Mesophotic Coral Ecosystems

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?

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

Figure 2.6.

Leptoseris

coral-dominated MCE in the

‘

au Channel, offshore of Maui, Hawai

‘

i, depth of 70 m (photo NOAA’s Hawai

‘

Undersea Research Laboratory).

Figure2.7.

A 0.25m

mosaic of a Caribbeanmesophotic reef (depth

60m).Notethehighcoverageanddiversityofspongesinthequadrat,

which is typical of many Atlantic MCEs (photo Marc Slattery).

Figure 2.8.

A green algal-dominated MCE in the

‘

au Channel,

offshore of Maui, Hawai

‘

i, of

Halimeda distorta

, 75 m depth (photo

NOAA’s Hawai

‘

i Undersea Research Laboratory).

Habitat-forming organisms

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).