Arran’s rocky reefs

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Scotland’s Year of Coasts and Waters has officially been extended to 2021, given many events and promotions couldn’t go ahead this year as a result of the pandemic. To recognise this, and to raise the profile of many exciting habitats and species we find in Arran’s seas, many protected by the South Arran Marine Protected Area (MPA), the Community of Arran Seabed Trust (COAST) is running a year-long media campaign to raise awareness.

Each month, they will focus a particular habitat or collective group of inter-linked species and here for February is their second instalment. There is more information on COAST social media or visit www.arrancoast.com.

 


Rocky reefs

If you have walked along any of Arran’s coastline you will know just how varied and spectacular it can be and this coastal scenery can give us some idea of what the adjacent seabed might look like.

Where the rock and boulders of the shore extend underwater, they form rocky reefs which sometimes end abruptly in shallow water or, at other locations, disappear into the depths. In contrast to Arran’s living reefs that were described in the Banner last month, rocky reefs are a result of the local geology and many of the plants and animals that live on the reef habitat actually spend their whole life attached to the rock.

The island of Pladda, within the South Arran MPA, is a distinctive landmark off the southern coast of Arran. While this substantial rock outcrop is high enough to appear above the sea’s surface, nearby there are other similar rocky outcrops that are submerged below the waves, forming part of Arran’s rocky reef habitat.


One such reef, known as Roraima Reef by local divers, appears to be as spectacular and mysterious as Sir Arthur Conan Doyle’s Lost World which itself was based on an actual location in South America which bears this name.

Roraima Reef is as dramatic as its name suggests, its sheer rock walls rising abruptly from the surrounding seabed. In places, this reef is carpeted by an assortment of vibrantly-coloured elegant anemones which look like tightly-packed flower heads. Elsewhere are clusters of the much larger orange or white plumose anemones, which can grow up to 30cm tall and are topped by a fluffy-looking circle of tentacles.

Other parts of this rocky reef are dominated by bulbous fingers of the soft coral ‘dead man’s fingers’ along with colonies of less familiar-looking marine animals such as sea firs and sea mats.

Around all the life attached to the rock are bustling mobile animals which include brilliant red starfish, beautiful sea slugs and sea snails and more familiar crabs and fish.

Roraima reef is really impressive, not only because of its dramatic underwater landscape but also in terms of the marine life that thrives on it.

Bedrock reefs like Roraima are unusual around Arran as the underwater rocky reefs are more commonly formed by boulders. As well as rocky reefs that fringe the island, isolated patches of bedrock and boulders occur further from the coast, interspersed with gravel, sand and mud. The prominent volcanic dykes that are such a distinct part of Arran’s coastline are also present underwater, forming substantial underwater reef walls in some places.

In shallower water, seaweeds generally dominate the reefs with large brown kelp plants forming underwater forests together with a dense understory of smaller red seaweeds. The level of light in the sea reduces significantly with increasing depth and as seaweeds, like plants on land, need light to grow, changes in light levels result in distinct seaweed zones depending on how much light the different species need to survive. They do not grow so well, or at all, on less well-lit vertical rock faces or in deeper water. As light levels dwindle with increasing depth, seaweeds can no longer survive and the rocky reef habitats become dominated by animal life, much of which can look plant-like.

Animals such as the antenna hydroid, or delicate, colourful feather stars challenge our views of animal forms. Even more unusual are creatures like sea squirts which, like most of the animals attached to the rock, feed by capturing food such as plankton from the water.

Rocky reefs are an essential habitat for many different mobile species including commercially important lobsters and crab. A wide variety of fish species also choose to live on the reefs. These range from small fish, such as the bottom-dwelling leopard-spotted goby which shelters in spaces within the rock or under boulders, to larger fish such as wrasse that often live as a group in defined territories.

Wrasse feed around and over the reef and make use of rock crevices or purpose-built seaweed nests to lay their eggs. In recent years, wrasse species around Arran and elsewhere around Scotland and the UK have been the target of unmanaged fisheries that are supplying wrasse as cleaner fish to the salmon aquaculture industry and their numbers appear to be dwindling.

The variation in reef habitat that occurs around Arran provides many different opportunities for marine life. Some species, such as those on Roraima reef, thrive in locations exposed to greater wave action and faster water currents, whereas other species prefer much more sheltered conditions. The sometime subtle changes in conditions at different locations around the island means Arran’s rocky reefs support a wide diversity of marine life which our protected area helps preserve.

Sea Fir. Photograph: Lucy Kay. No_B06COAST01

Ballan wrasse and soft corals. Photograph: Howard Wood. No_B06COAST02

A diver and elegant anemones at Roraima Reef. Photograph: Paul Kay. No_B06COAST03

COAST logo. No_B06COAST04