Holywell Bay Cornwall: Amazing mussel ecosystems

Holywell Bay Cornwall: Amazing mussel ecosystems

The water was azure blue and the sand was sparkling gold in the sun at Holywell beach that day. We were there hunting for sea glass but we found amazing mussel ecosystem engineers instead. 

Hollywell Bay in Cornwall has a beautiful sandy beach with clear blue water. On our hunt for beautiful sand-worn glass, we discovered that the rocks on the west side of the beach were completely covered in pristine deep blue mussels. Tens of thousands of them had covered the rocks they were attached to in a carpet of deep purple and blue.

Back home near Boat Cove, we have mussel beds, but they’re rarely visible above the water. It was an amazing sight to see these blue mussel colonies contrasting in colour against the glistening quartz sand. I couldn’t help but take photos of them. 

Later, we were looking at the pictures we’d taken that day. They made us curious to know more about these beautiful sea creatures. This is what we learned about mussels.

What is a mussel?

Mussels are molluscs (mollusks) and are members of the family of invertebrates (animals with no backbone) called phylum Mollusca. 

Mussels are described by scientists as ‘bivalves’. This name is used to describe animals that have two shells hinged at the back. Other common marine bivalves are cockles, clams and oysters.

Marine mussels have a soft orange body inside a hard, but often brittle, oval-shaped shell. 

Seawater mussels belong to the marine family Mytilidae, and freshwater mussels belong to the family called Unionidae. Other common marine invertebrates related to mussels are octopuses and squid.

Mussels are natural water-filtering pumps. They clean water like a fish pond water filtering system. Mussels can filter 30 or more litres of water per hour. Incredible! Watch it happen in this video from the DEESTUARY project:

Mussels at work: A Time Lapse Demonstration.

What do mussels eat?

Mussels feed by filtering the water they live in for tiny creatures and other water-borne food. Marine mussels filter and eat the tiny plankton creatures from seawater. They also filter other particles in the water and can use them to build and repair their shells.

What does a mussel look like?

Marine mussels are usually wedge-shaped or pear-shaped. The shells can be smooth or ribbed or can sometimes have a hair-like covering. Mussels come in many colours and often have dark blue, dark green, or brown outer shells. The inside of a mussel shell is typically smooth and pearlescent, shiny like pearls.

How big is a marine mussel?

Marine mussels can range from 5 to 15 centimetres in length, that’s about 2 to 6 inches.  

How long does a mussel live?

A typical mussel’s lifespan is 2-3 years, but sometimes they can live to 10 years or more.

How do mussels stay still in the sea?

Mussels live near the shore, referred to as the intertidal zone, and to avoid being washed away by the tide, they need to hold on to solid objects.  

Mussels attach themselves to rocks, wood or metal structures using their tufts of anchoring threads, often referred to as their ‘beard’. 

These threads have a soft, stretchy core which has a tough outer coating which makes them strong but still flexible. This means the mussel can withstand rough seas and strong tides without being washed away.

These mussel ‘beards’ are made up of byssus threads. Byssus threads are incredible as they are made of a proteinaceous (made of protein) substance which means they can self-heal if they become damaged. Other sea creatures also contain self-healing proteinaceous substances such as barnacles.

Which sea animals eat mussels?

Many sea birds eat mussels when the tides are low enough to expose the mussel beds or if storms have washed the mussels onto the shore. Gulls, oystercatchers, and ducks commonly eat mussels. Marine creatures like flatfish, crabs, starfish and dog whelks also feed on them.

How does a mussel make its shell?

A mussel grows its shell in stages. The mussel will draw in carbonates (rock-like material) and calcium (also contained in bone) in the seawater and convert these into a calcium substance called amorphous calcium carbonate (ACC).

A mussel creates and repairs its shell in a similar way to how humans grow and repair bones.

Blue mussel - blue and silver shell from the top - two halves open from the hinge side with yellow body inside - two halves open from the opening side with yellow body showing

Mussels create the ACC calcium substance by filtering seawater through their body tissue. They then move it through their body and convert it into a harder substance called crystalline calcium carbonate (CCC). This CCC is what mussel shells are mostly made of. However, mussels are very clever, they keep some ACC calcium material in their body which they can use to repair their shell if it’s damaged. How clever is that!

How do mussels reproduce?

Female mussels produce vast numbers of eggs! A 7 cm (3 inch) mussel can produce 5 million eggs, and larger mussels can produce up to 40 million eggs. Male mussels produce 10,000 times more spermatozoa than the volume of eggs produced by females. 

The females release their eggs as plankton only hundredths of a millimetre in size. Approximately 95% of the eggs die or are eaten by other sea life. 

Surviving eggs will settle on rocks, seaweed, or other hard object and grow into baby mussels that are only a couple of millimetres long. These miniature baby mussels are called spat. The number of spat that survives every year varies significantly. 

The spat then begin to grow their byssus ‘beards’, which they use to attach themselves to the objects they are living on. Once a mussel is fixed by its byssus threads, it can grow by up to 1 cm (1/3rd inch) a month.

Can you farm mussels?

Humans have farmed mussels throughout history, first recorded in Europe in the 13th century (1200 onwards). Commercial mussel farming developed into a large industry at the beginning of the 20th century (1900 onwards).

Spain and Italy were the first countries to create large mussel farming industries and continue to maintain large mussel farms. In 2022, China was one of the largest producers of farmed mussels. The UK, Holland, Chile, and New Zealand also have large mussel farming industries.

There are four main types of farmed mussels: 

The blue mussel (Latin name: Mytilus Edulis). 

The blue mussel is the one that we can find here near Boat Cove, Dawlish, UK. Its colour can also be purple. Blue mussels can grow up to 10cm long in the wild, but farmed mussels are harvested at smaller sizes as shown in the french mussel farm image above.

The Mediterranean mussel (Latin name: Mytilus galloprovincialis) 

The Mediterranean mussel is one of the larger types, often reaching 15 cm (6 inches). This mussel has a tougher shell than other farmed mussels like the blue mussel. 

This mussel prefers warmer waters which is why it is farmed in the mediterranean sea, which is warmer than the north Atlantic. The Mediterranean mussel is the most common mussel that can be found from the northern coast of Spain down to the Mediterranean sea and east as far as the Black Sea. 

The New Zealand greenlip mussel (Latin name: Perna canaliculus) 

This southern hemisphere mussel lives all along the coast of New Zealand. New Zealand has a major mussel farming industry around its northern island. 

The New Zealand greenlip mussel can reach 20 cm (8 inches) long. This means it is a much bigger mussel than the European species. 

The Chilean mussel (Latin Name: Mytilus chilensis)

The Chilean mussel (Latin Name: Mytilus chilensis) top view of outside of shell
Chilean mussel. https://commons.wikimedia.org/

The Chilean mussel is commonly farmed in South America. This mussel grows to a similar size to the Mediterranean mussel.

How do you farm mussels?

Mussel farmers collect baby mussels (spat) every year and use them to grow crops of mussels. The farmers place ropes over existing mussel beds for spat to settle on. Spat is also collected by harvesting seaweed which the spat has settled on. 

The most popular method of growing mussels is by using ropes suspended in the water. These ropes are fixed below the water by hanging them from buoys. They are then left for 12-24 months to allow the spat to grow into mussels big enough to be harvested.

Over time these ropes get heavy with the weight of adult mussels. A rope can have 20kg of mussels per metre growing on it. 

Mussels can also be grown using rafts that can produce as much as 30 tonnes per raft. 

The seabed can also be used to farm mussels. Growing mussels beds directly on the seafloor in shallow water is a popular farming method around the world. 

Mussel farming requires very little management as mussels don’t need to be fed like a land-based crop. Mussels get all they need to grow by filtering seawater. 

Do mussels only live in the sea?

There are approximately 1,000 types of freshwater mussels which are called naiads. Freshwater mussels can be found living in streams, lakes, and ponds all over the world. 

Mussels are great at cleaning the water that they live in. They can filter and clean dirty water by eating the food in it and converting dissolved material into shell-building material. 

Mussels are so good at cleaning the environments they live in that they’re now being used to improve inland water ecosystems. You can read about one of these mussel-growing environmental programmes happening in the Delaware Estuary in the USA. Mussels For Clean Water Initiative (MFCWI).

Partnership for the Delaware Estuary logo

Mussels really are amazing ecosystem engineers!  

Thanks for reading 

Main photo Hollywell Bay in Cornwall has a beautiful sandy beach with clear blue water. 

Go Deeper

You can learn more about mussels from the links below:

Segmented molecular design of self-healing proteinaceous materials. 2015. By V. Sariola, A. Pena-Francesc, H. Jung. Available at www.nature.com

Mussels. The Fish Society. 2022. Available at: www.thefishsociety.co.uk

Mussel. Scientific name: Mytilus edulis. The Wildlife Trusts. 2022. Available at: www.wildlifetrusts.org

Elements of Marine Ecology (Fifth Edition). Francis Dipper. 2022, Pages 257-318. Available at: sciencedirect.com

Mussels. British Sea Fishing. 2022. Available at: britishseafishing.co.uk

What is a freshwater mussel? Freshwater Mollusk Conservation Society (FMCS), 2022. Available at: molluskconservation.org

Freshwater Mussels. DEESTUARY. 2022. Available at: delawareestuary.org

DEESTUARY is the Partnership for the Delaware Estuary, a National Estuary Program, leads science-based and collaborative efforts to improve the tidal Delaware River and Bay, which spans Delaware, New Jersey, and Pennsylvania.


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