Darwin Mounds - Composition

Composition

The mounds are mostly sand, currently interpreted as 'sand volcanoes'. These features are caused when fluidised sand 'de-waters' and the fluid bubbles up through the sand, pushing the sediment up into a cone shape. Sand volcanoes are common in the Devonian fossil record in UK, and in seismically active areas of the planet. In this case, tectonic activity is unlikely; some form of slumping on the south-west side of the undersea (Wyville-Thomson) Ridge being a more likely cause. The tops of the mounds have living stands of Lophelia and blocky rubble (interpreted as coral debris). The mounds provide one of the largest known northerly cold-water habitats for coral species. The mounds are also unusual in that Lophelia pertusa, a cold water coral, appears to be growing on sand rather than a hard substratum. Prior to research on the mounds in 2000, it was thought that Lophelia required a hard substratum for attachment.

The deep-water coral systems located upon the mounds, are especially fragile. Unlike shallow-water coral reefs, they are not adapted to cope with minor disturbances such as wave action. The mounds also support significant populations of the xenophyophore Syringammina fragilissima. This is a giant single-celled organism (a protozoan) that is widespread in deep waters, but occurs in particularly high densities on the mounds and the tails. Individual xenophyophores can grow to be larger than 20 cm and are often very fragile. The corals themselves provide a habitat for a wide diversity of other marine life including sponges, worms, crustaceans and molluscs. Among these starfish, sea urchins and crabs. Various fish have been observed, including blue ling, roundnose grenadier, and the orange roughy.