Marine Sublittoral (Subtidal)

Snakelocks Anemone - Anemonia viridis (R. T. Mills)

Habitat categories and codes
S Sublittoral shores (subtital)	SR Sublittoral rock	SR1 Exposed infralittoral rock
		SR2 Moderately exposed infralittoral rock
		SR3 Sheltered infralittoral rock
		SR4 Exposed circalittoral rock
		SR5 Moderately exposed circalittoral rock
		SR6 Sheltered circalittoral rock
	SS Sublittoral sediment	SS1 Infralittoral gravels and sands
		SS2 Infralittoral muddy sands
		SS3 Infralittoral muds
		SS4 Infralittoral mixed sediments
		SS5 Circalittoral gravels and sands
		SS6 Circalittoral muddy sands
		SS7 Circalittoral muds
		SS8 Circalittoral mixed sediments

This section includes habitats of the seabed in the sublittoral, or subtidal zone that extends seawards from the MLWS tide mark. It incorporates the sublittoral fringe, an area that is occasionally emersed during the lowest spring tides. Elsewhere the sublittoral zone is permanently submerged by water. The sublittoral section is subdivided into habitat categories on the basis of factors such as vertical zonation (infralittoral and circalittoral zones), substratum type (rock or sediment), and levels of exposure to wave action and tidal currents. Mixed substrata of rock and sediment are not differentiated as a separate habitat category but are included instead with rock. Note that the sublittoral sections of any coastal constructions, including artificial substrata of built stone, concrete, metal, wood or plastic, should be considered in the coastland section (CC1-2) . Any estuarine channels that retain water at low tide (freshwater or brackish) should be categorised under sublittoral sediment. Tidal rivers - CW2 and lagoons and saline lakes - CW1 are treated as separate categories in the coastland section.

The term 'exposure', as used in relation to the marine environment, refers to wave action or tidal currents, and not to air. Instead, the terms 'emersion' (exposed to air) or 'submersion' (submerged by water) are used.

Sublittoral Rock

This section covers habitats of the rocky seabed in the sublittoral, or subtidal zone. Rock includes bedrock and stable accumulations of loose rock (boulders, cobbles and pebbles - see Table 2, page 80), in addition to subtidal peat deposits. There is no separate category for mixed substrata of rock and sediment; these are included instead with sublittoral rock. Sublittoral rock is subdivided into habitat categories on the basis of vertical zonation, and the degree of exposure to wave surge and tidal currents. The infralittoral is the shallow sublittoral zone that extends seawards from the MLWS tide mark, to the lower limit of kelp growth, or of dense seaweed growth, as determined by the depth of light penetration through the water (usually about 10-25 m below chart datum). It includes the sublittoral fringe, or extreme lower shore that is occasionally emersed. Elsewhere, the infralittoral zone remains permanently submerged.

The circalittoral is the deeper sublittoral zone where the rocky seabed is dominated by communities of animals, and plants are occasional or absent. It extends downwards from depths of 10-25 m below chart datum. The character of the circalittoral fauna varies enormously and the species present are determined by factors such as wave action, tidal stream strength, salinity, turbidity, sand scour conditions and rock topography. Communities are typically dominated by mixtures of a range of different species, as opposed to single species. Salinity can range from full sea water to variable or reduced salinity conditions.

Built stone and other artificial structures, including fish farms, in the sublittoral zone should be considered under coastal constructions (CC1-2) . Note that artificial substrata generally support the same communities of marine species as natural substrata. Sea caves are not treated as a separate habitat in the sublittoral section but should be included in the appropriate sublittoral rock category. They are characterised by species that are tolerant of wave surge. Vertical rock faces, overhangs and caves are characterised by animal-dominated communities that may occur across a range of exposures. Communities of anemones, soft corals ( Alcyonium digitatum ), short bryozoan turfs and red seaweeds typically dominate vertical rock faces and overhangs in the infralittoral zone. Circalittoral vertical rock faces support a varied fauna depending on the degree of water movement. Sponges, anemones and cup-corals are particularly characteristic of overhangs and caves in the circalittoral zone.

Links with Annex I : Sublittoral rock categories may contain examples of the annexed habitats, 'reefs (1170)' and 'submerged or partially submerged sea caves (8330)'.

Exposed infralittoral rock SR1

Saccorhiza polyshides with foliose red algae (EcoServe)

This category includes areas of the rocky seabed in the infralittoral zone that are exposed or extremely exposed to wave action, or to strong tidal currents. Kelps usually dominate. The sublittoral fringe is characterised by dense growth of Dabberlocks ( Alaria esculenta ) or, occasionally, Furbelows ( Saccorhiza polyschides ). Below this, Laminaria hyperborea dominates and, as depth increases, kelp forest (dense growth) thins out to become kelp park (sparse growth). An understorey of foliose red or brown seaweeds is characteristic; these may form a dense band below the main kelp zone at the bottom of the infralittoral. Sea urchins ( Echinus esculentus ) and Common Mussel ( Mytilus edulis ) may also be present. Animals become more prominent and replace kelps in areas with the strongest water movement. Surge gullies and shallow sublittoral caves (with vertical bedrock walls and overhangs) are typically colonised by communities of encrusting or cushion sponges, colonial ascidians, short turf-forming bryozoans, anemones, barnacles and calcareous tubeworms. Red seaweeds may be present if there is sufficient light, but kelps are absent. The floors of caves and gullies that are abraded and scoured by moving boulders, cobbles and pebbles are usually colonised by coralline crusts and barnacles.

Moderately exposed infralittoral rock SR2

This category includes areas of the rocky seabed in the infralittoral zone that are subject to moderate wave exposure, or to moderately strong tidal currents on more sheltered coasts. Kelps usually dominate; Laminaria digitata typically forms a dense narrow band in the sublittoral fringe above Laminaria hyperborea forest or park. Kelp generally thins out with depth. Seaweeds, particularly red types such as Delesseria sanguinea, Phycodrys rubens, Corallina officinalis and Palmaria palmata , are common in the understorey and the variety of delicate filamentous types increases as exposure decreases. Characteristic animals include hydroids ( Obelia geniculata ), calcareous tubeworms ( Pomatoceros triqueter ), starfish ( Asterias rubens ) and top shells ( Gibbula cineraria ). Grazing by sea urchins ( Echinus esculentus ) may significantly alter and impoverish these communities. If grazing pressure is intense, there may be little other than barren algal-encrusted rocks beneath the kelp, with animals confined to crevices and spaces beneath boulders that are too small to be penetrated by sea urchins. Dense aggregations of brittlestars ( Ophiothrix fragilis, Ophiocomina nigra ) will give rise to similarly barren communities because of their smothering effect. Communities may also be influenced by disturbance if the substratum is unstable, or by abrasion or covering by coarse sediments. Where this occurs, the kelps, Laminaria saccharina and Saccorhiza polyschides , or Sea Oak ( Halidrys siliquosa ) are often prominent and replace the typical Laminaria hyperborea communities.

Sheltered infralittoral rock SR3

This category includes areas of the rocky seabed in the infralittoral zone that are wave and tide-sheltered, and subject to silting. Sheltered infralittoral rock is characterised by kelp-dominated communities with Laminaria hyperborea and/or Laminaria saccharina. Among the other associated seaweeds, there is usually a high proportion of silt-tolerant and delicate filamentous types. Grazing by sea urchins ( Echinus esculentus ) and chitons ( Leptochiton spp.) may impoverish these communities, particularly in the lower infralittoral zone. In the turbid waters of estuaries, kelps and seaweeds are usually replaced by animal-dominated communities comprising barnacles ( Balanus crenatus ), Common Mussel ( Mytilus edulis ) and shore crabs ( Carcinus maenas ).

Exposed circalittoral rock SR4

This category covers the rocky seabed in the circalittoral zone on wave exposed coasts or in areas with strong tidal currents, including tidal channels and tide-swept narrows. It is characterised by robust faunal communities that are capable of withstanding strong water movement. Low-lying crusts, cushions and turfs typically dominate but communities of large soft corals such as Alcyonium digitatum, and sponges with massive growths ( Haliclona viscosa, Cliona celata, Pachymatisma johnstonia ) may also be present. Jewel Anemone ( Corynactis viridis ), Devonshire Cup-coral ( Caryophyllia smithii ), and a range of crustose or low-growing bryozoans, anemones, sponges and hydroids are prominent on wave exposed rock. Current-swept and sand-scoured areas are usually colonised by hydroids ( Sertularia spp.), bryozoans ( Flustra foliacea ), anemones ( Urticina felina ), barnacles ( Balanus crenatus ) and calcareous tubeworms ( Pomatoceros triqueter ). In areas subject to very strong tidal streams, communities are characterised by various combinations of hydroids ( Tubularia indivisa ), barnacles ( Balanus crenatus ) and cushion-forming sponges ( Halichondria spp.). Tide-swept steep or vertical rock often supports dense masses of soft corals ( Alcyonium digitatum ), in addition to large sponges ( Pachymatisma johnstonia, Cliona celata ) and hydroids ( Tubularia indivisa ). Grazing by sea urchins ( Echinus esculentus ) can reduce the diversity of communities and lead to a proliferation of encrusting species.

Moderately exposed circalittoral rock SR5

This category covers areas of the rocky seabed in the circalittoral zone that are subject to moderate wave exposure, or to some degree of tidal current in more sheltered situations. Communities vary considerably depending on factors such as water quality (the amount of suspended silt or sand), tidal current strength, topography and rock type. Mixed faunal turfs in areas with moderately weak currents and low levels of suspended silt are characterised by branching and cup sponges, hydroids, anemones, feather stars, sea fans, axinellid sponges and turf-forming bryozoans. Areas influenced by sand typically support a prominent turf of bryozoans ( Flustra foliacea ) and hydroids ( Sertularia spp., Hydrallmania falcata ). Moderately exposed circalittoral rock or mixed substrata may also support honeycomb reefs with Sabellaria spinulosa, beds of mussels (dominated by Mytilus edulis, Musculus discors or Modiolus modiolus ), or dense beds of brittlestars (typically dominated by Ophiothrix fragilis and/or Ophiocomina nigra ). Dense beds of a variety of ascidians ( including Polycarpa and Molgula spp.) may colonise rocks where silting occurs. High grazing pressure by sea urchins ( Echinus esculentus ) reduces the extent and diversity of faunal communities.

Sheltered circalittoral rock SR6

This category covers areas of the rocky seabed in the circalittoral zone that are sheltered from wave action and significant tidal currents. Under still and sheltered conditions, silting is common. The rock is often well grazed and is typically dominated by encrusting algae ( Aglaozonia, Pseudolithoderma extensum, coralline crusts ). Large solitary ascidians ( Ascidia spp., Ascidiella spp., Corella parallelogramma, Ciona intestinalis ) are usually prominent and brachiopods ( Neocrania anomala, Terebratulina retusa ) may occur. On mixed substrata, there may be clumps or more extensive beds of Horse Mussel ( Modiolus modiolus ).

Sublittoral Sediments

This section includes habitats of the seabed in the sublittoral, or subtidal zone where the substratum comprises mainly unconsolidated material. Sediments may range in size from shingle (mobile cobbles and pebbles) to fine muds; mixtures of sediments of various different grades may also occur. For details of sediment particle sizes see Table 2 (Page 80). Sublittoral sediment is subdivided into habitat categories on the basis of vertical zonation and sediment type. The infralittoral is the shallow sublittoral (subtidal) zone which extends seawards from the MLWS tide mark, to the lower limit of extensive plant growth as determined by the depth of light penetration through the water (usually about 10-25 m below chart datum). With the exception of the sublittoral fringe, or extreme lower shore, which is occasionally emersed, there is permanent cover of water. The circalittoral is the deeper sublittoral zone that is characterised by communities of animals, and where plants are occasional or absent. It extends below the lower limit of the infralittoral zone, beginning at a depth of about 10-25 m below chart datum. Note that zonation of the sublittoral is less distinct on sediment than it is on rock. Salinity conditions can range from full sea water to freshwater in the case of some estuarine channels.

Infralittoral gravels and sands SS1

This category covers gravels and sands in the infralittoral zone where conditions range from exposed to sheltered. They occur along sections of the open coast and in sea inlets. Infralittoral gravels and sands support faunal communities that are influenced by high levels of disturbance from wave action or tidal currents, or by communities of loose-lying coralline algae known as maerl beds. In shallow water, coarse clean gravels are colonised by a robust fauna of venerid bivalve molluscs ( Spisula elliptica ), anemones ( Cerianthus lloydii, Edwardsia timida, Halcampa chrysanthellum ) and brittlestars ( Ophiura spp.), while coarse sands contain amphipod crustaceans ( Bathyporeia spp.) and robust polychaete worms ( Nephtys cirrosa, Lanice conchilega ). Maerl beds in coarse clean sediments on the open coast or in tide-swept channels are usually dominated by Phymatolithon calcareum and/or Lithothamnion coralloides. Eelgrass ( Zostera spp.) beds may also be present. In estuarine conditions, faunal communities are characterised by robust species that are tolerant of brackish water conditions including amphipod crustaceans, polychaete worms and mysid shrimps.

Links with Annex I: This category may contain examples of the annexed habitats, 'sandbanks which are slightly covered by sea water all the time (1110)' and 'estuaries (1130)'.

Infralittoral muddy sands SS2

This category covers muddy sands in the infralittoral zone where conditions range from exposed to sheltered, and tidal streams are weak. Muddy sands are characterised by a variety of animal-dominated communities including, in particular, polychaete worms ( Arenicola marina, Lanice conchilega ), bivalve molluscs, burrowing brittlestars ( Amphiura spp.) and sea urchins ( Echinocardium cordatum ). Other characteristic species include razor shells ( Ensis spp.), hermit crabs ( Pagurus bernhardus ) and starfish ( Asterias rubens ). Muddy sands may support beds of oysters ( Ostrea edulis ) or maerl ( Phymatolithon calcareum, Lithophyllum spp.). Seagrass beds with eelgrasses ( Zostera spp.) and Beaked Tasselweed ( Ruppia maritima ) may also be present. The latter occurs only in estuarine or reduced salinity conditions.

Links with Annex I: This category may contain examples of the annexed habitats, 'sandbanks which are slightly covered by sea water all the time (1110)' and 'estuaries (1130)'.

Infralittoral muds SS3

This category covers sandy muds and soft muds of the open coast or marine inlets where conditions range from fully marine to estuarine, and where tidal streams are weak. Infralittoral muds are characterised by communities of polychaete and oligochaete worms and bivalve molluscs ( Abra spp.); these may occur together with seapens ( Virgularia mirabilis ), anemones ( Cerianthus lloydii, Sagartiogeton spp.), burrowing brittlestars ( Amphiura spp.), sea slugs ( Philine aperta ) and sea cucumbers. In estuarine situations with variable and reduced salinity, infralittoral muds typically support communities of oligochaete ( Tubificoides spp.) and polychaete ( Aphelochaeta marioni, Nephtys hombergii ) worms. Seagrass beds similar to those of infralittoral muddy sands - SS2 may also develop on infralittoral muds.

Links with Annex I: This category may correspond to the annexed habitat, 'estuaries (1130)'.

Infralittoral mixed sediments SS4

This category covers areas of the seabed in the infralittoral zone that comprise various mixtures of sediments (gravel, sand and mud), often with shells and stones on the surface. The varied substratum may result in a wide variety of plant and animal communities being present. Under sheltered conditions, seaweeds can attach to shells and stones on the seabed, or grow as loose-lying mats on the sediment surface. Characteristic species include the kelp, Laminaria saccharina, the bootlace weed, Chorda filum, and various other red and brown seaweeds including many filamentous types. Maerl beds with Lithothamnion and Phymatolithon spp. may develop on muddy gravels. Oyster beds with the native species, Ostrea edulis, may occur on mixed sediments in very sheltered conditions. Faunal communities of bivalve molluscs and anemones may also be present depending on the nature of the sediment. In estuarine conditions, mixed sediments are often quite species-rich and support epifaunal communities of slipper limpets ( Crepidula fornicata ) and Common Mussel ( Mytilus edulis ), as well as infaunal communities of polychaete worms and amphipod crustaceans.

Links with Annex I: This category may correspond to the annexed habitat, 'estuaries (1130)'.

Circalittoral gravels and sands SS5

This category includes gravels and sands of the seabed in the circalittoral zone on coasts that are exposed or moderately exposed to wave action, and where tidal currents range from strong to weak. Characteristic components of the fauna include razor shells ( Ensis spp.), brittlestars ( Ophiura albida ), sea cucumbers ( Neopentadactyla mixta ), polychaete worms, scallops ( Pecten maximus ), hermit crabs ( Pagurus bernhardus ) and starfish ( Astropecten irregularis ).

Circalittoral muddy sands SS6

This category includes muddy sands of the seabed in the circalittoral zone that are characterised by a variety of animal-dominated communities including, in particular, polychaete worms, bivalve molluscs ( Abra alba, Pecten maximus, Nucula nitidosa ), sea urchins ( Echinocardium cordatum ), brittlestars ( Amphiura and Ophiura spp.) and low densities of seapens ( Virgularia mirabilis ). Other common components include starfish, tube anemones and hermit crabs.

Circalittoral muds SS7

This category includes areas of the muddy seabed in the circalittoral zone where communities of burrowing megafaunal species such as prawns ( Nephrops norvegicus ) often become established under relatively stable conditions on the open coast or in marine inlets. Seapens ( Virgularia mirabilis ), brittlestars ( Amphiura spp.), auger shells ( Turritella communis ) and tube anemones ( Cerianthus lloydii ) may also be present.

Circalittoral mixed sediments SS8

This category includes areas of the seabed in the circalittoral zone where sediments comprise various mixtures of gravel, sand and mud, often with shells and stones on the surface. As with infralittoral mixed sediments - SS4 , the mix of hard substrata (stones and shells) and finer sediments means that both epifaunal and infaunal communities are present with the result that species diversity can be high. The development of honeycomb reefs ( Sabellaria spinulosa ) and Horse Mussel ( Modiolus modiolus ) beds helps to stabilise the sediment surface. Other characteristic species include tube anemones ( Cerianthus lloydii ), calcareous tubeworms ( Pomatoceros triqueter ), hermit crabs ( Pagurus bernhardus ), starfish ( Asterias rubens ) and brittlestars ( Ophiura albida ).