Mesophotic Coral Ecosystems

Habitat-forming organisms

The dominant habitat-forming communities in the mesophotic zone can be comprised of coral, sponge and macroalgal species (Figures 2.6–2.8). MCEs, similar to shallow-water reefs, include habitat-forming scleractinian corals that exploit a symbiotic relationship with zooxanthellae (genus Symbiodinium ), a type of microscopic algae (see also section 4.5). This single-celled organism lives within the cells of the coral’s gastrodermis. The coral provides a safe home and essential compounds for the algae, and in return the algae supply the coral with nutrients from photosynthesis (hence the need for light). The algae are generous guests, and on shallow reefs can provide as much as 100 per cent of the organic material needed by the host’s coral tissue (Muscatine 1990). However, mesophotic coral zooxanthellae often cannot produce enough energy given the light limitations, thus mesophotic corals may also rely on planktonic food captured by their tentacles (Davies 1977, Lesser et al. 2010). As coral and algal cover decline with decreasing light at depth, the benthic communities of MCEs may shift towards communities dominated by particle-capturing species, such as sponges and gorgonians (e.g. Bridge et al. 2012b, Slattery and Lesser 2012). Ecological work in the Caribbean has shown that mesophotic sponges rely less on photosymbionts, and more on plankton feeding. In some Caribbean MCEs, sponge biodiversity and biomass exceed that of shallow reefs by almost ten to one (Slattery and

Lesser 2012), and growth rates are higher (Lesser and Slattery 2013). Thus, faster growth and enhanced competitive strategiesmay allow mesophotic sponges to thrive while coral reefs worldwide are on the decline (Slattery et al. 2011). This may not be the case outside the Caribbean, such as in the Pacific Ocean (Pawlik et al. 2015a, b, see Slattery and Lesser 2015). In addition, the different selective pressures (e.g. predation) between shallow and mesophotic reefs have resulted in significant phenotypic differences in sponges with increasing depth (Slattery et al. 2015). Macroalgae, or seaweed, can also form vast beds and meadows over rocky or sandy substrate in the mesophotic zone, or grow intermixed with mesophotic corals. Although some native macroalgae, such as the brown alga Lobophora , can be invasive — overgrowing corals in areas where native herbivores are removed (Lesser and Slattery 2011, Slattery and Lesser 2014) — luxuriant stands of native macroalgae also occur naturally and are important ecologically. For example, species such as the mesh-shaped alga Microdictyon create bottom complexity, which forms significant habitat for reef fish (Abbott and Huisman 2004, Huisman et al. 2007). Calcified green algae, such as the meadow-forming Halimeda spp., can live for several years and are important sand producers (Spalding 2012). Thirteen different dominant macroalgal mesophotic communities have been documented in the Hawaiian Archipelago alone, suggesting that rich and diverse assemblages of macroalgal species may exist at mesophotic depths, and many are distinct from shallow-water populations (Spalding 2012).

Figure 2.6. A Leptoseris coral-dominated MCE in the ‘ Au ‘ au Channel, offshore of Maui, Hawai ‘ i, depth of 70 m (photo NOAA’s Hawai ‘ i Undersea Research Laboratory).

Figure 2.8. A green algal-dominated MCE in the ‘ Au ‘ au Channel, offshore of Maui, Hawai ‘ i, of Halimeda distorta , 75 m depth (photo NOAA’s Hawai ‘ i Undersea Research Laboratory).

Figure2.7. A 0.25m 2 mosaic of a Caribbeanmesophotic reef (depth 60m).Notethehighcoverageanddiversityofspongesinthequadrat, which is typical of many Atlantic MCEs (photo Marc Slattery).

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 14