WIND WHISTLES THROUGH the rigging on mv Halton as the scientific divers go through their final safety checks with the dive supervisor.
“Divers ready” The last call comes from the skipper, Bob Anderson, as the boat is manoeuvred to the shotline, which has been carefully deployed to place the divers exactly on top of the study area.
Making their way to the gate, with numerous scientific instruments clipped to themselves, each buddy-pair is dedicated to a specific task.
As the boat approaches the shotline, Bob calls to the divers “go, go, go!” and, one giant stride later, six divers are descending the line, focused on collecting data that will help to unveil another mystery of our oceans.
It is said that more is known about the dark side of the moon than is known about our seas, and even around the UK innumerable discoveries have yet to be made. Our knowledge remains woefully inadequate, and even some of the most basic questions go unanswered.
Researchers in the Heriot Watt Scientific Dive Team are working hard to change this and reveal what may turn out to be some of the highest biodiversity hotspots in UK waters.
Leading this expedition in Shetland around some of the UK’s most northerly islands, head scientist Dr Bill Sanderson has his own particular interest.
Much of his recent research has focused on a mussel known as the horse mussel. Twice to three times bigger than the common or edible mussel, the one you would usually eat with white wine and garlic, these large molluscs gather in massive aggregations on the seabed, creating what are known as biogenic (living) reefs.
“These complex structures provide habitat to an astonishing array of organisms that make their home in
every nook and cranny between both live mussels and their dead shells,” explains Bill.

THE SHEER AMOUNT of life supported by these incredible living structures is astounding. To give an example, a recent horse-mussel bed, found by the team just off the north coast of Scotland near Wick, covered the equivalent of about 380 football pitches. It was the biggest known horse-mussel reef found to date.
In an area of 0.25sq m, roughly the size of a sheet of A3 paper, there can be 270 different species and in excess of 5000 individual animals.
Factor in some patchiness and 380 hectares, and you have tens of billions of organisms living on a single horse-mussel reef. That’s what you call a biodiversity hotspot!
Diving in Shetland’s cool water is exciting and, as is often the case in this part of the British Isles, the underwater visibility is superb, at least 15m if not more. Accompanying the scientific divers on a biogenic reef dive had filled me with expectation.
As I make my way along the extensive mussel bed, I marvel at the fact that I’m actually struggling to see a mussel.
This isn’t because they are absent, but because the entire bed is covered in several layers of brittlestars.
These delicate creatures form dense carpets, creating an extraordinary amount of colour. Their bristly arms wave in the water column, catching floating particles and passing them on to their centrally placed mouths.
Edible crabs move among the brittlestars, along with large whelks and common and northern sea urchins. The whole seabed is alive with animals, as scientific divers peer out of small clouds of silt, collecting mussel samples.
Making my way back to the shotline with my buddy Paul Kay, I come across another diver pair reeling in a tape with a 2m length of tube.
This is Flora Kent, a PhD student under Bill’s supervision. Her research is focused on horse-mussel beds as nursery grounds for commercially important fish species.
Flora’s task involves swimming out from a fixed point (the shotline), on a line of a set distance measured by the tape. She swims along, holding the tube in front of her and counting all the fish within the 2m-wide section along a 25m measuring line, or transect.
Flora carries out the same procedure at every site she dives. This gives her a standardised sample that is comparable when looking at the data from different dive-sites.
Another seabed type of conservation importance, and one in which the scientific dive team are extremely interested, is composed of maerl.
Maerl is a red algae that forms a hard calcium skeleton, similar to that of tropical corals. It grows slowly into pink or purple round nodules and short, stumpy branched shapes.
These maerl pieces are individually small but can occur in vast numbers and cover extensive areas of seabed.
Like all algae, maerl needs sunlight to grow, so it occurs only in relatively shallow water.
Many of the white sandy beaches found in the tropics are made up of dead coral fragments. On the west coast of Scotland some of the beautiful white “coral” beaches are made up of dead fragments of bleached maerl.
Maerl beds are crucially important and, similar to horse-mussel reefs, provide habitat and shelter for many other species, as well as nursery areas for numerous juvenile species.
The Heriot Watt team are interested in locating these beds, mapping their size and recording the levels of associated biodiversity.
The sites had been surveyed the week before diving, using a drop-down video camera to identify areas where maerl beds might be present. This allowed divers with limited bottom time to concentrate on specific sites and use that time to best advantage.
Diving maerl is an unusual experience. Descending the shotline onto the seabed, I am met by dense, vivid patches of live maerl, its beautiful purples and pinks in striking contrast to the blue/green of the surrounding water.
Multi-coloured starfish and urchins move slowly over the top of the maerl, with swimming and hermit crabs going about their daily business of foraging for food. A closer look shows many smaller animals among the maerl fragments.
Lost in this macro world, I hear a loud “bang, bang, bang”. I turn to see some divers not far away, so I swim over to see what’s going on.
One pair is collecting core samples, using a plastic tube about 30cm long and 10cm wide. This is pushed or hammered into the seabed, hence the noise, and then plugged at the top and pulled out with the core full of sediment.
A plug is quickly pushed in at the bottom and a core sample taken. This allows the scientific team to see which animals are living in the seabed.
Not all the diving is conducted in such great conditions. Another lead scientist on this expedition, Professor Hamish Mair, has been carrying out dive surveys in Shetland for more than 30 years.
The team is heading for a site he had surveyed back in the 1980s at the bottom of one of Shetland’s voes, and is more akin to diving the sea-lochs on the west coast of Scotland.
The freshwater input, which contains peat from the surrounding moorland, has dyed the water dark-brown, giving it the appearance of strong tea.
Dropping down the shotline onto the seabed, we are met by a muddy bottom and many empty horse-mussel shells.
Visibility is less than 10m and it’s dark, because of the peaty water filtering out the daylight. From the corner of my eye I see a torch-beam go up a couple of metres and then descend to the seabed. Then it happens again.

I SWIM OVER TO FIND Bill Sanderson taking “random quadrat counts”.
A quadrat consists of a metal frame, a 50 x 50cm square split into 5 x 5cm squares. This is laid haphazardly onto the seabed, allowing him to count the live mussels within each square.
I head back to the shotline, and in the gloom can see two lights moving back and forth across the seabed.
Natalie Hirst, an independent marine ecologist working as a consultant for Heriot Watt Scientific Dive Team, is undertaking a video transect along a set distance measured by the tape from the shotline. The video will be analysed back in the office to see what and how much marine life is living there.
“It’s important to study both pristine horse-mussel beds as well as those that seem to have been impacted sometime in the past, and to assess whether or not recovery is happening,” comments Hamish Mair.
“It’s probable that this bed we’re studying now was affected a while back by some anomalous massive freshwater inflow or some low-oxygen event. Through our studies here we hope to clarify the picture.”
Many more techniques and technologies are used to collect data.
Dr Jo Porter from Heriot Watt and Dr Piotr Kuklinski from the Natural History Museum are collaborating on an ocean-wide project.
They have developed and deployed two sets of large settlement panels at numerous sites from the Mediterranean to the Arctic, including Shetland during this expedition.
These panels are used to investigate the settlement of a group of marine organisms called bryozoans. These tiny creatures occur just about everywhere in our seas, but are often overlooked.
“This two-year-long experiment aims to help us to understand how changes in the seawater caused by climate change will affect the way that animals make their chalky skeletons,” explains Jo.
“Perhaps the most important implication of this research is that, in
a changing climate, our oceans will become more acidic and many animals will struggle to make their chalky skeletons and shells, but how well they’ll cope, we don’t know.”

THERE IS JUST AS MUCH, if not more activity before, between and after dives, as scientific equipment and dive kit is reorganised for the next survey.
Samples need to be analysed and stored prior to being taken back to the laboratory at the end of the expedition, and data needs recording and entering into the computer.
The expedition goes smoothly, and the team achieves many successful dives, gathering a large amount of valuable data. It’s a good co-operative effort by the team, including skipper Bob Anderson and his crew.
There’s a constant buzz of energy and excitement on the boat, with everyone pulling together to complete the work.
Although at times the mood is serious and intense, with some challenging tasks, there’s time for many laughs and a great deal of fun to be had along the way.

IN RECENT YEARS the Heriot Watt Scientific Dive Team has made some amazing discoveries. It has found and mapped Orkney’s biggest maerl bed, Scotland’s biggest horse-mussel bed,
the largest flame-shell bed of 75 hectares in Loch Alsh, and mapped some of Scotland’s most sensitive habitats, such as the serpulid worm reef in Loch Creran (Key To the Loch, April).
Although much of its work has been conducted in the UK, it has also supported the designation of Marine Protected Areas (MPAs) overseas, such as the San Andres Biosphere Reserve, around an island off the coast of Columbia, and a Panama Special Management Zone.
Such work plays a crucial role in providing information to managers of marine resources, assisting in the creation of MPAs where the correct level of management can be implemented.
The maintenance of healthy oceans and the preservation of critical eco-systems essentially enables the provision of numerous goods and services to a large percentage of the world’s population, including food and climate regulation.
However, “I wouldn’t eat ropey old horse-mussels” says Bill Sanderson. “Never eat anything as old as your granny – besides, the habitat supports much more palatable species.”
This kind of research provides an invaluable contribution to sustainable management – and, of course, makes for wonderful diving among some of the planet’s most fascinating habitats.