Ecological Consultancy Services Limited (EcoServe)

Biology of the deep sea coral Lophelia

Lophelia builds a hard branched skeleton of calcium carbonate. At the tips of branches tiny polyps extend their tentacles into the water to catch small animals. Living coral ranges in colour from white to orange-red. Unlike most tropical corals, the polyps are not interconnected by living tissue, and do not contain single celled plants in their tissues which provide them with food.

The older coral at the base of the branches dies but continues to support the living coral. The coral is attacked by sponges, worms and other organisms which can weaken the skeleton and cause it to fragment. Detached coral can form a surface for new coral and other animals to start growing on.

The coral probably reproduces by ‘budding’ off new polyps and by producing free-living larvae which float in the water until they find a suitable surface to attach to and grow on.

Lophelia reefs can grow to 35 m high, be hundreds of metres wide, and reach 13 km long off Norway. In Norway, some reefs are 8,000 years old. The extent, age and biodiversity of living coral reefs off Ireland is largely unknown.

The deep sea coral Lophelia typically lives between 200 m to 2000 m depth, where there is no sunlight, and temperature decreases from about 10^o C at 600 m, to 4^o C below 1400 m depth. Where it occurs in shallow water in Norway, temperatures are also less than 10 ^oC. There is no seasonal variation in temperature.

The ‘spring bloom’ of phytoplankton and subsequent zooplankton blooms, provide the main source of nutrient input to the deep sea. This seasonal ‘rain’ of dead plankton is visible on photographs of the seabed and stimulates a seasonal cycle of reproduction in many deep sea animals.

Deep water currents from the Mediterranean, Labrador and Norwegian Seas occur off the south and west of Ireland. They can be detected by their salinity, temperature, and oxygen concentrations, but it is unlikely that these variations affect species distributions in deep waters. However, annual variations in the direction of these currents, and concomitant changes in nutrient supply and temperature, may cause significant annual variations in growth and reproduction rates in deep sea species.

Although the residual current along the continental shelf off Ireland flows to the north, it moves back and forth with the tide, forms eddies, and may flow in different directions at different depths. In the abyssal plain at 3000 m depth, currents appear to be gentle, less than 10 cm per second. However, the effects of seabed topography and wind driven surface currents on the slopes and ridges of the continental shelf create stronger currents; for example, up to 1 m/s (2 knots) in the Porcupine Sea Bight and off the Faroes. Thus the deep sea environment is cold, seasonal, and has water currents variable in speed and direction.

The research to produce this article is a contribution by Ecological Consultancy Services Ltd (EcoServe) to the Year of the Ocean 1998.