Deep-sea grenadier fish, Nezumia sp. (© NOCS 2006) Deep-sea grenadier fish, Nezumia sp. (© NOCS 2006)
Deep-sea grenadier fish, Nezumia sp. (© NOCS 2006)
Deep-sea grenadier fish, Nezumia sp. (© NOCS 2006)
Deep-sea grenadier fish, Nezumia sp. (© NOCS 2006) Deep-sea grenadier fish, Nezumia sp. (© NOCS 2006) Deep-sea grenadier fish, Nezumia sp. (© NOC 2006)
Deep-sea grenadier fish, Nezumia sp. (© NOCS 2006)

Darwin Mounds

 A sidescan image of a single mound from the Darwin Mounds area. Image courtesy of Andy Wheeler, University College Cork.

The Darwin Mounds constitute a special case of sandy deep-sea habitats. The ‘Darwin Mounds’ were discovered in 1998 by scientists from the then Southampton Oceanography Centre (now National Oceanography Centre, Southampton) during an environmental survey to the north and west of Scotland (part of the AFEN cruises – see here for more information). The Darwin Mounds (named after the research vessel RRS Charles Darwin) were initially detected with SOC’s deep tow sidescan sonar system TOBI (Towed Ocean Bottom Instrument). Subsequent investigation in 1998 and twice during summer 2000, used SOC’s WASP vehicle, a seafloor photography and video system and also piston corers.

Map showing the location of the Darwin MoundsLocated at a depth of about 1000m in the north-east corner of the Rockall Trough, immediately south of the Wyville-Thomson Ridge (see figure), the comparatively small carbonate 'Darwin Mounds' cover an area of approximately 100 km2and contain some hundreds of mounds. Individual mounds are typically circular in outline with a height of up to 5m and a diameter of approximately 100m. The Mounds occur as a more or less continuous field in an arc-like form. The arc follows the local bathymetric contours for about 30km. There are hundreds of mounds in the field. Their sandy construction suggests that they are sand volcanoes - the result of fluidised sand 'de-watering', possibly as a result of slumping on the SW side of the Wyville Thomson Ridge.

The Darwin Mounds are remarkable for their ‘tails’– teardrop-shaped areas some hundreds of metres in length which can be found downstream (south-west) of most of the mounds. They are readily observed on sidescan sonar but have no topographic signature and are not visible at the seabed. The tails are characterised by high density populations of xenophyophores, (giant protozoans) that may be as big as 20 cm in diameter!

A Xenophyophore from the Darwin Mounds

The Darwin Mounds support a substantial population of the deep-water coral Lophelia pertusa. The tails are characterised by very high abundances of giant one-celled animals (protozoans) called xenophyophores, Syringammina fragilissima. Individual xenophyophores can grow to more than 20cm and are often fragile. The corals, and probably the xenophyophores, provide a habitat for numerous associated species, including deep-sea demersal fish. The animals associated with the Darwin Mounds differ substantially from the surrounding seabed. There is a significant increase in biological density and diversity on the mounds. The coral colonies on the mounds vary from one to a few metres across. The number of colonies on any one mound ranges from a few to hundreds. Lophelia pertusa appears to be the dominant coral species. The dominant deep-sea fish on and around the mounds are the cut-throat eel (Synaphobranchus kaupi) and the round-nosed grenadier (Coryphaenoides rupestris).

The density and diversity of benthic invertebrates also increases on the mounds, with suspension feeders such as sponges and brisingiid starfish using the corals as perches, and large echiuran worms using the coral as a refuge. Other animals noted in the area include echinothuriid sea urchins (Calveriosoma hystrix and Sperosoma grimaldii), pencil sea urchins (Cidaris cidaris), sea stars, gastropods and hermit crabs (Parapagurus pilosimanus).

Protection of the Darwin Mounds

Due to potential damage of this fragile habitat from deep-sea trawling, on 23rd Mar 2004 the Darwin Mounds were designated as the UK’s first offshore Special Area of Conservation and a complete ban on bottom trawling for an area 1380km 2 was ordered. (see here for story).

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