It was found within a marine protected area (MPA) to the south of the Isle of Arran and is only the second known flame shell bed remaining in the Clyde marine region.
It has been described as a “biodiversity powerhouse” and an important weapon in the fight against global warming.
Conservationists have welcomed the discovery, which they say is a positive sign for both the Clyde and for Scotland.
Dr Dan Harries, a marine scientist at Heriot-Watt University, said: “It is very encouraging to hear of the discovery of an extensive and previously unknown area of flame shell reef in the Clyde.
“These reefs support diverse and abundant communities of marine organisms, so it is not just about the discovery of the flame shells themselves – it is a discovery of an entire marine community of exceptional biodiversity.”
Flame shell beds support hundreds of different species and provide key nursery grounds for juvenile fish and commercially important scallops.
Research on the other Clyde reef identified 265 different animal species living within the habitat.
They also act as ‘blue’ carbon stores, which helps increase resilience to climate change.
Lucy Kay, who was one of the divers who found the reef during a recreational outing, is MPA project officer for the Community of Arran Seabed Trust.
“Important discoveries like this are helping improve our collective knowledge of Scotland’s seas,” she said.
“This discovery highlights the invaluable contribution of community groups and citizen scientists in helping survey and monitor the marine environment around our shores, much of which is currently done on a voluntary basis.”
The Clyde was once home to at least seven large flame shell reefs, covering massive areas – at Sanda-Southend, Skelmorie Bank, Stravanan Bay, Tan Buoy, Great Cumbrae, Inchmarnock, Otter Ferry and Lamlash Bay.
A single remnant of one reef remains at Otter ferry in Loch Fyne, where high power subsea electric cables render the ground too dangerous to fish.
Professor Jason Hall-Spencer, of Plymouth University has described the discovery of this new flame shell bed off Arran as “absolutely amazing and very welcome news”.
The academic, who spent years studying flame shell beds while based at Millport Marine Biological research station on the Isle of Cumbrae, said: “This discovery re-ignites the possibility that, with adequate protection, the once widespread Clyde flame shell beds could one day fully recover along with wider marine and fisheries improvements.”
Flame shell beds are scarce around the UK.
They are listed as a priority habitat under the UK’s post-2010 Biodiversity Framework and a Priority Marine Feature in Scotland’s seas.
Significant parts of the world’s largest flame shell bed – found in Loch Carron, Wester Ross – were destroyed by legal bottom-towed fishing activity in 2017.
This resulted in an emergency closure before the area was designated as an official marine protected area.