The sublittoral of the islands largely comprises an extension of the topography seen above the surface with vertical or near vertical underwater cliffs plunging for 40-50m before a more gradual slope, largely of massive boulders takes over extending down to a depth of around 70m. Below the sea surface, away from the islands, skerries, etc. a remarkable underwater topography is concealed. Large expanses of rugged bedrock and boulder outcrops from the sea bed, the result of erosion by rain and ice over the last 50-60 million years before sea levels rose to their current levels drowning much of a dramatic landscape, the remnants of which is the current archipelago. Between the main islands there are extensive areas of gravelly/sandy sea bed with a mega-rippled surface reflecting the effects of the relentless Atlantic swell even at depths of 70m or more. The various islands form the visible part of a circle of rock outcrops traced out on the sea bed that lies in around 70m of water, forming a plateau from which the sea bed falls away rapidly to a sandy/boulder plain in around 140m of water.

Immediately below the sublittoral fringe of dabberlocks, the steeply sloping bedrock cliffs are covered by a rich,Fucus distichus dense forest of kelp surrounding the islands. One of the remarkable features of St Kilda’s marine environment is the depth to which this kelp forest extends. The low turbidity of the oceanic waters means that sufficient light is able to penetrate the water to support dense growths of kelp (kelp forest) to a depth of over 30m. The density of kelp plants begins to reduce below this depth, to form a ‘park’ of individual plants and it is not unusual to find them at depths of over 40m (the deepest recorded was at 45m), compared to around 15m in other parts of the west coast and as little as 5m on the Scottish east coast.

The dense kelp forest that surrounds the islands like a vast protective curtain is dominated by cuvie and supports a rich underflora of red algae together with a variety of encrusting fauna including various sea anemones such as, Corynactis viridis, Metridium senile, Parazoanthus anguicomus and Sagartia elegans that create vast, colourful carpets over the rock. Other encrusting animals including various sponges (e.g.Myxilla incrustans, Leucosolenia complicata, Leuconia nivea, Pachymatisma johnstonia and Clathrina coriacea), bryozoans(e.g. Crisia spp., Bugula flabellata, Scrupocellaria spp., Porella compressa, Flustra foliacea and Securiflustra securifrons), the colonial sea squirt, Botrylloides leachi and the corals, Caryophyllia smithii and Alcyonium digitatum all compete for space. Mobile species are limited but the large pink topshell, Calliostoma zizyphinum is ubiquitous together with a variety of small but colourful sea slugs. Similar dense assemblages are associated with the deeper areas where the kelp is less abundant. In some of the most exposed areas below the dabberlocks where it is too turbulent for cuvie to develop or where it has been lost due to storms the kelp forest comprises the opportunistic, fast growing sugar kelp and furzebellows.

In the circalittoral, below the kelp zone where the light has diminished to levels capable of supporting only encrusting red algae, the rock surfaces are dominated by dense aggregations of encrusting animals that themselves support populations of scavenging and grazing organisms such as sea slugs, gastropods and echinoderms.

Anotable feature of the underwater cliffs is the caves, tunnels and gullies. The walls and ceilings are colonised by dense sponge crusts such as Myxilla incrustans together with hydroids such as Tubularia indivisa and bryozoans (e.g. Escharoides coccinea) and where there is sufficient light specialised, surge-tolerant red algae, including Parasmittina trispinosa and Schmitzia hiscockiana. In areas of reduced surge, anemones such as the Phellia gausapata, jewel anemones, KelpParazoanthus anguicomus and Sagartia elegans are abundant often occurring in massive monospecific aggregations, along with thin encrusting sponges, and bryozoans and feather stars (Antedon bifida). Within the deeper caves there is an apparent wave exposure gradient with species more typically found in more sheltered environments such as the fan worm, Sabella pavonina and the burrowing anemone, Cerianthus lloydii present in the inner reaches. Rarely recorded nocturnal species also occur in the innermost reaches of some caves including the crab, Bathynectes longipes and the sea anemone, Arachnanthus sarsi. The floors of many of the caves are covered in large, rounded boulders that act as massive rock mills grinding out pits in the bedrock beneath. In more shallow areas these boulders support a sparse fauna of tubeworms, barnacles and seasonally-occurring communities of turf-forming hydroids such as Obelia dichotoma, whilst in deeper water they also support the Devonshire cup coral, common sea urchins and squat lobsters (Galathea strigosa and Galathea nexa).

Sea anenomes and spongesExtensive areas of rocky outcrops and boulders are found between 60–80m that are still affected by surge and complex deep water eddies and currents. These are highly unusual conditions to occur at such depths and is reflected in the unusual species assemblages found here. The community is characterised by erect sponges such as Axinella spp. and Phakellia spp., as well as bryozoans, including Porella compressa. The erect coral-like bryozoans, Pentapora foliacea and Coronapora truncata are found on these deep rocks and cobbles. Occasionally at depths in excess of 70m there are patches of pink encrusting algae, emphasising the remarkable water clarity. The crevices amongst the boulders are ideal for brittlestars such as Ophiocomina nigra and squat lobsters – Munida spp.

Ross coralWithin the centre of the plateau that is bounded by the rocky outcrops, lying in water at a depth of –70m, there is an extensive sediment plain with a mosaic of fine rippled sand with only occasional stones and boulders and coarse gravelly sand in megaripples. These areas lack any conspicuous fauna of note but do support a diverse infauna of nematodes, sipunculids, annelids, crustacea, molluscs, bryozoans, echinoderms and tunicates.

Anumber of notable species have been recorded in recent surveys. The jewel anemone, which has a predominantly southern and western distribution, is present in super abundance. The rarely recorded snail, Sinnia patula is a southern species that is intimately associated with the soft-coral dead man’s fingers. Other southern species at or about the extreme of their range include the sponges, Tethyspira spinosa and Plocamilla coriacea, the soft coral, Parerythropodium coralloides, the sea slugs, Crimora papillata, Eubranchus doriae and Antiopella hyalina and the brown alga, Carpomitra costata. In addition there are also a number of northern species reaching the southern limit of their ranges such as the sea anemones, Parazoanthus anguicornis which is particularly abundant, and Phellia gausapata, the spider crab, Lithodes maia and the starfish, Stichastrella rosee.