda are among the most northerly in the world. Zooxanthellate scleractinian corals occur to depths of 50–70 m where they form a largely unexplored deep fore-reef slope, before giving way to rhodolith fields on the side of the seamount (Fricke and Meischner 1985). The zooxanthellate corals of the mesophotic deep fore-reef system of Bermuda thus face the physiological challenge of being at both their latitudinal and bathymetric limits.

In 2007, preliminary surveys of the Bermuda fore-reef were conducted using mixed-gas technical diving between 45 and 60 m. The structure of the deep fore-reef was strikingly different to shallower terrace-reef communities. Notably the Diploria-Montastraea-Porites species assemblage that dominates coral cover around much of Bermuda was absent on the deep fore-reef. Instead, the deep fore-reef was dominated by Montastraea cavernosa, Agaricia fragilis, and Madracis decactis, usually as small (~20 cm diameter) colonies (Fig. 1a). Occasionally, however, large (~1 m diameter) colonies of the branching coral Madracis carmabi (Vermeij et al. 2003) were found at 50–55 m depth. This is the first recording of this scleractinian on the Bermuda reef system (Fig. 1b).

Fig. 1
figure 1

a Colonies of Montastraea cavernosa (MC) and Agaricia fragilis (AF) and b Madracis carmabi at 55 m depth (scale 50 cm)

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Analysis of genetic diversity of zooxanthellae from deep corals by PCR-RFLP of the small subunit rRNA gene revealed that although previous surveys have shown almost all coral species contain a single clade of zooxanthellae at shallow depths in Bermuda (Savage et al. 2002), several of these corals harbour multi-clade populations at depth. These included M. decactis, Dichocoenia stokesi, and Meandrina meandrites, that contain clade B at shallow depths, but mixed populations of clade B and C at 45–60 m. Recently, mixed populations of zooxanthellae have also been reported in species of Madracis on the deep reefs of Curacao (Frade et al. 2008).

Better understanding of mesophotic habitats derived from previous studies of deep reefs (e.g., Bak et al. 2005), and future research are required to assess whether they provide potential refugia from global threats of bleaching and disease that have damaged shallower coral communities.