Hidden life
Sandy shelves, Ishelufa ezinesanti, Imigede yenhlabathi, Sandbanke
Shifting sands where animals live between sand grains and fish we eat hide on the seabed
What are Sandy Shelves?
Sandy Shelves are areas of sandy seabed on the shelf that may appear barren, but in fact, are home to diverse animal communities hiding on and in the seabed.
Extending from shallow inshore waters beyond the surf zone to the deep outer shelf, the sandy shelf is characterised by fine, medium or coarse-grained sediment. Like sandy beaches on the shore, sandy shelves are dynamic systems where bottom currents move sand particles from one place to another. Sunlight doesn’t reach the deeper sandy shelves and there are few places for roots or holdfasts to attach, making these ecosystems unfavourable for seaweeds. Without these primary producers, animals who live on sandy shelves rely on microalgae, plankton or waste products falling from the surface or flowing into the system with bottom currents.
Who lives on Sandy Shelves?
Animals living on sandy shelves have either adapted to life in an exposed environment above the sand or are hiding below the seabed surface. On the open plains of the shelf, there are few hiding places to avoid predators and many animals use camouflage to blend into their surroundings. Just as tracks on land give us clues as to who passed by; the trails, tracks and burrows of animals can be seen on sandy seabeds. Many animals hide in burrows beneath the sand or have adapted to existing in plain sight by developing specialised behavioural or physical characteristics like camouflage or using shells to protect themselves.
Invertebrates (animals without a backbone) dominate sandy shelves. Many small animals, especially worms and small crustaceans (like crabs and shrimps) live between the sand grains and these animals are known as infauna. Suspension feeders like sea pens (a soft coral species specially adapted to anchoring in soft sediment) and other soft corals (that look like small, colourful trees) collect food particles suspended in the water column in their branches and make habitats for other species. Some animals are deposit feeders, like sea cucumbers and urchins, and sift through the sand for organic matter and waste products. Sea snails, burrowing clams and horse mussels, crabs and other small crustaceans (amphipods and isopods) feed on tiny food particles trickling down from the surface waters.
The large number of invertebrates on sandy shelves make these ecosystems ideal for predators and scavengers. Sit-and-wait predators like anemones, have stinging cells in their tentacles to catch prey. South Africa has many beautiful starfish species that live on sandy shelves. Some are specialised predators that hunt for urchins Photo to describe
Sandy shelves are also home to many fish predators. Dragonets are a common fish species living here that feed on small prey items like crustaceans and worms. Skates, rays, guitar sharks and catsharks are often associated with these ecosystem types. Other predators on sandy shelves include important fish species like hake, white stumpnose, St Joseph sharks and kingklip. Some fishes, skates, rays and sharks dig beneath the sand to unearth prey on sandy shelves.
Sea Pen found near Aliwal Shoal 
Kingklip burrow beneath the sand 
Why are Sandy Shelves important?
Sandy shelf ecosystems are important in supporting commercially valuable fish stocks like the Cape Hake species, Kingklip and some line fish species. Maintaining fish stocks secures the fishing industry, providing jobs to more than 30 000 people and providing food resources to many households.
Juveniles of other commercially valuable fish species hide from predators in the habitat-forming invertebrates like soft corals such as seapens and thistle corals, which are scattered throughout the sandy expanse. These soft corals create structure and are important in providing habitats for eggs and protecting young fish in their branches. Many deposit-feeding animals mix and turn over the sandy seabed in this ecosystem and play an important role in the recycling of nutrients. Scientists are also studying the role of these ecosystems in storing carbon and helping us cope with climate change.
Sea pens are a type of coral that provides homes to many animals
How do we care for Sandy Shelves?
Manage demersal (bottom-contact) fisheries – Fisheries such as bottom or demersal trawling and demersal longlining can affect sandy shelves. Trawl fishing catches everything that is trapped in the net passing over or just above the seabed, which includes not only target species but also many non-target bycatch species. Additionally, trawling gear can damage the seabed and communities living on the surface of the sand, thereby impacting habitats and biodiversity. Species that may indicate the presence of Vulnerable Marine Ecosystems such as sea pens should be identified and monitored. Areas where these animals are abundant need to be mapped and managed through Marine Protected Areas or other effective spatial management measures. Consumer choices with regard to seafood make a difference in the demand for certain species and can help encourage the fishing industry to better manage habitats.
Manage mining and petroleum activities: seismic surveys, drilling and infrastructure installation can cause physical damage to sandy ecosystems. There is a risk of pollution during drilling operations, which can harm animals, pollute and introduce alien species, thereby, impacting biodiversity. Mining operations are also noisy and underwater noise from mining operations and seismic surveys may severely affect marine life by disrupting their ability to hear, communicate, navigate and feed. It is important that mining and petroleum activities are avoided in the most important biodiversity areas. Vulnerable Marine Ecosystems, key migration routes, or breeding areas of animals are often part of Critical Biodiversity Areas where impacts on biodiversity should be avoided. South Africa uses Marine Spatial planning to guide the use of marine areas and support a sustainable oceans economy.
Protect portions of sandy shelves in Marine Protected Areas (MPAs): Even though Sandy shelves may seem mostly empty and uninhabited, much of the life here is hidden and very small. These ecosystem types need protection. Protecting some areas will help us understand the natural state, the processes that support life and to help maintain the benefits we get from sandy shelves. Marine Protected Areas that help protect South Africa’s sandy shelves include the Namaqua National Park, Agulhas Bank Complex, Amathole and Amathole Offshore and Aliwal Shoal MPAs. Our special guitar sharks are protected on sandy shelves in the Protea Banks, Agulhas Bank and iSimangaliso MPAs.
How do we learn more about Sandy Shelves?
South African scientists are working to understand the biodiversity patterns and the potential impacts of trawling, mining and pollution on sandy seabed habitats and their ability to sustain the important activities that provide food and jobs for many South Africans. Seabed samples and underwater visual surveys are used to support these studies. Pollution impact studies, including that from drilling petroleum wellheads, have been undertaken to guide the management of Sandy Shelves and ensure that healthy seafood continues to be provided by these ecosystems.
Bomikazi Tshingana
Benthic Ecology PhD student at the University of Cape Town
Studied: Bachelor of Science (Walter Sisulu University), Master of Science, Honours (University of Cape Town)
What is interesting about the ecosystem you are working in?
South African scientists are working to understand the biodiversity patterns and the potential impacts of trawling, mining and pollution on sandy seabed habitats and their ability to sustain the important activities that provide food and jobs for many South Africans. Seabed samples and underwater visual surveys are used to support these studies. Pollution impact studies, including that from drilling petroleum wellheads, have been undertaken to guide the management of Sandy Shelves and ensure that healthy seafood continues to be provided by these ecosystems.
What are the challenges with working in your ecosystem?
The nearshore zone is a very active and dynamic environment so we have to strategise and plan field sampling in advance. It requires a lot of patience when sampling, when one methodology does not work then we need to adopt another one. We often sample using a tool called a benthic (meaning bottom) grab where a set of metal jaws is lowered to the seabed to collect a “bit” of sand along with the animals living within the sediment. Scuba divers also collect sediment cores and suction samplers can be used to vacuum up a sample of the seabed. Sediment samples have to be sieved to remove the sand and collect the small animals living within the sand and these are studied under a microscope in the laboratory. Identifying these animals can be difficult but you never know what you might collect!
What are you working on and why is it important?
I am looking at the factors that affect soft-sediment macrofauna (animals bigger than 1mm in size that live in the sediment) in the nearshore zone and trying to document and understand the patterns of hidden biodiversity. This includes understanding the influence and importance of bays. One of my study areas is False Bay, an area with many cumulative pressures. My work is important because it can feed into the management of these ecosystems to make sure that they are well taken care of and continue to provide benefits to people. The species hidden in my ecosystem can be indicators of change and help us to track change in sandy shelves. This is particularly important in managing water quality and mitigating pollution.
Acknowledgements: The ACEP Deep Forests, ACEP Agulhas Bank Connections, ACEP Deep Connections and the Coastwise projects are acknowledged for helping build the knowledge base for better management of Sandy Shelves.
MzanSea 