The two-tone cup coral (Phacelocyanthus flos, Caryophyllidae) consists of integrated clusters of small (0.5–1.5 cm) corallites whose skeleton is connected by a common base (Figs. 1 and 2). This species lives attached to and encrusts the underside of overhangs, cave ceilings, and hard structures at depths from 20–355 m (Cairns 1979). P. flos is azooxanthellate according to Humann (1993). During various deep dives around the islands of Curaçao and Klein Curaçao (12°N, 69°W), extensive monospecific formations of living and dead P. flos were observed between depths of 50–80 m. On Curaçao, near the Caribbean Marine Biological station CARMABI (Van Duyl 1985), coverage of a vertical overhang with P. flos was estimated at 40×7 m (width x height) between 65 and 75 m depth. On the satellite island of Curaçao, Klein Curaçao, a P. flos wall, approximately 100×10 m between 55 and 65 m was discovered. In between P. flos corallites, invertebrates such as sponges, gorgonians, crinoids, and other solitary scleractinian species were observed. Diving restrictions only allowed for limited excavation to investigate the thickness of the P. flos layer. Three holes of approximately 10–15 cm deep chiseled through the P. flos covered areas indicated layers of new P. flos overgrowing old ones. If the structures are constructional, further excavations should reveal that P. flos is one of the first reported tropical scleractinian corals at these depths that significantly contributes to active reef formation.

Fig. 1.
figure 1

Phacelocyanthus flos corallites that are interconnected by a common base at 65 m depth at Klein Curaçao

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Fig. 2.
figure 2

Detail of P. flos corallites: this sample was collected from the same location as the P. flos polyps in Fig. 1. Note that the living polyps grow on the skeletons of polyps living earlier. Scale bar: 1 cm

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