The divers spotted flame shells, a small scallop-like species, in Scapa Flow, just two years after a scarce giant fan mussel was recorded by divers in the same area.
Led by Dr Joanne Porter of Heriot-Watt University, 12 divers from Seasearch, the volunteer dive project co-ordinated by the Marine Conservation Society (MCS), were diving in Orkney from the MV Halton, skippered by Bob Anderson.
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Seasearch surveyor and underwater photographer George Brown, from Inverness, recognised a shell and pebble structure on the seabed as that made by flame shells (Limaria hians).
He said: "Only close examination of the structure revealed it was indeed inhabited. These beautiful flame shells were found in Scapa Flow at a depth of 16 metres. Interestingly, there are two other records of Limaria in Orkney, both dating back to the 1950s. So these could be the first finds for over half a century."
Despite its flamboyant bright orange tentacles, the Flame shell is very difficult to spot within its nest.
Calum Duncan, MCS Scotland Programme Manager and Seasearch Coordinator in Scotland said: "This is certainly one of Scotland's underwater biological treasures. It's always exciting when a new record of a rare, threatened or declining species such as flame shell is made."