Deep-Sea Corals

Lophelia coral from the Blake Plateau off Georgia.

Many of us are familiar with the bright colors and diverse shapes and forms of corals that form shallow-water tropical reefs.  Those corals thrive in warm, clear water, where associated microscopic plants called zooxanthellae add color to the corals and convert light energy into food for the corals through photosynthesis.  These shallow coral reefs provide many services to human populations by building protective barriers against the sea; by supporting populations of snapper, grouper, lobster and other food; by providing aesthetically-pleasing recreation; and many other ways. 

Corals commonly occur in areas besides the tropics, and they provide ecosystem services to humans wherever they occur.  Off the coast of the southeast U.S. (and many other places around the world), deep-sea corals provide habitat for some important fishery species and the animals that these valuable species feed on.  Off the Carolinas, Georgia and Florida, extensive hard rocky bottom provides a firm attachment for a variety of branching stony corals and soft corals such as black coral, bamboo coral and others.  The rugged bottom and the additional relief provided by the coral attracts populations of fish, shrimp and crabs that support important fisheries.

These corals feed by extending their tentacled polyps into the water from delicate branches that rise above the bottom.  The deep-sea hard corals are particularly fragile, and colonies reproduce asexually through fragmenting, when broken-off branches form new colonies that are genetically identical to the parent colony (clones).  Corals also have sexual reproduction through mass spawning, and eggs shed into the water are fertilized, develop into drifting planktonic larvae, and disperse great distances in the ocean currents.  The relative contribution of the two reproductive modes to local populations of deep-sea corals is unknown.  Genetic study of samples of coral will be used to determine how corals reproduce and how they can be replenished if damaged.

How Old is That Coral?

A thicket of Lophelia coral. The light-colored parts are the growing portions.

Corals grow in many directions along the branches of the colony.  Branches increase in length, but also in thickness at the corals grow, by laying down successive layers of skeletal material from which the polyps extend.  The successive layers can be seen as growth rings on a cross-section cut through a coral branch, much like the rings seen on the trunk of a tree.   We do not know how often the rings are formed, but it may be that annual cycles of productivity in the ocean result in annual deposition of growth rings on corals.  One of our objectives on this cruise is to obtain coral samples to determine if rings are formed annually, by analyzing elemental isotope composition of sections cut through the coral branch. 

Oxygen (red) and carbon (black) isotope profile from coral in cross section. The most recent growth is on the left, proceeding backwards through the life of this coral fragment. Numbers indicate potential annual growth bands.