Our underwater living 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 happen 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 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 January is the first of the articles. There is  more information on COAST social media or visit www.arrancoast.com.

If the word ‘reef’ conjures up a vision of warm, tropical seas full of colourful corals and fish, you could be forgiven as this is the sort of reef we more commonly see on TV and in news stories.

It may therefore come as a surprise to learn we have equally wonderful underwater seabed habitats in the cooler waters around Arran and other parts of Scotland.

The recent discovery of an extensive flame shell reef off south Arran, reported in the Arran Banner last month, brought an example of Scotland’s amazing living reefs into the limelight. But flame shells are just one of a number of different living reefs around our coasts.

These living reefs, or beds, are only made by a few marine animals and plants and are specialised marine habitats. As with coral reefs, the living organism creates the reef structure, sometimes secreting tubes that combine to form the reef or by binding seabed material together to form a raised structure. In the case of the organ-pipe or serpulid worm reefs that are found at less than a handful of locations in Scotland, the reef structures can grow up to 75cm high and 1m across.

Maerl beds are probably the best-known of the living reefs around Arran, being one of the particularly important habitats in the Lamlash Bay No Take Zone and the South Arran Marine Protected Area (MPA). They are formed by species of free-living red seaweed that have a hard calcium carbonate skeleton and look like small twiglets. Where many of these maerl nodules occur together, they form a bed – a complex three-dimensional structure, with the live maerl providing visually stunning pink colouration to the seabed.

A complex, multi-dimensional structure is one of the characteristics of living reefs and is why they provide such an important contribution to biodiversity. The reefs form a multi-story home for many other plants and animals that can live on and amongst the reef, ultimately meaning these habitats support a greater diversity of life than would be found if they were not present. The cacophony of life living on these reefs is spectacular: seaweeds, sponges, anemones, sea squirts, marine slugs and snails, sea urchins, starfish, octopus, crabs and fish are just some of the more visible species that can be found, with many more cryptic species living within the reef.

It’s not just the variety of species that is important. Reefs function as an essential habitat for different life stages of many species, providing ideal spaces for them to live, feed and reproduce. Many living reefs are important nursery grounds for commercially important fish and shellfish species. The young of some species, such as queen scallops, are actively attracted to live maerl, and horse mussel beds can act as a nursery for juvenile common whelk.

The abundance of life on the reef also provides food for mobile predators such as fish, making these habitats an important food outlet for some species in our seas.

The recently discovered flame shell reef is the only other living reef that we currently know of in Arran’s MPA but there are other living reefs within the Clyde area. Horse mussel and blue mussel reefs are present in a few locations in this region and there is a new trial project to re-instate oyster beds in the Firth.

In the past, native oyster beds were really important fisheries in Scotland, however, overfishing and an infestation of oyster parasite devastated these beds. There are now a number of projects across Scotland looking at actively restoring oyster beds so we can once again benefit from their ecosystem and biodiversity value, as well as their potential to support sustainable local fisheries.

Scotland’s living reefs encompass the weird and the wonderful. Sadly, many of them are far less extensive than they used to be. They are highly sensitive to physical damage from bottom towed fisheries and many of the reef builders are slow growing and long-lived – maerl grows only a few millimetres a year and horse mussels can live for up to 50 years – so reef structures can take a long time to develop.

We need to protect our living reefs. They play an extremely valuable role in the wellbeing of our marine ecosystems and as part of the marine food web, in turn supporting businesses such as fisheries and wildlife tourism that rely on productive seas.


Velvet crab on maerl. Photograph: Howard Wood NO_B53reef01

Serpulid Reef. Photograph: Howard Wood NO_B53reef02

Pipefish on maerl. Photograph: Howard Wood NO_B53ref03

Flame shell habitat. Photograph: Paul Kay NO_B53reef04