Marvellous Mud

In 2021, Scotland celebrates the Year of Coasts and Waters, and COAST celebrates 25 years of the community of Arran’s efforts to protect our seas.

During November, while COP26 was well and truly in full swing, we showcased how Scotland’s inshore sediments and muddy habitats help combat climate change and biodiversity loss.

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“Mud, mud, glorious mud….”, the opening words of The Hippopotamus – the song by comedy duo Michael Flanders and Donald Swan – were not written in celebration of anything to do with the sea, but are entirely appropriate for how we should view one of our less well known and probably least celebrated marine habitats.

In the wake of COP26 and last month’s feature focus describing the importance of blue carbon habitats to help absorb carbon, it is timely to be showcasing marine mud habitats. They are home to a wealth of weird and wonderful marine life, have a vital role in the functioning of our seas and are one of our critical blue carbon habitats.

Mud develops in areas where there is little water movement from currents and waves, allowing the sediment particles to settle and consolidate. Around the Clyde, mud can be found on the shore in estuaries and sheltered coastal areas, forming sometimes extensive areas of mudflats, home to many small marine creatures and, as a result, often a haven for feeding birds. This sort of seashore habitat is limited on Arran as there are not that many large, sheltered, intertidal marine inlets around the island, Lochranza being the exception. But there is a lot of mud in deeper water around the coast and it is one of the particularly important habitats in the South Arran Marine Protected Area.

The importance of Mud

Mud is composed of much finer sediment particles than the sand and gravel habitats, and can range from slightly coarse sandy mud to full blown soft, gloopy mud, with a range of marine species inhabiting the varied sediment types.  As with sands and gravels, animal species dominate mud habitats because there is little hard material for marine plants to attach to and it is also often too deep for there to be sufficient light for seaweeds to survive. Our muddy habitats are often described as ‘burrowed mud’ because of the abundance of burrowing marine life that lives within them.

Much of the marine life of mud habitats is hidden from view with large numbers of burrowing  animals such as worms, brittlestars, crabs, shrimps, heart urchins and even fish that live within the mud itself. Animals are also found living on or at the surface of the mud, and these include tube worms, other fish, and anemones and their relatives.

Special Species

Some of Scotland’s most spectacular seabed marine life can be found living in mud.

*Crustaceans* One of the better known mud dwellers is the large and commercially important orange-coloured langoustine or prawn which lives in shallow burrows in marine mud. Another distinctive species is the mud runner crab, with its stalked eyes and extraordinarily long arms.

*Fireworks anemone* This aptly named species is generally found in more sheltered parts of sea lochs, and lives in a long tube that is buried in the mud. The long and numerous, predominantly white or white and brown striped tentacles can extend up to 30cm across, and give the impression of a firework exploding out of the seabed; they almost seem to glow when seen against the dark water and dull-coloured mud.

*Sea pens* These are colonial animals so named because of their resemblance in shape to quill pens. The slender sea pen tends to be the most common and is present within Lamlash Bay and elsewhere around Arran. They have rows of small anemone-like polyps that extend out from a central stem, enabling the animal to feed by capturing small particles of food from the surrounding water. Slender sea pens grow up to 60cm long, whereas the tall sea pen reaches heights of over 2m. Nestled on the upper part of the stem of tall sea pens a small crustacean called Astacilla longicornis may be found. In contrast to the size of the tall sea pen, Astacilla only grow a few cm long but despite this small size they show extraordinary care for their developing young, with the females carrying the small juveniles on their body and antennae. The third sea pen species found in Scotland’s marine mud is the phosphorescent sea pen. The smallest of our sea pen species, it is stouter and more fleshy, and gets its name from the fact that it can emit luminescent light from the anemone-like polyps when disturbed. 

The Fries’ Goby (Lesueurigobius friesii) is a rare fish that shares a burrow with the commercially important langoustine. Although only growing up to 13cm, these fish feed on worms, small crustaceans and molluscs. They are identifiable by their yellowish-brown spots on their body and dorsal fins.

Human Benefits

The burrowing activities of mud creatures helps exchange and cycle oxygen, nutrients and minerals between the water and the sediment, making mud an extremely productive habitat that provides food and shelter for many species, as well as feeding areas for commercial fish species such as cod, haddock and whiting.

As we talked about last month, muddy sediments are of great benefit to humans as they help battle climate change. They are by far the most important blue carbon habitat of them all! A paper by Smeaton et. al published this March estimates that 524.4 million tonnes of organic and 2582 million tonnes of inorganic carbon is held within the top 10cm of surface sediments in the UK; the carbon trapped within them is unevenly distributed with muddy inshore and coastal sediments being the hotspots for organic carbon storage. The paper states “Within the well-defined organic carbon (OC) hotspots, muddy sediments store the greatest quantity of OC; the muds offer potentially valuable opportunities for targeted future management and protection of sedimentary Carbon stores within the UK Exclusive Economic Zone.”

Threats to mud and sediments

Burrowed mud habitats are vulnerable to a number of human pressures but physical disturbance and pollution are particularly problematic. High levels of nutrients or organic material can cause the mud to become very low in oxygen, to a point where the characteristic mud communities can no longer survive. Many of the larger mud species are considered to be long-lived and slow growing and so can be severely affected by physical impacts from activities such as bottom trawling which also affects the overall abundance and diversity of the marine life communities. While these impacts have been known of for many years, there is now also increasing focus on the impact of bottom trawling on blue carbon habitats such as mud, because the physical impact of trawling releases carbon from the seabed sediment.

Get involved to help our muddy ecosystems

Burrowed mud is identified as a Priority Marine Feature, and is recognised as an OSPAR threatened and declining habitat. Scotland has particular responsibilities for looking after mud habitats – it has some of the largest areas of burrowed mud in Europe and the majority of mud habitat that is found in UK waters. The habitat is protected in eight locations around Scotland, by a suite of Marine Protected Areas, of which South Arran is one. MPA’s have an important role to play in helping look after our marine mud habitat, although action is also needed on a much larger scale to ensure that our biologically rich and important blue carbon marine mud habitat is protected into the future. A modern inshore limit is one such measure that would provide vital protection from physical disturbance:

Cover photo courtesy of Lucy Kay