Rocky Shelves


Rocky shelf, rotsagtige rak, Ishelufa ezinamatye, Imigede esamatshe

Magical forests of animals providing homes for South African fishes and hope for future medicines

What are Rocky Shelves?

Rocky shelves are the reefs and hard grounds on the middle or outer continental shelf. At depths known as the twilight zone, this area receives some light but has less light than the sunlit zone above and more light than the dark slopes and margins where light is absent. Scientists refer to deep rocky shelves as mesophotic reefs.

At this depth where light is low, rocky shelves rely mostly on nutrients flowing through ocean currents or falling from the surface layer. This can be from plant or animal plankton and from waste from dead plants, seaweeds or animals. 

Attaching themselves firmly to the hard seabed of rocky shelves are animal forests of lace corals, soft corals like sea fans and sponges. These coral gardens form habitats or homes for other creatures that live in the twilight zone.

Who lives on Rocky Shelves?

Limited light reaches the seabed  of rocky shelves, and creatures living there are adapted to  the dim twilight. Because of the low light availability; primary producers who rely on energy from the sun are not common and filter or suspension feeders dominate rocky shelves. Lace corals, sea fans, soft corals, basket and feather stars and sponges filter plankton and organic matter originating from the sunlit zone. These animals are important in structuring the rocky shelf ecosystem and they form micro-habitats for other creatures. Brittle stars, starfish and crustaceans live among the arms of sea fan and thistle corals while bottlebrush and other soft corals often protect fish eggs and larvae in their arms.

Invertebrate predators and scavengers like starfish, urchins, small crustaceans and lobsters, feed on the many filter feeders attached to the hard seafloor of rocky shelves. Vertebrate predators like fish and sharks feed on the abundant invertebrate life found on rocky reefs. These rocky reefs support a number of special fish that are endemic, meaning that they are found only in South Africa. Such fishes include the world’s largest seabream, the red Steenbras, the iconic Seventy Four and beautiful dageraad.

Why are Rocky Shelves important?

Just like fynbos is special on land in South Africa, the lace coral and sea fan habitats in the twilight zone off the coast of South Africa are highly diverse and home to many endemic species – found only in our country.

Deep rocky shelves make important contributions to food and job security by supporting fisheries. Rocky shelves provide habitat for commercially important fish like carpenter (silverfish), panga, slinger and rock cods as well as rock lobster like the endemic South Coast Rock Lobster. Maintaining the structure and health of these ecosystems helps ensure such resources for the future.

The corals, sponges and sea fans all grow to create a complex habitat for line fish. This means that these twilight forests support our line fishery. Healthy rocky shelf ecosystems include nursery areas and provide spawning habitat for many harvested species including squid, iconic local fishes such as red steenbras, black mussel cracker or poenskop, seventy four and Miss Lucy (red stumpnose).

Some marine invertebrates like sponges and sea slugs living on rocky shelves have potential for future medicines. Compounds active against certain forms of cancer have been found in some species of sponge found on our rocky shelves. The silver or frilled seaslug and many sponges have yielding promising compounds. Even traditional healers and sangomas use animals from the mesophotic reefs in medicine and ritual. Basketstars are used in ceremonies to maintain strong bonds between people with the many arms of the basketstar reflecting the symbolic powers of these animals.

How do we care for Rocky Shelves?

Some of the ways we can care for deep rocky shelves are to:

Manage activities that can impact the seabed– Mining operations, trawling and anchoring damage the seabed and remove sensitive, slow-growing vulnerable marine species important to ecosystem functioning. These activities must be managed by the government and they should be closely monitored and regulated. Protected areas prohibit these activities and should be managed to prevent illegal activity. Our choices with regard to seafood make a difference in the demand for certain species and may reduce the frequency of trawling (See SASSI).  Skippers, tour operators and others, need to be aware of where they drop anchor, taking into consideration the reef below.  

Support efforts to map and manage Vulnerable Marine Ecosystems (VMEs)- Like shallow sub-tropical coral reefs, the fan corals and black coral trees on rocky shelves take a long time to grow. Some black coral trees found in South Africa can be more than a hundred years old! The animals that build the structures on rocky shelves are very sensitive to damage from any activity that can harm the seabed including trawling, mining and anchoring. It is essential that these ecosystems are mapped in order to prioritise areas for protection and avoidance of activities that can impact the seabed (#DiepRespek).. 

Protect healthy examples of rocky shelves: Rocky shelves are protected in several Marine Protected Areas such as Robben Island, Amathole Offshore, Addo Elephant National Park, Protea Banks, uThukela and iSimangaliso MPA.

How do we learn more about Rocky Shelves?

South African scientists have only recently begun exploring rocky shelves, discovering deep coral and sponge gardens colouring the seafloor. To study these ecosystems researchers often use special camera systems, as they do not damage the sensitive seabed. South African scientists capture video and photographs of the ecosystem using Remotely Operated Vehicles (ROV) and Towed Camera systems.

Luther Adams

Benthic ecologist, MSc Biological Sciences at the University of Cape Town

Studied: Bachelor of Science (University of the Western Cape), Bachelor of Science, Honours (University of the Western Cape) and recently submitted his MSc thesis (University of Cape Town).

What is interesting about the ecosystem you are working in?

One of the most interesting aspects of working in these unexplored ecosystems is the sheer diversity of organisms on the seabed. In a relatively small area of seabed, I found dense aggregations of feather and brittle stars, different red algal species capable of thriving in low light conditions, countless forms of sponges, unimaginable colour combinations of soft corals and forests of canopy-forming fan corals.

What are the challenges with working in your ecosystem?

One of the greatest challenges working offshore in South African waters is the mighty Agulhas current– especially on the southeast coast. A strong current coupled with gale-force winds from the Southern Ocean cause towering waves that can create precarious conditions which can limit time at sea. In the end, working with instead of against the elements usually ensures a safe working environment for exploration in the twilight zone.

What are you working on and why is it important?

I have spent the past four years analysing video footage and images from Remotely Operated Vehicles (ROV), exploring and describing deep reef ecosystems off the Kei River mouth on the East London continental shelf. The completely unexplored study area ranged between 30m to 100m deep, much deeper than conventional SCUBA diving would allow us to survey. Essentially, the ROV became my eyes that enabled me to explore the deep and discover new reef ecosystems. These discoveries not only add to our growing knowledge of our ocean but also allows us to conserve and protect these unknown deep-sea habitats. My work has contributed to the planning and implementation of the Kei zone of the Amathole Offshore MPA.

Mzansea: revealing south Africa's marine ecosystems Logo

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