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MESOPHOTIC CORAL ECOSYSTEMS

Flower Garden Banks National Marine Sanctuary is well known for its beautiful coral caps, areas of reef-building corals on top of the salt dome formations within the sanctuary. These represent the most studied and visited portions of the sanctuary, but only account for about 1% of the area within sanctuary boundaries.

As technology has improved, so has exploration of areas beyond recreational scuba limits, in the mesophotic (middle light) or "twilight" zone. There, we have encountered light-dependent corals in what are now called Mesophotic Coral Ecosystems (MCEs).

These habitats were initially explored by Dr.'s Richard Rezak, Thomas Bright, and David McGrail in the 1970s and 1980s. We continue this exploration and habitat characterization today.

A small black and white striped fish swims amid orange sponges and corals in deep habitat at East Flower Garden Bank
A jackknife fish swimming in a Mesophotic Coral Ecosystem at East Flower Garden Bank. Photo: FGBNMS/NURC-UNCW

What are MCEs?

Mesophotic Coral Ecosystems (MCEs) are defined as light-dependent coral communities that occur in the deepest half of the photic (light-receiving) zone in tropical or sub-tropical waters.

Yellow fan-shaped coral and green bottle brush-shaped corals in deep habitat at West Flower Garden Bank
Mesophotic Coral Ecosystem 108 meters (354 ft) deep at West Flower Garden Bank. Photo: FGBNMS/NURC-UNCW

They are further described as areas from 30-40 meters (98-130 ft) to 150 meters (492 ft) deep. Any deeper than this and there is not enough light to support photosynthesis, a requirement of the zooxanthellate corals (corals with symbiotic algae) found in MCEs.

Two trumpetfish hiding in bushes of white branches of black coral in deep habitat at East Flower Garden Bank
A trumpetfish hiding amid black corals (Plumapathes pennacea) in the mesophotic zone at East Flower Garden Bank. Photo: FGBNMS/NURC-UNCW

In other words, these are coral communities that occur deeper than typical stony coral reefs, but where there are still low levels of light. They provide a connection between shallow coral ecosystems and deepwater coral assemblages, and habitat for a variety of species, including commercially and recreationally valuable fish.

There is some overlap between the coral cap and MCE zones at Flower Garden Banks National Marine Sanctuary, since the shallow stony reefs extend to about 50 meters (164 ft).

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Why All the Interest?

Mapping of areas in and around the sanctuary over the past decade showed that there was structural habitat of some kind all around the sanctuary's salt domes, and even in the open areas between them.

Bathymetric map showing contours of seafloor in and around Flower Garden Banks National Marine Sanctuary
Red lines on map show sanctuary boundaries around East and West Flower Garden Banks. Red contour areas show locations of the diveable coral reefs. Orange and yellow contour areas show structural habitat in the mesophotic or "twilight" zone.

In the same time period, scientific diving techniques and underwater vehicle technology developed enough that studying these areas became more practical. It also provided the means to take photographs and samples from otherwise inaccessible areas.

A red crinoid perched amid white gorgonians in deep habitat at West Flower Garden Bank
A crinoid spreads its arms from its perch on top of a gorgonian in the mesophotic zone at West Flower Garden Bank. Photo: FGBNMS/NURC-UNCW

By pairing these technologies, we were able to explore and learn more about what species make up these MCE communities, and the support they provide to the surrounding ecosystem.

Feathery corals and several small fish in deep habitat at Bright Bank
A crinoids, black corals, gorgonians, sponges, and threadnose bass at nearby Bright Bank. Photo: FGBNMS/NURC-UNCW

As it turns out, MCEs occur in about 98% of the sanctuary, and much of the areas surrounding other nearby banks, yet we know very little about them.

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What Have we Learned?

Prior to exploration of these areas, we always said that the sanctuary had no gorgonians (octocorals including sea fans, sea whips, sea plumes). As it turns out, we were wrong!

Red fan-shaped coral in deep habitat at West Flower Garden Bank
Gorgonians in a Mesophotic Coral Ecosystem at West Flower Garden Bank. Photo: FGBNMS/NURC-UNCW

In the mesophotic (middle light) zone there are many different corals, including species of black coral and gorgonians (octocorals). As exploration continues, we are building regional identification catalogs.

Bushy white and red corals in deep habitat at Stetson Bank
Unidentified corals and sponges in the mesophotic zone around Stetson Bank. Photo: FGBNMS/NURC-UNCW

You can take a look at some of the corals and other species we've encountered in the MCEs at a collaborative website called www.mesophotic.org.

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We have also compiled some species posters from the sanctuary MCEs that you can download. Identification work continues, so many of the photos remain unlabeled.

Deepwater Antipatharians Poster, High Resolution (41.7MB pdf)

Deepwater Antipatharians Poster, Low Resolution (560kb pdf)

Deepwater Fishes Poster, High Resolution (40MB pdf)

Deepwater Fishes Poster, Low Resolution (376kb pdf)

Deepwater Invertebrates Poster, High Resolution (31.2MB pdf)

Deepwater Invertebrates Poster, Low Resolution (508kb pdf)

Deepwater Octocorals Poster, High Resolution (28.2MB pdf)

Deepwater Octocorals Poster, Low Resolution (560kb pdf)

Deepwater Sponges Poster, High Resolution (21MB pdf)

Deepwater Sponges Poster, Low Resolution (8MB pdf)

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Small, knobby corals in foreground; boulder of brain coral in background.  Long, fingery branches of purple sponge anchored in knobby corals and standing upright.
   
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