An unappreciated group of filter-feeding animals found around our coastlines could clean up our waters and nourish a billion people. Is it time we championed bivalves?
Simmering in pots around the world is a food that could spark a revolution. Most of us have probably eaten it at some point, but it’s an overlooked part of our diet. This natural source of protein is laden with essential nutrients that could fulfil the dietary needs of nearly one billion people in the most vulnerable populations on the planet. It could be a viable alternative to intensively-farmed meats such as beef. And it comes with smorgasbord of environmental and sociological gains.
The animals that are the source of this food require no feeding, need no antibiotics or agrochemicals to farm. And they actively sequester carbon. They can even protect fragile ecosystems by cleaning the water they live in. Welcome to the remarkable and unglamorous world of the bivalve.
This biological corner of our oceanic ecology is not as attention-grabbing as fish or mesmerising as a deep-sea octopus. Instead it includes the evolutionarily simpler family of shell-dwelling creatures consisting of mussels, clams, oysters and scallops. These hinge shelled molluscs have quietly embraced the lower reaches of the food chain as filter feeders, sustaining themselves on microscopic organic matter present in the waters of their immediate environment.
Their lowly status, however, perhaps means their potential has been largely overlooked. But as the world attempts to find ways of feeding a growing population with less environmental impact, many experts believe we may need to make these shellfish a larger part of our diet.
“Bivalves have the remarkable potential to provide people with food that is not only environmentally sustainable but also nutrient dense,” says David Willer, a zoologist at the University of Cambridge in the UK.
With a higher protein content than many meats and plant crops, and high levels of essential omega-3 fatty acids and micronutrients, like iron, zinc and magnesium, this specific group of shellfish has the potential to ameliorate many global food issues. This is particularly relevant to child malnutrition, as many of these nutrients are especially crucial to growth, and the planet stands to gain from their increased consumption too. Bivalves can be both wild-harvested and actively farmed offshore and in coastal areas, with a fraction of the environmental impact of more traditional animal proteins.
While oysters might be thought of as a luxury good in the West, bivalves, including clams, mussels and scallops, can be farmed cheaply (Credit: Getty Images)
“We know that meat and fish have a greater environmental impact than plant-based foods,” says Willer. “But the environmental footprint of bivalve aquaculture is even lower than many arable crops in terms of greenhouse gas emissions, land and freshwater use.” Given that animal protein production is so often cited as a significant carbon culprit down both to the carbon footprint of feed and fertiliser production, and the methane emissions of the animals themselves, this is forcing a shift in the landscape of environmentally friendly eating decisions.
And the numbers are there to back it up. According to Willer, we are looking at 340 tonnes of greenhouse gases produced per tonne of edible beef, compared with just 11 tonnes of emissions per tonne of bivalve protein. Those are of course generalised global figures which do not take into account different types of beef production, but it’s not just about the cattle: salmon, one of the most popular fish consumed in developed countries, takes 5kg of wild fish feed for every 1kg of salmon produced.
“If just 25% of this ‘carnivorous fish’ aquaculture was replaced with an equivalent quantity of protein from bivalve aquaculture, 16.3 million tonnes of CO2 emissions could be saved annually – equivalent to half the annual emissions of New Zealand,” continues Willer.
Next up in our bingo of bivalve benefits come the Eutrophication Potential and Ecosystem Service Value of these filter feeders – rather impenetrable names for some very palatable bonuses.
Eutrophication happens when nutrients washed into watercourses cause a sometimes toxic flush of algal growth, called “algal bloom”. If rain or groundwater washes excess fertilisers used on farmland away, or industrial and urban effluent leaches into the soil, it might find its way into waterways. Algae are better able to quickly capitalise on this phosphorus boom and soon spread over the surface of the water. In extreme cases, this impacts the aquatic ecosystem by blocking out sunlight so other plants die, and killing off wildlife as oxygen levels plummet.
As it’s a massive global problem, Eutrophication Potential is a standardised measure of how different activities harm watercourses. So how do bivalves score? Unlike farmed fish, they actually have a negative impact as they gobble up all that excess algae and other organic particulates cleaning up the waterways and oceans. Bivalves help to protect watercourses from the effects of eutrophication.
This was put to great use to solve a smelly problem in the UK’s Liverpool Docks in 2010, when eutrophication of the harbour waters occurred after the flood gates remained shut following the decline of the Manchester Ship Canal.
Rope-grown bivalves can form habitats for other creatures, like the sea squirts growing here (Credit: Getty Images)
Antony Knights a marine ecologist at the University of Plymouth explains: “Each mussel can filter 1.75 litres of seawater per hour, and given that they colonise at a density of around 500 mussels per metre squared, it becomes clear how they can have a positive impact pretty quickly.”
“By reintroducing mussels and the regular re-opening of the floodgates, the algal blooms in Liverpool Docks were cleared and the problem of eutrophication was solved without any environmentally disruptive intervention,” he continues.
“This is a great example of mussels providing ecosystem services – a biological function that nature provides that is of use to humans – that is performed by mussels and other bivalves,” says Knights.
Mussels are also hugely important to marine ecosystems because they increase biodiversity by acting as ecosystem engineers. “The reefs they form create habitat for other ocean wildlife to colonise and use, such as barnacles, seaweeds and nursery grounds for fish,” he says.
This ecosystem service is being used at an international scale through the World Harbour Project – an initiative designed to invigorate the biodiversity of blank harbour walls across the globe by introducing bivalve colonies which provide habitat for myriad other species in addition to the associated benefits they bring through their filter feeding activity.
Yet here lies the double-edged sword of the ecological niche of bivalves. Because they are filter feeders, whatever is in the water – good or bad – ends up inside them, which is a problem due to the relatively unusual way in which we eat this food.
Bivalves can be grown in a variety of habitats, from rope farms out at sea, to purpose-built frames in brackish water (Credit: Getty Images)
“Essentially they take on the characteristics of the environment in which they grow,” says Rachel Hartnell, a seafood safety scientist at the UK Government’s Centre for Environment, Fisheries and Aquaculture Science (Cefas). This can mean that they concentrate harmful micro-organisms (usually from sewage or agricultural run-off), toxins (from naturally occurring algae, or bacteria) and chemicals present in the water. “That can pose a human health risk especially as we often eat bivalves raw or lightly cooked and eat the whole animal including the gut,” says Hartnell. “Because of this we need to understand the hazards, assess the harm and ensure that programmes are in place to manage any risks before the product reaches the consumer.”
Yet if this risk is managed, bivalves have the potential to contribute to better nutrition and the improvement of the GDP in developing nations in a significant way, due to their high export value compared to traditional fish species. Bivalve farming can also contribute to gender equality in coastal communities, as women are able to work in aquaculture because the farms typically are located close to the coast. This means women can balance earning an income with childcare needs, which is not the case when it comes to offshore fishing.
Such is the potential economic benefit that the Food and Agriculture Organization of the United Nations (FAO) and World Health Organization (WHO) are jointly promoting bivalve farming to more vulnerable nations through training programmes with Cefas, as the FAO’s Rome-based fishery officer Esther Garrido Gamarro explains.
“We piloted joint FAO-WHO guidance on bivalve sanitation projects in Angola, Madagascar, Mozambique and Namibia, and ran a workshop on bivalve sanitation for 15 African nations in November 2019,” she says. “This year we will be doing the same for those Asian and Pacific nations who are yet to develop the sector.”
Bivalve farms can contribute positively to gender equality in coastal communities (Credit: Getty Images)
The fact that we end up eating whatever bivalves eat can be managed in a positive way too: enter vitamin bullets for shellfish. This technique developed by Willer and research partner David Aldridge turns bivalves into supercharged, fortified food to tackle malnutrition.
“We came up with a way of encapsulating nutrients in a feed that is palatable to bivalves,” Willer says. Oysters fed 3% vitamin A and D microcapsules had elevated levels of both vitamins in their tissue after just eight hours. A serving of just two of these vitamin-boosted bivalves provides enough vitamin A and D to meet human dietary RDAs.
“Over two billion people worldwide are micronutrient deficient, scaling-up this technology and application to other bivalve species including clams and mussels could provide a low-cost and highly sustainable mechanism to contribute toward tackling nutrient deficiencies globally,” says Willer.
Across the world there is an estimated 1.5 million sq km (579,000 sq miles) of coastline suitable for growing bivalve shellfish. According to Willer, developing just 1% of this could produce enough bivalves to fulfil the protein requirements of more than one billion people.
Some might find bivalves an acquired taste and others might pale at the thought of cooking with them, but perhaps it’s time to rethink the potential of these filter-feeding animals. Their environmental credentials alone are impressive – the fact they can be nourishing and even empowering is surely enough to give them another go. The bottom line is there is huge potential in this unassuming collection of creatures.