Scotland's Marine Atlas: Information for The National Marine Plan

SUBTIDAL ROCK

Subtidal rock habitats comprise bedrock, boulders and cobbles occurring below low water mark. The communities found here are strongly affected by the availability of light; shallow areas are typically dominated by seaweeds and in deeper areas below the photic zone (about 50m) communities comprise exclusively animals. Other factors influence the composition of communities including wave action, tidal stream strength and salinity.

Seaweed communities are dominated by several species of large kelp, Laminaria digitata, Laminaria hyperborea and Laminaria saccharina. On the east coast of Scotland in the more turbid waters of the North Sea these may extend down only a few metres before a more sparse community of small foliose and encrusting red seaweeds take over. On the outer west coast, and especially on some of the more remote islands such as St Kilda, the kelp can extend down to depths in excess of 40m.

On the east coast of Scotland subtidal rock is rare and occurs only in isolated pockets. There is much more on the west coast with many of the sea lochs having extensive areas of bedrock extending to considerable depths. There are also extensive areas of bedrock to the west of the Hebrides and around Shetland. The extensive, relatively shallow bedrock to the west of the Hebrides support a dense kelp forest that is important in dissipating a lot of wave energy, thus protecting the vulnerable west facing coasts of the Outer Hebrides from erosion.

Broad habitat subtidal rock

Soft coral with dead man's fingers © Paul Kay	Kelp forest understorey of encrusting red algae, sponges, soft corals, etc. © Paul Kay
Orange jewel anemones © Sue Scott

Priority Marine Features

Northern sea fan and sponge communities
White cluster anemone (Parazoanthus anguicomus)
Pink sea fingers (Alcyonium hibernicum)
European spiny lobster (Palinurus elephas)
Tide-swept algal communities
Blue mussel beds (Mytilus edulis)
Horse mussel beds (Modiolus modiolus)
Flame shell beds (Limaria hians)
Serpulid aggregations (Serpula vermicularis)

Northern sea fan and sponge communities

Northern sea fan communities comprise dense aggregations of the northern sea fan ( Swiftia pallida) and the cup coral ( Caryophyllia smithii) together with a wide range of other species occurring on upward-facing and vertical rock surfaces. S. pallida plays host to the nationally rare sea fan anemone Amphianthus dohmii. These communities are characteristic of the moderately exposed rocky habitats of the west coast and Outer Hebrides and occur below ~20m. Ninety-seven percent of current UK records of Swiftia pallida are from the west coast of Scotland and the Outer Hebrides and St Kilda where it is at the southern limit of its range and rising sea temperatures may lead to its reduction or loss.

There are a number of important Northern sea fan and sponge communities:

Caryophyllia smithii and Swiftia pallida on circalittoral rock

Recorded from a number of locations on the west coast and Outer Hebrides. It is a very species rich community supporting a range of soft corals, sea firs, sea mats, sea squirts etc.

Mixed turf of hydroids and large ascidians with Swiftia pallida andCaryophyllia smithii on weakly tideswept circalittoral rock

Sparsely recorded with a restricted distribution in a few sea lochs and inlets on the west coast, especially around the Isle of Mull. It represents a particularly diverse community of dense cup corals, sea firs, sea squirts, sea fans, sponges, soft corals, sea mats and feather stars, as well as sea cucumbers, squat lobsters and fish.

Deep sponge communities

Deep sponge communities are found off the west of the Hebrides and the north-east coast of Shetland as well as a few other locations along the west coast. They are very diverse communities with the majority of UK records from Scotland.

Northern sea fan and sponge communities

Northern sea fan Swiftia pallida

The northern sea fan occurs in many sea lochs and inlets along the west coast of Scotland and the Outer Hebrides, from Loch Laxford in the north to Sound of Jura in the south, and out to St Kilda. It usually occurs at depths of 15-60 m but may occur up to 200m. Virtually all (97%) of UK records are from Scotland and as such is of national importance. It is also host to the nationally rare sea fan anemone Amphianthus dohrnii.

Pressures

These communities are vulnerable to physical damage from bottom fishing gear and siltation from fish farms. It is also possible that with increasing temperature as a result of climate change sea fans could decline or disappear as they are currently on the southern limit of their distribution.

Caryophyllia smithii and Swiftia pallida on circalittoral rock © Paul Kay	Mixed turf with northern sea fan © Paul Kay
Deep sponge communities © Marine Scotland Science	Northern sea fan Swiftia pallida © Paul Kay

White cluster anemone Parazoanthus anguicomus Usually found at depths down to at least 400m but can occur as shallow as 20m. This is a scarce sea anemone found at a number of scattered locations around northern and western Scotland that represent a large proportion of all known records. It may be more widespread and is almost certainly under recorded in deep water. It is considered nationally scarce in the UK. Pressures Populations growing on stones and cold water corals may be threatened by demersal fishing activities. © Paul Kay
Pink sea fingers Alcyonium hibernicum Found on shaded vertical rock surfaces between 1-30m depth. This species has a very restricted distribution and is confined to British waters but it can be common in those locations where it occurs. Almost 50% of global records are from Scotland around western coasts, north to Mull and particularly in the Firth of Lorn, the Outer Hebrides and St Kilda. Pink sea fingers soft coral is considered nationally scarce in the UK and Scottish populations are probably of global importance. Pressures Pink sea fingers may be vulnerable to the impacts of climate change and pollution. © Paul Kay
European spiny lobster Palinurus elephas Usually occurs at depths of 5-70m but can occur down to around 160m. It was once considered common in Scotland. It is now found along the north and west coasts of Scotland with occasional records from the east coast, but numbers have been declining, especially towards the northern extremes of its range. It is now considered scarce. Pressures European spiny lobster is vulnerable to overfishing. European spiny lobster © Paul Kay

Tide-swept algal communities

This includes the tidal rapids of sea lochs, areas of water between islands and also the strong tidal regions found between mainland Scotland and islands (e.g. the Pentland Firth). Large volumes of sea water are forced through narrow channels by tidal processes culminating in fast currents that, in certain locations, can exceed 10 knots. Although the turbulent conditions are largely associated with the top 5m of the water column, some tidal streams can be felt at depths in excess of 30m.

Many of these habitats are found on the west coast around the sills and narrows of sea lochs. These areas are inhabited by a wide range of animals such as sea mats, sea firs, sea squirts, sea anemones and sponges. Where the substrate is particularly stable, kelp parks may develop which in turn provide suitable habitat for a diverse epiphytic fauna and flora of sponges, sea squirts and foliose seaweeds. In deeper water, where light is limited, the sea bed is dominated by animals and it is not uncommon to see dense beds of brittlestars. Tide swept channels are particularly well represented in Scotland and are extremely important for a great diversity of marine organisms including a range of biogenic reef habitats such as horse mussel beds, flame shell beds and maerl.

There are a number of important tide-swept algal dominated communities:

Fucoids in tide-swept conditions

A species rich complex habitat occurring in sea lochs, embayments and between islands in the Inner and Outer Hebrides, Orkney, Shetland and the west and north-west of Scotland.

Halidrys siliquosa and mixed kelps on tide-swept infralittoral rock and coarse sediment

Very few records in Scotland but considered to be under-recorded. It is structurally complex and species rich supporting a diverse community of red seaweeds, sea firs, sea mats, sea squirts, starfish, topshells and sea anemones.

Kelp and seaweed communities in tide-swept sheltered conditions

A large proportion of records in UK are from Scottish sea lochs, the Outer Hebrides, Orkney and Shetland. It is dominated by kelp ( Laminaria digitata, L. hyperborea and L. saccharina) with an understorey of red seaweeds, sponges, sea squirts and sea mats.

Laminaria hyperborea in tide-swept infralittoral mixed substrata

A large proportion of UK records are from sea lochs of west coast mainland and Orkney, Outer Hebrides and Shetland. The kelp canopy supports a diverse range of red seaweeds along with sponges, sea squirts, sea mats and sea anemones, echinoderms and molluscs.

Pressures

Until recently tide-swept channels have remained largely unaffected by human developments and activities. However, seaweed harvesting, scallop dredging and any activity that reduces water flow will adversely affect these habitats.

Fucoids in tide-swept conditions © SNH	Halidrys siliquosa and mixed kelps on tide-swept infralittoral rock and coarse sediment © SNH
Kelp and seaweed communities in tide-swept sheltered conditions © Paul Kay	Laminaria hyperborea in tide-swept infralittoral mixed substrata © Sue Scott

Biogenic reefs

There are a number of habitats created by the accumulation of shelled organisms that transform sedimentary sea bed into 'rocky' habitat. Biogenic reefs are formed by blue mussels ( Mytilus edulis), horse mussels ( Modiolus modiolus), flame shells ( Limaria hians) and serpulid worms ( Serpula vermicularis) all of which are considered priority marine features.

Blue mussel beds
Mytilusedulis

These are typically found on shallow mixed sediments. The mussel beds stabilise the underlying sediment creating a highly complex habitat for a diverse community of plants and animals living on, within or under the bed. The hard substrate they provide in otherwise sedimentary areas increases the overall biodiversity of the area. They have an important role in nutrient cycling and for filtering sea water and removing potentially toxic algae and are an important source of food for a range of other species including wildfowl, seabirds and humans.

There are two subtidal blue mussel bed communities:

Mytilus edulis beds on sublittoral sediments

Found in areas of moderately strong to strong water movement in relatively shallow areas in both full and variable salinities. The mussel beds act to stabilise the sea bed sediments creating a habitat that support a dense community of animals. It is recorded from only a few locations around the coast.

Mytilus edulis beds on reduced salinity infralittoral rock

Typically found in the tide swept entrances to sheltered or very sheltered basins in sea lochs and voes. The majority of UK records are from Scotland.

Pressures

Blue mussel beds are vulnerable to various activities including anchoring, demersal fishing operations and coastal developments and pollution.

Mytilus edulis beds on sublittoral sediments © Paul Kay

Mytilus edulis beds on reduced salinity infralittoral rock © Paul Kay

Horse mussel beds
Modiolusmodiolus

Horse mussels can form dense raised beds and occur down to around 100m depth. They significantly modify the underlying habitat and provide substratum and refuge for a wide variety of species, including brittlestars, featherstars, crabs, whelks, sponges, sea firs, sea mats and sea squirts and are important settling grounds for commercially important bivalve molluscs such as scallops. Scotland holds around 85% of the known horse mussel beds in the UK and are found in sea loch and embayments from Shetland and Orkney and down the west coast and Outer Hebrides.

There are a number of important horse mussel bed communities:

Modiolus modiolus beds with hydroids and red seaweeds on tide swept circalittoral mixed substrata

Found on the open coast and in tide-swept channels between 5-50m depth. The mussel beds support a diverse range of red seaweeds, sea firs, tube worms, molluscs and sea anemones. Recorded from Shetland, Orkney, the Hebrides and west coast sea lochs, the majority of records are from Scotland.

Modiolus modiolus beds on open coast circalittoral mixed sediments

Typically found in current swept areas at depths of 50-100m, the horse mussels support a rich diversity or organisms, especially polychate worms, bivalves and brittlestars. The only known records from Scotland are from Sullom Voe, Shetland and Hoy Sound, Orkney.

Modiolus modiolus beds with fine hydroids and large solitary ascidians on very sheltered circalittoral mixed substrata

Typically at depths of between 5-30m in very sheltered conditions, the mussel beds support a variety of brittlestars, scallops, crabs and gastropods. Found in Shetland, Orkney, the Hebrides and west coast sea lochs, it is not found anywhere else outside Scotland.

Modiolus modiolus beds with Chlamys varia, sponges, hydroids and bryozoans on slightly tide-swept very sheltered circalittoral mixed substrata

Found in very sheltered conditions at depths of between 5-30m, the mussel beds support a variety of organisms including the variable scallop, brittlestars, feather stars, crabs, sponges, sea firs, sea mats, and sea squirts. Only a few records occur from the west coast sea lochs, Skye and Bluemull Sound, Shetland.

Pressures

Horse mussel beds are particularly sensitive to physical damage from mobile fishing gear which may severely damage or even destroy beds.

Modiolus modiolus beds with hydroids and red seaweeds on tide swept circalittoral mixed substrata © Sue Scott	Modiolus modiolus beds on open coast crircalittoral mixed sediments © Crown Copyright Marine Scotland Science.
Modiolus modiolus beds with fine hydroids and large solitary ascidians on very sheltered circalittoral mixed substrata © SNH	Modiolus modiolus beds with Chlamys varia, sponges, hydroids and bryozoans on slightly tide- swept very sheltered circalittoral mixed substrata © SNH

Flame shell beds Limariahians These are most commonly found on mixed muddy gravel and sand in tide-swept narrows in sea lochs. They are formed by the intertwining of the byssus threads of Limaria with seaweed, maerl, shells and stones creating a unique habitat that consolidates the underlying sediments providing a surface to which other animals and plants can attach. They are scarce in the UK, the majority of records are from west Scotland and are considered of national importance. The most extensive beds are in Loch Sunart and Loch Fyne. Large beds once found in the Firth of Clyde have declined considerably. Pressures Flame shell beds are sensitive to damage from mobile fishing gear, dredging and boat anchoring as well as coastal developments resulting in changes in water flow patterns. © Sue Scott

Serpulid aggregations Serpulavermicularis Large clumps of the calcareous tubes of the worm Serpula vermicularis rise from the sea bed to in excess of 1m high and 2m diameter in some of the oldest aggregations. They provide a hard substrate on an otherwise sedimentary sea bed and support a highly diverse epifauna of sponges, sea squirts, brittlestars and sea urchins etc.. Serpulid aggregations are nationally rare, only known from two places in Scotland, Loch Creran and Loch Teacuis, and from only two other places in the world. The reefs in Loch Creran are considered the best example of this habitat in the world. Pressures They are extremely sensitive to physical disturbance and abrasion, loss of suitable substratum and changes in water flow and salinity. They are vulnerable to demersal fishing activities and anchoring of boats and fish farm cages.

© Paul Kay