Underwater sealoch cliffs

The inner basins of Scotland's sealochs are a special place for marine life, where the still waters partly mimic conditions in the very deep sea. Here divers come face-to-face with creatures, in water less than 50m deep, which humans could otherwise see only by descending into deep water in a submersible. Nowhere else in Britain is underwater bedrock found in such sheltered conditions, and these sheer cliffs are especially rich in life

A dramatic habitat

In places, the dramatic glaciated scenery that characterises the west coast of Scotland continues underwater in the sealochs, forming a special home for marine life. Deep basins, scoured out by glaciers during the last ice ages, are protected from the waves of the open sea by tidal narrows and shallow sills. The sides of the basins, especially where headlands project into the loch, can fall vertically to depths of 30m or more, ending in a seabed of soft mud.

On the surface, a layer of dark, peaty freshwater floats above the salt water, and in the absence of waves, does not mix easily. The seawater below is effectively insulated, making it relatively stable in salinity and temperature. In the extreme shelter, fine silt settles on gently sloping underwater bedrock. Few animals or larvae can tolerate this clogging layer of fine silt, but vertical and overhanging rock remains clean and becomes covered with life, especially where tidal currents feed animals fixed to the rock.

Life on the cliff

The shallow parts of the cliff, less than 10m below the surface, are challenging places to live. Any animals and seaweeds that settle here must be able to withstand being regularly immersed in fresh or brackish water, and considerable bigger temperature shocks than in deeper water. But some creatures do well - small green urchins Psammechinus miliaris are abundant, grazing kelp and rock surfaces. Resisting the urchins sharp teeth are the hard white tubes of keelworms, and a tough coating of encrusting coralline algae like a layer of pink paint. The large sugar kelp Laminaria saccharina hangs down the upper parts of the cliff, sheltering myriads of little two-spotted gobies

On the deeper cliffs, it is too dark for most seaweeds, but animals grow in abundance in the more stable conditions. White seasquirts festoon the cliffs, with the bright red seasquirt Ascidia mentula, and occasional bloody Henry starfish Henricia sp, in shades of bright purple or orange, adding splashes of colour. The long tubes of peacock fan worms Sabella pavonina stand out from the rock, trapping passing food morsels in a magnificent feathery fan. In some west coast sealochs the sealoch anemone Protanthea simplex, with frosty white tentacles and peachy column, is abundant. This anemone is a particular Scottish speciality, otherwise known only from much deeper water off Rockall, and on coldwater coral reefs off Scandinavia. This anemone can detach itself from the cliff and drift to new places. It is easy to overlook the small brachiopod Neocrania anomala, a creature of ancient lineage the size and colour of a penny stuck to the rock. In the quiet water, sponges grow in strange shapes, and an unusual bright blue sponge Hymedesmia paupertas is easy to spot. Red, white and orange featherstars, including some deep water species, thrive where currents are slightly stronger.

Small cracks and crevices are occupied by brittlestars and perhaps a comical Yarrell's blenny, while wider ones are refuges for squat lobsters and goldsinny wrasse. A large fissure may even harbour a wolf fish, and shoals of young herring or sprats pass by.