A brainless and faceless "fish" was one of 15 species discovered during a series of Scottish marine surveys this year.

The prehistoric amphioxus, was found in waters off Tankerness in Orkney. It has a nerve cord down its back and is said to be regarded as a representative of the first animals to evolve a backbone.

Giant mussels with shells measuring up to 48cm (18in), were also discovered around the Small Isles and are said to have the largest sea shells in Scotland.

More than 100 specimens of the Fan Mussels were found around the islands, meaning the area has the largest aggregation of the fish in UK waters. The mussels are said to have golden threads similar to human hair, which are so fine they are able to attach to a single grain of sand.

In Caithness, the country's largest horse mussel bed was found in waters near Noss Head. The species, known as clabbydhhu in Gaelic, which translates as "enormous black mouth", are slow-growing molluscs that can live for up to 50 years.

Other finds from the marine surveys, which covered more than 2,000 sq miles, included flame shell (Limaria hians) beds in Loch Linnhe, Argyll, as well as new communities of northern feather star, a brightly coloured species with 10 feather-like arms fanning out from a central disc, which were discovered off the Sound of Canna.

The Scottish government said the findings would further the country's knowledge of the biodiversity of its seas. Scotland's environment secretary, Richard Lochhead, said: "In an age where the lands of the world have been mapped out and recorded, it's amazing how many discoveries are waiting to be found under the waves.

"Spanning from the weird to the wonderful, discoveries this year have included the bizarre amphioxus and the beautiful yet elusive brightly coloured flame shell. The waters around Scotland are rich in such fascinating biodiversity and it's our responsibility to protect this fragile environment."

Dr Dan Barlow, head of policy at WWF Scotland, said: "These surveys highlight that Scotland's seas and coasts are home to a truly amazing range of weird and wonderful wildlife. By providing vital information on what lies beneath the waves, these surveys will help inform decisions on better ways to protect this important resource now and long into the future.

"From helping inform the appropriate deployment of marine renewables to supporting the rollout of a network of Marine Protected Areas, these survey findings will prove invaluable in helping ensure the recovery of Scotland's seas. It is important that the government builds on this survey work to further our knowledge of the marine environment."

Susan Davies, director of policy and advice with Scottish Natural Heritage, added: "Scotland's seas really are a fantastic asset. The findings from these surveys will help us to manage them sustainably and ensure future generations can also enjoy the benefits of a healthy and diverse marine environment."

• This article was amended on 30 December to correct a sentence reading: "The fish has a nerve chord down its back."