Gorgonians are a group of soft corals that belongs to the large group Cnidaria. This includes hard corals and jellyfish, as well as sea anemones and other species. Gorgonians can be found in all areas of the ocean, including shallow coastal areas and deep sea canyons, temperate, tropical, and polar zones.
Gorgonians’ skeleton can be soft or rigid, horny or calcareous. These organisms can form dense communities that structure the seabed, constituting “animal forests” offering a refuge to a myriad of marine species.
One of the most beautiful underwater scenes in the Mediterranean
There are five main species, all living up to 100m below sea level, of gorgonians in the Mediterranean. One of them is, ParamuriceaclavataIt forms amazing populations due to its red colour and the size of its colonies which can exceed 1 metre high.
It is popular among amateur divers and underwater photographers as one of the most beautiful underwater landscapes within the western Mediterranean. The planula, a free-swimming and crawling larva common to Cnidaria, is the source of the colonies. The planula is egg-shaped and uses hairlike projections called “cilia” to propel itself through water. It grows slowly (about 2 to 3 cm/year) and can live for many decades.
“Animal forests” weakened by human activities…
Like other gorgonians’ beds, purple gorgonians’ beds are fragile and susceptible to human activities in coastal regions. These populations are often affected by climate change in the Mediterranean.
Anchors and fishing nets can cause injury to the living tissue or tear away fragile colonies. Creatures that live on top of other organisms – known as epibion organisms – then proceed to colonise the beds’ exposed areas, threatening the colony’s survival.
Excessive input of sediment in marine environment – referred to as hypersedimentation and linked to coastal constructions and the changes of the courses of rivers – is also a threat to the populations of gorgonians.
Although they were spared from the warming Mediterranean waters, ParamuriceaclavataLocally, fishing is a major factor in affecting the environment. Recreational fishing is also a major contributor. These giant gorgonians have colonized a vast rock shelf that is home to many sea breams, which are prized by amateur fishermen.
Up to 180 boats were counted 29 October 2016During the sea bream breeding period. Many giant colonies were evicted by anchors and fishing lines. If we want to allow this unique natural heritage to coexist alongside human activity, it is essential that we limit the number of visitors to this conservation area.
… and by climate change
Not only are gorgonians at risk from human activities in the coast area, but so is the rest of the world. Climate change has caused the Mistral, the strong wind off the Provencal coast, to become weaker over the last decade. This has led to thermal anomalies in water column.
The result of this is the submergence of the surface warm water layers (temperature >22 °C) for stretches of several weeks. The latter can be fatal to the gorgonian populations, which are usually exposed to cooler temperatures (around 13 to 15 °C).
1999 was a year of a Thermal anomaly largeThe western Mediterranean, stretching from Spain to Italy, was affected by the gorgonians. A total of 20 species, including gorgonians and bryozoans, were also affected.
The anomaly was characterised by the presence of a column of warm water (23 to 24°C) at a depth ranging from 40 to 60 metres for a month. In 2003, 2006, and 2009, other events of this nature were also observed, which affected the gorgonians to a lesser or greater extent.
Factors such as the gorgonians’ genetic makeup or microbiota will dictate how they fare in the face of heat stresses. In the next decades, Significant climatic changes are expected to impact upon these species’ location and distribution.
Discovering a unique deep-sea community
Although the Mediterranean’s gorgonian population is well documented down to 50m, very little information is available about those who live further below. A unique settlement was found off the coast of Africa a few decades back. Côte bleueAt a depth of 50-60 metres, in the northern bay of Marseille. This settlement occupies nearly 2,500 hectares of rock shelf.
It is distinguished by its high density of gorgonians but also by the presence giant colonies. These corals, which can be seen hovering between 1.50 to 1.80 metres, are likely around a century old.
Recent studies have sought to explain the reasons behind the species’ gigantism and tightly knit networks (REFUCLIM programme) and distribution (GIGOR programme). The results show that these gorgonians are genetically different from other gorgonians at shallower depths.
Their environment is special. Not only are the gorgonians regularly subjected to strong inputs of organic matter linked to the proximity of the Rhône’s mouth, but the bottom currents are generally weak. Combining these two parameters allows gorgonians achieve extraordinary sizes.
One of the issues that is at stake here is the possibility of these deep settlements becoming Refugee populations, which are relatively spared from global warming, making them all the more important for the species’ survival.
The study of deep-sea gorgonian populations is a major area of research in the years to come, not only to initiate appropriate protection measures, but also to better understand their connections with the populations inhabiting the sea’s shallower depths.