Oasis of the deep: cold-water corals of Atlantic Canada

Learn more about the cold-water corals and sponges of Atlantic Canada.

Transcript

Two-thirds of the surface of the earth is covered by the ocean and ninety per cent of that lies beyond the continental margins in depths greater than 2 kilometres.

Living in these deep waters are animals that have adapted not only to the lack of sunlight and the cold temperatures which rarely go above 2 degrees Celsius but also to the pressure of the water which can reach 1,000 kilograms per square centimetre!

Yet in this seemingly inhospitable environment there is a wealth of life, including a group of animals known as cold- or deep-water corals.

In the deepest waters yet explored in Atlantic Canada — some 2.5 kilometers — sea pen fields project above the featureless ocean bottom.

About a dozen species have been identified and not all look like the old-fashioned quill pens from which they take their name.

Unlike other octocorals, a sea pen's polyps are specialized to specific functions: a single polyp develops into a rigid, erect stalk called the rachis, and loses its tentacles, forming a bulbous "root" called the peduncle, which anchors the sea pen in the soft sediments of the sea floor.

Sea pen stalks can be over a metre long!

The rocky ledges and walls along the continental margin and other hard bottom areas support the sea fans or gorgonian corals.

These are amongst the largest of the coral species and are attached to the bottom by a holdfast – similar to the root of a plant.

From this comes a large, flexible, branching tree-like structure that supports the polyps and then orients these towards the prevailing currents.

These colonies can be over a century old.

Fishers refer to these as “trees” which has led to the term “forest” being used to describe the extensive concentrations of these species which can occasionally occur.

Their bright colours and large size make them an unforgettable feature of the deep-water ecosystems.

Also growing on hard ocean bottoms are the Bamboo corals, which although they are octocorals, develop a bony internal skeleton.

Their skeleton is formed by hard calcareous segments intersected with small bands of protein. When dead, the coral skeleton is exposed and resembles bamboo. When alive the long lightly-branched fronds sway in the currents.

One of the deepest recorded occurrences of all deep-water corals is a bamboo coral which was discovered at almost 5 kilometres below the surface of the ocean.