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These gardens rival flower gardens in terms of color and beauty!

by Cheryl Morrison, USGS; Nancy Prouty, USGS; and Brendan Roark, Texas A&M University 

From April 30 to May 27, 2013, scientists from the U.S. Geological Survey (USGS) DISCOVRE team worked with colleagues from other organizations in the Deepwater Canyons 2013: Pathways to the Abyss expedition aboard the National Oceanic and Atmospheric Administration (NOAA) Ship Ronald H. Brown. Using the Woods Hole Oceanographic Institution's remotely operated vehicle (ROV) Jason 2, they investigated the ecology of deepwater canyons off the U.S. mid-Atlantic coast. Little is known about life in the canyons, which are pathways for nutrients, sediments, and pollutants from the continental shelf to the deep sea. A high point of the expedition was the discovery of a new chemosynthetic community at a seafloor gas seep (see "Life in the Abyss," this issue), but the team observed other intriguing things as well, such as the "coral gardens" described in the May 11 log, reprinted here with additional photographs. This log originally appeared on the NOAA Ocean Explorer website.

Underwater photo of a deepwater coral and a crinoid.
Close-up of a crinoid attached to bubblegum coral (Paragorgia). The coral's polyps are extended in feeding position. Both animals capture small organisms and organic matter in the water column as it drifts by.
Photo of a Primnoa sample.
Wet lab photograph of a Primnoa sample with a shark or skate egg case attached, collected in 2012.

Two octocoral (sea fan) species we see often in the mid-Atlantic canyons are Paragorgia arborea (a.k.a. bubblegum coral) and Primnoa resedaeformis (red tree coral). These species are some of the largest and most widely distributed of the deep-sea octocoral species. When they occur in high densities, they are often referred to as “coral gardens.”

Foundation Species

Like trees in the forest, corals are a foundation species, creating three-dimensional habitat that other organisms call home. In other words, these deep-sea corals are ecosystem engineers since they create the structure that an entire ecosystem relies upon, therefore concentrating and enhancing biodiversity.

We see brittle starfish of many types; crustaceans such as shrimps, copepods, amphipods, ostracods and barnacles; anemones; and polychaete worms, just to name a few. Fishes are also attracted to the habitat created by deep-sea corals, and in some cases, fishes rely on the corals for part of their reproductive cycle. For example, catsharks commonly lay egg cases on the branches of Paragorgia and other octocorals.


A simple map of the seafloor showing location where a study was done.
Baltimore and Norfolk Canyons off the U.S. east coast were the focus of exploration by the Deepwater Canyons 2013 expedition. Modified from "High-Resolution Multibeam Mapping of Mid-Atlantic Canyons to Assess Tsunami Hazards" in Sound Waves - September-October 2011.

Large forest trees are hundreds of years old. We know this because we can count annual growth rings in cross-sections of tree trunks. Deep-sea corals can also be hundreds of years old. In fact, some coral species can be as old as the oldest known trees, the bristlecone pines. For example, a black coral species, Leiopathes, has been aged at over 4,000 years, making them the longest-lived marine organism known.

The Primnoa coral we’ve collected during this cruise will be aged by two of our scientists, Brendan Roark (Texas A&M) and Nancy Prouty (USGS), who are experts in aging techniques.

The skeletons of Primnoa have growth rings that appear to be approximately annual, reflecting fluctuations in food supply. Estimated linear growth rates for Primnoa range from 1.5 to 2.5 millimeters per year, but growth is likely slower when the corals are young and old. Paragorgia, on the other hand, has a less dense skeleton that does not have reliable growth rings. Instead, estimates of ages have been obtained by photographing the same individual twice, the second photo a year later, then comparing the length of branches on the overlaid digital images, as well as detecting the presence of bomb-produced radiocarbon (carbon-14) over the last approximately 50 years.

Potential for Regeneration

A skeleton of a branchy coral.
A coral skeleton, likely of the genus Primnoa, collected by the remotely operated vehicle (ROV) Jason 2 and photographed in the wet lab on the National Oceanic and Atmospheric Administration (NOAA) Ship Ronald H. Brown. Its branching pattern and growth structure resemble those of trees on land.

Both corals and trees spend their lives in one place. Where they settle down when young is where they will remain for life. Opportunity to move comes very early in life for both of these types of organisms. In most tree species, the creation of new individuals involves the production of flowers by females, followed by pollination, fruit and seed development, then finally dispersal to new locations that may be assisted by animals that eat seeds then move, or wind dispersal.

Although much less is known about the reproductive process in corals, many octocorals spawn gametes [reproductive cells] into the water column, where they meet, fertilize, and become larvae that either drift in currents or swim to some degree. Although these larvae may have the potential to follow currents and travel large distances (hundreds of kilometers), genetic studies of shallow water corals have demonstrated that many larvae don’t travel very far from their parents.

We will try to gauge how far larvae of Paragorgia and Primnoa travel using genetic techniques that quantify how unique populations are. The more connected populations are via exchange of larvae, the more similar their genetic profiles will be. We hypothesize that populations within one canyon will be more similar to each other than populations between Norfolk and Baltimore canyons.

Cross-sectional view of a coral branch, looking close at the center of the branch and showing growth layers.
Close-up of the coral branch at left, showing a cross-section of its growth bands. At very high resolution, one can count these annual growth bands to determine age. This particular species deposits material made of calcium carbonate (light) and material made mostly of protein (dark) to form its skeleton.

Unfortunately, like forests, these life history traits also make the coral gardens especially vulnerable to destruction. The long life spans and slow growth rates shared by both trees and deep-sea octocorals suggest that regeneration of these habitats could take decades or centuries. In other words, destroyed habitat may not be replenished in our lifetimes.

Given all of the unknowns about the factors that influence reproductive success and failure for deep-sea octocoral species, it is very difficult to predict how this process may play out, and whether techniques used to help regenerate forests, such as artificial propagation, may also work for deep coral ecosystems. Some of the pieces of the puzzle involving the life histories of Paragorgia and Primnoa may be solved through this research, and may help us understand and protect these complex ecosystems.

The principal investigators on the USGS DISCOVRE team are Amanda Demopoulos, Christina Kellogg, Cheryl Morrison, and Nancy Grumet Prouty. During the expedition, invaluable help was provided by Jill Bourque (Cherokee Nation Technology Solutions, contracted to the USGS), Katharine Coykendall (USGS), Michael Gray (USGS), Jennifer McClain-Counts (USGS), and Marcus Springmann (USGS). Read daily logs from the expedition and the blog written by students.

Red bubblegum coral
Red bubblegum coral (Paragorgia) and several colonies of Primnoa occupy a boulder in close proximity to an anemone and sea star, at approximately 440 meters depth in Norfolk Canyon.

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