The Westcountry's seas are among the fastest warming on the planet – wiping out cold water species and bringing a host of "alien" visitors to our shores, scientists have said.

Creatures found in warmer water are advancing northwards by 30 miles per decade – ten times faster than changes on land – with the seas off Devon and Cornwall declared a "global hotspot" for the phenomenon.

After a decade of painstaking examinations and research, scientists at the Plymouth-based Marine Biological Association (MBA) are now able to reveal their startling findings. They say cold water northern species including limpets, barnacles, marine snails and kelps are declining in abundance, with some completely disappearing from the marine habitat.

In contrast, those found in warmer waters are extending their range and advancing northwards ten times more rapidly than species on land.

A range of foreign visitors are also appearing in South West waters, some of which could trigger severe biodiversity loss.

Dr Nova Mieszkowska, who led the study, said: "The seas off Devon and Cornwall are some of the fastest warming in the world.

"It is a really interesting place to study the divide between warm ocean waters to the south in Europe and cold waters to the north.

"Plymouth in particular is in an ideal location close to this breakpoint to study animals and plants that are located close to their temperature tolerance limits – either upper limits for native cold water species or lower temperature limits for native warm water species.

"Our research shows a real shift in dominance. The warm water species are accelerating, whereas there has been a decline in the numbers of many from cold water.

"People might think 'who cares about a barnacle?' but there is a whole food web of dependence that is affected. We are now starting to look at what these changes will mean to the wider marine community."

The MBA and government agencies compared data from the past 50 years with new studies of the same sites in order to gauge the effects of climate change.

The study, called the MarClim Project, assembled information from more than 5,000 surveys at more than 1,000 locations.

Its findings include the rapid invasion of a Japanese seaweed called Sargassum muticum, which has spread around the coastline of England, Wales and Scotland.

Dr Mieszkowska added: "You can see it grows rapidly each spring and crowds rockpools, cutting out light for seaweeds living below. Nobody has ever successfully removed this species from a shoreline without it returning."

Another Japanese species, a kelp called Undaria pinnatifida, can now be found on rocks by the Tinside Lido in Plymouth, and in the waters off Torquay. Dr Mieszkowska said it had most likely spread from initial sites through boats visiting different marinas.

The research has also documented how a brown warm water seaweed – Bifurcaria bifurcata – has extended its range over 90 miles from Devon to Portland Bill in Dorset, while Osilinus lineatus, our largest seashore snail, has spread 28 miles east from Lyme Regis.

Meanwhile, a northern brown seaweed known as Alaria esculenta, or dabberlocks, has disappeared from much of the South West. Likewise a cold water barnacle (Semibalanus balanoides) is now almost extinct around Plymouth Sound and neighbouring shores.