Burrowed mud

Deep, soft mud is a surprising home for some special marine animals, like Norway lobster, fireworks anemone and sea pens.

On open coasts, muddy seabeds are found only in deeper water below 50m. But mud-dwelling creatures can be seen in much shallower water in the sheltered basins of sea lochs.

Fine mud particles only drop out of suspension and gather on the seabed where there’s very little water movement. So this kind of seabed is mainly found in deep water or sheltered conditions. Muddy seabeds are found at locations around Britain’s coast, but Scotland has most of them – on our west coast and in the northern North Sea.

But the inner basins of deep, fjordic sea lochs and some Shetland voes have a version of this habitat that’s special to Scotland. At the bottom of these basins are stable conditions similar to those of much deeper water on more open coasts. Animals more typical of deeper depths can live here as shallow as 15m.

Sea loch mud specialities

Sea pens, which are related to sea anemones and corals, are perhaps the most characteristic and obvious animals in the mud. Anchored by a bulbous base, the sea pen’s many small polyps trap plankton and other food particles in the water.

Sea pens found in the mud include:

  • slender sea pen (Virgularia mirabilis) – the most common sea pen species
  • phosphorescent sea pen (Pennatula phosphorea) – it glows when disturbed
  • tall sea pen (Funiculina quadrangularis) – common in very deep water, but occurs as shallow as 25m in some fjordic sea lochs

The tall sea pen can form white, feathery ‘forests’, and is sometimes host to the curious deep-water brittlestar (Asteronyx loveni). The brittlestar hangs on with one or two of its five long, coiled arms, and uses the others to catch food.

Another speciality of sea loch mud is the fireworks anemone.

Burrows and mounds between the sea pens are clues that other creatures live within:

  • Norway lobster (Nephrops norvegicus) – it makes a simple U-shaped burrow to sit in during the day, coming out at night to forage for food
  • Fries’ goby (Leseurigobius friesii) – this small fish may share a Norway lobster’s burrow
  • mud shrimps (Callianassa subterranean) and (Calocaris macandreae), and the angular crab (Goneplax rhomboids) – make more complex burrows with tunnels and side chambers
  • burrowing brittlestars (Amphiura spp.) – may be abundant in places, with just the ends of their long, slender arms showing above the mud
  • echiuran worms – these form large mounds, making characteristic scoop marks on the mud surface
  • giant naked foraminiferans – these single-celled organisms lurk in the mud and look like stringy cottage cheese

The activities of all these animals help to aerate the mud, which would otherwise become anoxic (lacking in oxygen) just below the surface.

On mud at 65m in Loch Goil, there is an unusual community with abundant sea squirts, including the rare Styela gelatinosa, along with terebellid worms and the seven-rayed scallop Pseudamussium septemradiatum. This odd assemblage of animals may be a relict from the last ice age.

Burrowed Mud map

Conservation of burrowed mud habitat

Burrowed mud habitats are vulnerable to a number of human pressures, but especially physical disturbance and pollution. For example, high levels of nutrients or organic material can cause the mud to become very low in oxygen, to the point that the typical communities can no longer survive. Disturbance from large scale engineering operations or from towed fishing gears can also affect the quality of burrowed mud seabeds.

Burrowed mud has therefore been one of the protected features included in nature conservation marine protected areas (such as the South Arran, Wester Ross and Small Isles MPAs). Elsewhere, the management of activities such as fishing and aquaculture can play a role in conserving this important habitat.