One fish, two fish, three fish, SHELLFISH?
Rita Steyn
The aptly named blue-eyed bay scallop, Argopecten irradians. Harvested legally only during certain seasons, this scallop is a highly prized recreational fishery animal. Photo: Rita Steyn.
What Are Shellfish?
Despite the name, shellfish are not fish in the true sense of the word. Shellfish are a group of aquatic invertebrate animals that are either mollusks, crustaceans, or echinoderms. Mollusks are shelled animals like oysters and mussels, crustaceans are animals with an exoskeleton like shrimp, prawns and lobsters, and echinoderms have a spiny skin, like sea urchins and sea cucumbers. The term shellfish is also widely used in the food industry and is often described as a low calorie, high protein food, however, shellfish allergies are the most common food allergy in adults.
Every continent in the world that has traces of human settlements near coastlines and river mouths bears evidence of the consumption of shellfish. From France to Florida, one can find ancient mounds of oyster shells, indicating that humans harvested these animals and depended on them as a valuable food resource. Beyond food, shellfish have been used for ornamentation and even currency. Shell money was a viable form of payment even until the turn of the 19th century.
Food and Aquaculture
Seafood is the largest traded food commodity in the world, and almost half of the world’s population (3 billion people) depend on either wild-caught or farmed seafood as their main source of protein. Certain types of shellfish, such as lobster, abalone, and oysters, are now considered luxury food items because they are rare; they were once so abundant that they were considered a poor man’s food. In 2018, people in the United States consumed approximately 5.3 billion pounds of fish and shellfish. The global seafood market is worth over 150 billion US dollars, and the European Union was the top exporter in that market, sending over 36 billion dollars worth of fishery related products worldwide.
This huge industry is supported, in part, by aquaculture. Aquaculture is sometimes called aquafarming or mariculture – because it involves the cultivation of aquatic organisms. Oysters, scallops, and mussels are among the most common types of farmed shellfish, with shrimp farming in both fresh water and marine environments following closely behind. Oysters were farmed by the ancient Romans as early as the 1st century BC, and archaeological evidence from South Africa tells us that humans were eating shellfish 164 000 years ago. Aquaculture is also a conservation vehicle, restoring ecosystems while providing food. Wild stocks have been decimated and can no longer meet the global demand for seafood. In some countries such as South Africa and the United States, the poaching of free living shellfish like abalone has lead to the creation of aquaculture for the purpose of re-seeding areas with the animals.
Shellfish Reefs
Shellfish can be found worldwide, but certain types of shellfish tend to aggregate in large groups, forming shellfish reefs. These structures are formed by millions of oysters or mussels clustering together in bays, estuaries and nearshore waters. Much like coral reefs, these shellfish reefs provide shelter and food for many other species. Shellfish are considered ecosystem engineers, building complex habitats, protecting nearshore communities from storms by forming natural breakwaters, and even cleaning up after themselves! A single oyster can filter up to 50 gallons (189 liters) a day, providing clearer, cleaner water which in turn supports more species, including seagrass and salt marshes.
A mussel reef exposed at low tide. Photo: Rita Steyn.
Shellfish reefs were once so abundant on the eastern coast of the United States that ships had to navigate around them, and now the Nature Conservancy estimates that over 85 percent of shellfish reefs have been lost worldwide. In 2018, shellfish reefs in Australia were called the country’s most threatened ecosystem, with less than 10 percent of oyster habitats remaining. The conservation of shellfish reefs can lead to carbon sequestration, or the creation of a carbon sink. Much like cutting down trees, the loss of those reefs turns the shellfish in to a carbon source, the release of carbon into the atmosphere. Reef presence is essential for long-term carbon burial in sandy environments, and the benefit of conservation extends far beyond the physical footprint of the reef itself.
Threats to Shellfish Reefs
Much like many marine environments, shellfish reefs and free-living shellfish face numerous threats. A combination of climate change and overfishing, shellfish are becoming scarce and shellfish reefs precious. Degradation of habitat due to warming, pollution, and development of coastal regions all cause a decline in populations. Warming also makes these animals more vulnerable to predation and increasing ocean acidity further compounds the problem. The shells of shellfish are made from calcium carbonate, and the amount of carbon available is reduced by more acidic oceans. Not only that, but a more acidic ocean slowly eats away at already formed shells, leaving these animals less protected. If we can address or eliminate these threats, we can restore these reefs.
Over-harvesting of certain shellfish like sea urchins in some parts of the world has led to cascading effects on the ecosystems. Urchins are grazers – they eat algae. Too few urchins means too much algae, overgrowing and shading out other parts of the habitat. But there must be a balance – too many urchins and they will overgraze even large kelp forests.
Don’t Be Shellfish – Lend a Hand!
You may be wondering what you can do to help make the world a better place for shellfish. Wonder no more! Here is a short list of actions you can take to conserve, preserve, and support these unique animals.
Choose wisely. If you eat shellfish, look for a sustainable source.
Educate those around you about the function and value of shellfish ecosystems.
If you can spare the cash, donate to restoration projects.
Volunteer your time – rebuild, re-seed, and restore oyster habitat.
Don’t clam up -speak up for and support climate action. Reach out to policymakers or tell your shellfish story in an effort to support shellfish.
There is still time to restore these reef habitats and the services they provide, benefiting people, places, and ecosystems.
Up close and personal with a sea urchin. Considered a delicacy in many parts of the world, and an important part of balanced ecosystems. Photo: Rita Steyn.