A species of mollusc that can grow to the length of a UK size nine shoe could be at risk due to unprotected areas of the sea near its bed, a new study has found.

Horse mussels, some measuring about 20cm in length, can live for several decades and can be found around the British Isles, with most seen in the North.

Marine experts have described their beds as “biodiversity hotspots”, providing foundations for soft corals and barnacles, shelter for many small sea creatures and habitat for shellfish.

They have been known to be in decline since the 1990s, and marine protected areas (MPAs) were introduced to protect their numbers.

But a new study by scientists at Heriot-Watt University shows the species is at risk due to a “failure” to consider gaps between protected areas of the sea where their beds lie.

Dr Jo Porter, based at Heriot-Watt’s Orkney campus, worked on the research with Dr Clara Mackenzie from Fisheries and Oceans Canada, scientists and nature managers, to sample horse mussel beds around Scotland and analyse their DNA.

She said: “We wanted to find out whether these areas are actually doing their job of protecting the horse mussels and how they operate as a network.

“This is especially important because, like many protected marine species, the larvae spend time travelling in the plankton before they settle.

“What we discovered is that the spaces and connections between MPAs are essential for the mussel beds.

“Some of the horse mussel beds wouldn’t be able to survive without the network, they are dependent on the unprotected spaces in between.”

The team analysed genetic markers of the some of mussels’ DNA, assessing how genetically connected and diverse the populations were: low genetic connectivity and diversity could make them less resilient to environmental changes.

Divers went to depths of more than 40m to collect the samples at some of Scotland’s 12 MPAs where horse mussels can be found.

Marine protected areas are an excellent tool for protecting marine species, but we know hardly anything about how they are connected,

Their research showed that the Noss Head horse mussel bed, which covers 385 hectares and is one of the largest horse mussel beds in the UK, could be at risk if other beds for the species are not protected properly.

This is because their genetics sampling suggested the bed is reliant on larvae coming from as far away as Shetland.

Dr Porter said the findings make the case for more research into how MPAs function.

“Marine protected areas are an excellent tool for protecting marine species, but we know hardly anything about how they are connected, despite many governments committing to so-called networks,” she said.

“Horse mussels are just the tip of the iceberg – there are many other species that are hugely important for conservation and biodiversity that are protected by marine protected areas.

“But, as with horse mussels, they could be far more vulnerable than we think, despite occurring in protected areas.”

Dr Mackenzie added: “Our findings suggest that Scotland’s horse mussel beds are an interconnected system of populations.

“It is imperative that conservation decisions account for the complexity of such relationships, rather than simply protecting representative sites.”