Öresund-farmed blue mussels
What you see in front of you in the aquarium are ropes for growing blue mussels. Out in the open waters of Öresund there are lots of such ropes, where blue mussels grow to clean the seawater from nutrients. The blue mussels also grow under jetties, on rocks and boulders – and on the pillars that are the foundation of the Öresund Bridge! In 2010, a project was started in Öresund, where they wanted to investigate whether it was possible to grow blue mussels here to obtain a better water quality. Municipalities, Region Skåne and the Marine Education Centre (formerly SEA-U) joined forces to establish large cultivation sites in the water, and it is from this project that we get our blue mussels.
Mussel farming on ropes in Öresund.
Photo: Michael-Palmgren-Marine Education Center
The water around a mussel colony can be much clearer than the surrounding water, as blue mussels are highly efficient filter feeders.
Photo: Michael-Palmgren-Marine Education Center
In other parts of the world, species of mussels are also used in attempts to purify water from pollutants.
Photo: USEPA-Environmental-Protection-Agency
From blue mussel to blue whale
Blue mussels are an example of a filter feeding animal. Filter feeders get their food by straining it out of the water, so that the food gets captured by e.g., thin hairs, tentacles or special teeth. The blue mussel has two tubes (siphons), one that pumps in water and plankton, and one that releases clean water. Between them, plankton and other small loose material in the water attach to the mussel’s ragged filter – and are ingested. A single blue mussel can filter several litres of water per hour. If you swim across a mussel bank, you can see that the water around the mussels is clearer than the surrounding water. But there are also many other species that are important filter feeders. Everything from small animals such as crustaceans, sea squirts and corals, to larger animals such as oysters, jellyfish, flamingos and some fish are filter feeders. But even the world’s largest animal – the blue whale – is actually a filter feeder! Can you think of more animals that filter their food?
All species of mussels are filter feeders, both in seawater and freshwater. This freshwater mussel contributes to improved water quality.
Photo: Alexander-Mrkvicka-CC-BY-SA
Both zebra mussels and bloody-red mysid shrimp, as seen in the picture, are filter feeders.
Photo: Creosoph-CC-BY-SA
Sea squirts are also a very important group of filter feeders. There are experiments with cultivating sea squirts in Sweden to improve water quality.
Photo: Diego-Delso-CC-BY-SA
In the larval stage of their life, some insects are also filter feeders. Most mosquito larvae live as filter feeders in the water before they pupate and transform into adult mosquitoes.
Photo: James-Gathany-CC-BY
One group of sea squirts is called Polyclinidae, and forms colonies. They also filter water to obtain food.
Photo: Diego-Delso-CC-BY-SA
Sponges are animals that also filter feed, by drawing in water with small food particles through their numerous pores.
Photo: Lamiot-CC-BY-SA
Many Anthozoas, such as corals, filter out small particles from the water, which get caught in the tiny tentacles of their polyps.
Photo: Narrissa-Spies-CC-BY-SA
Here you can see various groups of filtering animals - a mussel, corals, and sea squirts.
Photo: Nhobgood-CC-BY-SA
Porcelain crabs are small crustaceans that filter their food using comb-like appendages.
Photo: Nick-Hobgood-CC-BY-SA
Sea pens, a type of coral, are also filter feeders.
Photo: Anne-Hoggett-CC-BY
Brachiopods are a group of animals that resemble mussels in appearance. They use their lophophore, a feeding structure, for filter feeding.
Photo: Alex-Heyman-Public-Domain
Certain species of sea cucumbers are filter feeders. They capture their food using branched tentacles, which they then insert into their mouths.
Photo: Ed-Bierman-CC-BY
Entoproctas are a group of very small filter-feeding animals, resembeling cups or goblets.
Photo: Chloe-and-Trevor-Van-Loon-CC-BY
Horseshoe worms are also a group of very small filter-feeding animals.
Photo: Cricket-Raspet-CC-BY
Moss animals are microscopic and therefore filter out very small particles from the water.
Photo: 2020-Kumar-et-al.-CC-BY-SA
Living water treatment plants in the sea
Mussels, oysters, sea squirts and other filter feeders can be called the water treatment plants of the sea! When the sea receives an excess of nutrients through human release from agriculture, industry and sewage, it is called eutrophication. Eutrophicated water has a high growth of phytoplankton, and risks becoming oxygen-poor when dead plankton decompose. However, if there are plenty of filter feeders in the ocean, they can help eat the plankton in the water. In order for the nutrients not to go back into the water again, when the filter-feeding animals die, it is important to pick them out of the sea. Therefore, mussels, oysters and sea squirts are grown in many places, where they can be harvested to be eaten, made into animal feed, or used for biogas production. Since the mussels have to be picked up from the water in order to improve the environment – it is actually great, environmentally friendly and climate-smart to eat Swedish farmed blue mussels!
The oyster industry is significant in France; here, a French oyster farm is pictured. Oysters are also efficient filter feeders, and oyster farming can be used to counteract pollution in the oceans.
Photo: Alan-Hughes-CC-BY-SA
Even larger animals like fish can live as filter feeders. These Indian mackerel swim with open mouths, filtering larger plankton to eat.
Photo: Par-Dino-van-doorn-CC-BY-SA
The manta ray also sustains itself by filtering plankton.
Photo: Gordon-Flood-CC-BY
The baleen whale group includes the world's largest mammal, the blue whale, and smaller whales such as the Rorquals. Instead of teeth, baleen whales have baleen plates, which are coarse hair-like structures that act as filters. These baleen plates allow baleen whales to filter plankton from the water.
Photo: Colourbox
Certain bird species are also filter feeders, such as various types of flamingos! It is by filtering and consuming small crustaceans that flamingos acquire their pink color.
Photo: Mike-Prince-CC-BY
The blue mussels that cleanse the entire Öresund
Along the west coast of Sweden, mussels have been farmed for many years. There, the salinity is so high that blue mussels grow large enough to be suitable for human consumption. But in the Baltic Sea, where the problems with eutrophication are much larger, it is believed that the crops can mainly be used for animal feed or biogas. Today, Northern Europe’s largest contiguous mussel bank is located in Öresund It is estimated to have a total weight of 200,000 tonnes, and that it cleanses water equivalent to the entire body of water in the Öresund – every day!
In addition to filtering water from nutrients, filter-feeding animals can also cleanse the water for other things that can be harmful. A group of Dutch researchers has been able to show that filter-feeding sponges can cleanse the water of harmful viruses, something that can be used in fish farms to keep the fish healthy.