|On one of the ROV dives on the seamount north of the Azores at the depth around 2000 m we observed a great number of traces left by an unknown animal. It's our new deep-sea mystery...
Deep-sea corals Lophelia pertusa, live and dead
Fig. 15. Photo from ROV showing live Lophelia corals. (Photo: MAR-ECO)
Tops of seamounts, submerged from several hundreds to one thousand meter depth, are very often densely inhabited by a variety of organisms that feed from the water column, either filtering particles or suspending them.
This type of environment is commonly characterized by strong currents and water circulation. Water moves and carries food particles used by epifauna attached to the rocks. Filter-feeding is common for example for sponges and bivalves. Suspension-feeding is used by some brittle stars, sea lilies, sea stars, sea pens, anemones and a variety of corals. Most abundant among corals on seamounts are gorgonian and soft corals, and also solitary madreporarian corals that have a hard calcium carbonate skeleton like their shallow-water relatives.
The only reef building coral that lives at these depths is a deep-water coral Lophelia pertusa. Lophelia lives mainly along the continental shelf, at depths from 200 to almost 2000 m. Most records of this coral come from the north-east Atlantic but it has also been recorded in the Mediterranean Sea, along the coasts of eastern north America, Brazil, west Africa and on the Mid-Atlantic Ridge. Biology of this coral is known very poorly and there are only a handful of observations on live Lophelia in its natural habitat.
On our cruise, using the Remote Operated Vehicles (ROV), we repeatedly observed live Lophelia colonies on the seamounts that form a chain of the Mid-Atlantic Ridge. The deepest record was at 1340 m, south of the Charlie Gibbs Fracture Zone. We did not see very massive structures, the largest colony was probably 1 m across.
Lophelia reefs provide a habitat for many other species, such as sea lilies, sea stars, sea urchins, brittle stars, polychaete worms, sponges, bivalves and many others. Observed diversity and density of fauna associated with deep coral banks was remarkable!
After Lophelia corals die the structures they built still provide a home for many groups of animals. Even after the colonies destroy they form a layer of pebble that accumulates over hundreds and thousands of years and can be a pronounced feature of the habitat on tops of seamounts. This pebble is a substrate for various attached groups of animals. North of the Azores we saw a seamount covered with a thick layer of a coral pebble on the top and slopes with very few live bushes. Even long after their death Lophelia corals form a landscape on the seamounts in the North Atlantic.
Tracks on the sediment left by animals living on or inside it, mirror the activity of life in the habitat. Scientists use the term ‘bioturbation’ when they speak of the impact of organisms on the sediment. Different animals leave different tracks on the sediment. There is a whole classification of these tracks known as ‘lebenspuren’. For many kinds of the tracks, the animals producing them are known, but for a great variety they are not known.
Fig. 16. Photo from ROV showing mysterious burrows. (Photo: MAR-ECO).
On one of the ROV dives on the seamount north of the Azores at the depth around 2000 m we observed a great number of traces left by an unknown animal. The traces were almost straight or curved lines of regularly placed perforations, as if somebody used a sewing machine to create this landscape. The lines, some of them several meters long, were going in different directions. Diameter of holes varied but often was around 5 cm. Some of the holes were not circular, they rather had a dash-line pattern.
Naively we were sitting for a long time looking inside the holes hoping to see who makes them. With no success! The only idea that came to our minds was that these rows of burrows are made by some big crustacean, maybe a deep-sea blind lobster that we caught in trawl on this seamount? Perhaps each line is a burrow with multiple entries, or is it a succession of burrows with just a single opening, but then how and why can these lines be that straight? We were so confused after this dive…and still are.
Scientists to be credited: Andrey Gebruk, Ricardo Serrao Santos