Coral reefs in the deep sea by Santiago Herrera

Coral reefs in the deep sea by Santiago Herrera

Although coral reefs are often thought of as ecosystems that exclusively occur in the shallow and warm waters of the tropical seas, they are in fact also found in the dark and near-freezing waters of the world’s deep oceans.  Hard corals, like the ones in this picture, can be found living on the Kermadec seamounts.  Due to the near pitch-black conditions at which these animals live, having photosynthetic symbiotic microalgae like their shallow-water relatives do is not an option. Instead they rely on their ability of feeding on the suspended particles of organic matter and planktonic organisms that sink (or sometimes migrate) from the surface waters.

The deeper you go, the amount of suspended organic matter in the water column becomes smaller. This means that there are depth limits below which suspension-feeding species cannot meet their energetic needs and therefore cannot survive. Reef-forming hard corals, in general, seem to have larger energetic needs than soft corals. This could be in part explained by the relatively high cost of producing a hard skeleton. So it is no surprise that reef-forming hard corals are often restricted to the summits of relatively shallow seamounts. These corals build a tri-dimensional matrix that provides microhabitats for many different kinds of organisms. In this photograph there are at least 13 different species in sight (and many more hiding); can you spot them?

              Coral

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