Chapter 42. Cold-Water Corals
Contributors: Erik Cordes (Convenor and Lead Author)
, Sophie Arnaud-Haond,
Odd-Aksel Bergstad, Ana Paula da Costa Falcão, Andre Freiwald, J. Murray Roberts,
Patricio Bernal (Lead Member)
Commentators: Peter Harris (Group of Experts)
1.
Inventory and Ecosystem Functions
Globally viewed, cold-water corals cover a wide range of depths (39 - 2000 m) and
latitude (70°N – 60°S). In this Chapter, we will focus on the corals found below 200 m,
the average depth below which photosynthesis does not occur, to avoid overlap with
other chapters. The term “corals” refers to a diverse group of species in the Phylum
Cnidaria, including the scleractinian hard corals, octocorals including the sea fans and
soft corals, antipatharian black corals, and stylasterid lace corals. Although the majority
of the species-level diversity of scleractinians is in the solitary corals (Cairns, 2007),
some of the scleractinian corals may form extensive reef structures, occasionally
accumulating into large carbonate mounds, or bioherms. Many of the ecological
patterns discussed in this chapter are derived from the study of these structures, simply
because they have been the focus of the most extensive research in this developing
field. However, other types of cold-water corals can also form highly significant
structural habitat and these are also discussed. The most representative cold-water,
framework-building, scleractinian corals are Enallopsammia rostrata, Goniocorella
dumosa, Lophelia pertusa (Figure 1) Madrepora oculata, Oculina varicosa and
Solenosmilia variabilis (Roberts et al., 2006). The most common and widespread of the
large, structure-forming octocorals are found in the genera Corallium, Isidella,
Paragorgia,
Paramuricea, and
Primnoa (Watling et al., 2011) (Figure 2).
Cold-water corals (CWC) most commonly occur in continental slope settings, on deep
shelves and along the flanks of oceanic banks and seamounts. The majority of CWC
occur between the depths of 200 to 1000 m, with the bathymetric ranges becoming
shallower towards the poles (Roberts et al., 2009). However, there are numerous, dense
coral gardens (primarily octocorals and black corals) found on the slopes of seamounts
and the base of the continental slope to over 3000 m, and some soft corals and sea pens
are found on soft substrata down to abyssal depths (Yesson et al., 2012). The shallowest
occurrences of typically deep-water species are in high latitudes associated with the
rocky slopes and sills of fjords (L. pertusa off of Norway at 37 m depth, Wilson, 1979) or
narrow passes between islands (the octocorals Paragorgia arborea and Plumarella spp.
at 27 m depth in Alaska, (Stone, 2006)). Continental slopes exhibit a variety of specific
topographic irregularities that provide suitable substrate for cold-water coral larvae to
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settle. In many parts of the world ocean, the shelf edge is incised by gullies and
submarine canyons (Harris and Whiteway, 2011; Harris et al., 2014). Some prominent
examples are located at the canyon-rich slope of the Gulf of Lion off the coast of France
(Fabri et al., 2014), the Bay of Biscay under the national jurisdiction of France and Spain
(De Mol et al., 2011; Sánchez et al., 2014), the Gully off the coast of Nova Scotia
(Mortensen and Buhl-Mortensen, 2005), and the canyons off the eastern United States
(Watling and Auster, 2005; Brooke and Ross, 2014). Narrow straits between land-masses
may also provide suitable substrate, such as the Straits of Florida (Correa et al., 2012),
Gibraltar (De Mol et al., 2012), Sicily (Freiwald et al., 2009), and the Yucatan (Hebbeln et
al., 2014). Open-slope CWC mounds are known from the large reefs off the Norwegian
coast (Mortensen et al., 2001; Buhl-Mortensen et al., 2014), the Northeast Atlantic
along the Rockall and Porcupine Banks (Van der Land et al., 2014), the Southeast coast
of the United States (Stetson et al., 1962; Reed et al., 2006), the Gulf of Mexico (Reed et
al., 2006; Cordes et al., 2008), Southwestern Atlantic Ocean (Viana et al., 1998; Sumida
et al., 2004; Pires, 2007; Carranza et al., 2012), and off Mauritania (Colman et al., 2005).
These mounds are not randomly distributed over the slope but show a strong affinity
with distinct water mass boundaries passing along the slope (Mienis et al., 2007;
Arantes et al, 2009; White and Dorschel, 2010). Open-slope coral gardens appear to be
common along most of the continental margins of the world (Figure 3, Yesson et al.,
2012). Oceanic seamounts represent another important cold-water coral-rich
environment (see Chapter 51), such as the Tasmanian seamounts off South Australia
(Thresher et al., 2011), the seamount speckled Chatham Rise off the coast of New
Zealand (Tracey et al., 2011), seamounts of the central Pacific (Rogers et al., 2007), and
seamounts of the Mid-Atlantic Ridge system (Mortensen et al., 2008). A compilation of
framework-forming cold-water coral occurrences is displayed in Figure 4 based on the
UNEP-WCMC database (Freiwald et al., 2005) and more recent findings. The current
information on deep-water octocorals suggests that they are ubiquitous along
continental margins and seamounts on hard substrata, as well as occasionally on soft-
bottom in the case of the sea pens and a few species of bamboo corals. A combination
of octocorals collections and observations along with a predictive habitat suitability
model is displayed in Figure 3 (Yesson et al., 2012).
Cold-water corals have been known since the first descriptions in the 18
th
century and
the first deep-water research expeditions of the 19
th
century (Roberts et al., 2006). The
presence of large reef structures in deep water was not broadly appreciated by the
scientific community until the first submersibles were available in the late 20
th
century
(Cairns, 2007). Using these new tools, a more complete set of distribution records and
characterization of the habitat requirements of CWC were developed. Based on these
recent data, the use of habitat modelling has led to the discovery of numerous cold-
water coral sites and habitats. As an example, scleractinians were discovered on steep
submarine cliffs after modelling (Huvenne et al., 2011) and field observation in the
Mediterranean (Naumann et al., 2013) and the Bay of Biscay (De Mol et al., 2011;
Reveillaud et al., 2008). Similarly, an extensive screening of newly available mapping and
visualization technology in the Mediterranean revealed additional and more extensive
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