Harmful algal blooms, including invasive algal species, are often related to degraded water quality and are increasing in number and extent (Heisler et al. 2008). Blooms and invasions of fleshy macroalgae are common, but this is not the case for crustose calcareous algae. In Lac Bay, Bonaire, we found an encrusting unidentified species of the calcareous alga Ramicrusta sp. (Peyssonneliaceae, Rhodophyta) to be commonly overgrowing at least 14 species of scleractinian corals of the following genera: Acropora, Agaricia, Diploria, Favia, Montastraea, Porites (Fig. 1a, b), and Siderastrea. The alga also overgrows three species of gorgonians (Fig. 1c), two sponges, a hydrocoral (Fig. 1d), crustose coralline algae, and Halimeda. In most cases, Ramicrusta sp. grows upward from the base of apparently healthy scleractinians and gorgonians and causes death of the overgrown tissue. Over a period of several months, complete overgrowth of some colonies has been observed. Sponge tissue may survive partial overgrowth.

Fig. 1
figure 1

Ramicrusta sp. (indicated by arrows) overgrowing aPorites porites, bPorites astreoides, cPseudoplexaura sp. and dMillepora complanata. Color of Ramicrusta sp. varies from lightorange to deep red

Ramicrusta has not been reported elsewhere in the Atlantic, apart from Jamaica (Pueshel and Saunders 2009), and little is known about the ecology of this genus. Lac Bay is a small (ca. 8 km2), semi-enclosed bay with a back reef area (0.4 km2). Ramicrusta sp. may be invasive or, alternatively, stressful conditions in the bay may have caused a cryptic, native algal species to bloom.