scientific chapter |
Messing CG, Reed JK, Brooke SD, Ross SW
Reef-building, or hermatypic, scleractinian corals are widely treated as tropical organisms, confined to warm, sunlit, fully marine habitats by their obligatory endosymbiotic phototrophic algae (zooxanthellae ) and stenotopic environmental requirements. However, hermatypic scleractinians lacking zooxanthellae are also the primary architects of extensive, complex, biogenic build-ups at outer shelf to bathyal depths from subarctic (71° N) to subantarctic waters (55° S) at temperatures between 4°C and 12°C (Squires 1965; Stanley and Cairns 1988; Cairns 1995; Fosså et al. 2000; Mortensen et al. 2001; Roberts et al. 2006). Such assemblages, usually called deep-sea or cold-water coral reefs, may cover as much (or more) area of seafloor as the 284,300 km2 estimated for shallow warm-water reefs (Freiwald and Roberts 2005 cited in T. Williams et al. 2006). Most commonly, they consist of accumulated skeletal debris and captured sediment ; living colonies, when present, are restricted to a superficial, often discontinuous, veneer (Teichert 1958; Stetson et al. 1962; Wilson 1979; Mullins et al. 1981; Mortensen et al. 1995; Freiwald et al. 2002; Reed 2002b, Reed et al. 2006). Although Rogers (1999) suggested that such coralligenic topographic features fall within the definition of a coral reef based on their physical and biological characteristics, they lie at depths too great to constitute navigational hazards – the traditional diagnostic character of a reef. As such, they have also been called banks, mounds and bioherms, the latter a geological term reflecting their specifically biological origin. Off the southeastern United States, limestone ridges called lithoherms consist of layers of coral-containing skeletal debris and sediment lithified by successive episodes of sub-sea cementation, rather than unconsolidated sediments (Neumann et al. 1977; Messing et al. 1990; Paull et al. 2000; Reed et al. 2005c, 2006).