A casual walk along the intertidal shores of Scapa Flow provides a window into another world – a taste of what lies hidden beneath the waves. Amongst the tangles of seaweed there are moulted crab skeletons abandoned as its owner grew, purple fragments of sea urchin, and numerous shells, many very familiar to the beach-comber. But these are just the most obvious signs, the most resilient parts of a handful of creatures. So much more lies just metres away - don a wetsuit and a face mask and that whole other realm opens up – a kaleidoscope of sea anemones, sea slugs and echinoderms sporting far greater colour and diversity than that found behind you on dry land.
Life began in the sea and that extra time and the opportunities to diversify have led to an astounding array of forms. A broad variety of habitats can be found within Scapa Flow, some formed by different substrates (from soft muds to rocky shores and boulder fields); others formed by fierce tidal streams, wreck-‘reefs’, and the darkness of the Bring Deeps. With few exceptions, every animal phylum described by science is represented in Scapa Flow.
Many visitors make the pilgrimage to the Flow to dive the many wrecks, especially the long silent, rusting skeletons of the German High Seas fleet. Artificial ‘reefs’, such as the wrecks, provide a valuable substrate for the colonial soft-coral Alcyonium digitatum, more commonly known as the macabre ‘dead-man’s fingers’, as well as a mosaic of sponges and bryozoans. The deeper wrecks also attract a rich concentration of more mobile invertebrates - too dark for photosynthesising algae, animals dominate here – sea stars forage while the octopus waits in ambush. The muddier sediments around the wrecks are home to the unusual slender sea pen (Virgularia mirabilis) – a distant ‘cousin’ to the soft-corals. In Scapa Flow there are also ‘biogenic’ reefs, those produced by living organisms: the horse mussel (Modiolus modiolus) grows on various sediments and produces substantial reefs in sheltered waters under calm tidal conditions. Although highly susceptible to damage from trawling, accurate mapping and monitoring of horse mussel reefs is helping to protect and restore this valuable asset.
Strong tidal races run beside Graemsay, near Stromness, scouring the seabed. Far from lifeless, these currents also deliver large quantities of food particles. If an organism can find a firm place to attach or sufficient shelter between rocks, it can thrive. This deceptively barren seascape hosts important populations of hornwrack (not a seaweed but the bryozoan Flustra foliacea), the dahlia anemone (Urticina felina) and large barnacles. Exit the Flow to the west of Hoy or Mainland and we enter a world of extreme wave exposure – extensive forests of the kelp Laminaria hyperborea dominate with their holdfasts and fronds harbouring a dense community of small animals. Kelp forests are the North Atlantic equivalent of the coral reef ecosystem - supporting a large proportion of the overall biodiversity of the region.
Economically, the waters of Scapa Flow still attract a small but healthy fleet of ‘creelers’ targeting the edible crab (Cancer pagarus), known locally as partans, the velvet swimming crab (Necora puber) and lobster (Homarus gammarus). Similarly traps are set for whelks (Buccinum undatum) and langoustine (Nephrops norvegicus). Divers are employed locally to harvest scallops (Pecten maximus and Aequipecten opercularis) – with many of their favourite haunts located in Scapa Flow. On a more recreational basis, Orcadians can sometimes be seen on sandy shores walking backwards, knife-in-hand at the ready to dig out razor clams (Ensis spp.) at extreme low, spring tides – or ‘spoot’ tides due to the distinctive, tell-tale spout produced by the hidden quarry.
So, even though the relatively benign waters of Scapa Flow, whose seas have been sailed and fished in for millennia, may seem tame and perhaps rather gloomy on an overcast, dreary day, there remain fascinating animals rarely seen by humans and little understood.