Ceredigion is witnessing the rare sight of thousands of spider crabs along its shores this summer.
Ceredigion has seen a population explosion of spider crabs
It is claimed that it is the biggest influx of the species into the area in living memory.
Reasons put forward for the migration from south Wales to waters around New Quay, Llanrhystud and Borth vary from climate change to a population explosion.
But an expert in marine biology claims it is too early to be sure of the exact reason for the crustacean invasion.
Some claim that the crabs have migrated because of a rise in water temperature but it is too early to say if this is a factor
Dr John Fish, marine biologist
The creature lives on sand and amongst rocks down to about 50 metres in depth.
But it comes inshore in large numbers in summer to molt and mate forming huge writhing mounds of crabs and empty shells.
Dr John Fish, the head of biological sciences at the University of Wales, Aberystwyth,
has experience of working on Ceredigion's beaches stretching back for thirty years.
"I've seen more spider crabs on Ceredigion's beaches this year than in other years and fisherman have reported a large increase in numbers," said Dr Fish.
"Records for this area stretch back to the 1950s and there are authors who have stated that the spider crab was abundant in Ceredigion's waters during that decade."
The marine biologist said the crab population explosion could have occurred because of favourable climactic conditions for the survival of the current population.
"Some claim that the crabs have migrated because of a rise in water temperature but it is too early to say if this is a factor."
Borth beach has seen an increase in spider crab numbers
The crab grows up to 20cm across. Its spiny back is often covered with sponge and seaweed which the crab plants there for camouflage.
Keith Stone, the North Western and North Wales Sea Fisheries Committee's officer for the area between Cardigan and Barmouth, said there had been a definite rise in crab numbers.
"We have definitely seen an increase in spider crab numbers over the last two years and this year they have been almost marching up the beach," he said.
"It could be because of global warming, ocean currents or changing migration patterns."
Several of Ceredigion's beaches provide a favourite habitat for the spider crab.
"They like patches of sand in between rocks which you find at Cei Bach near New Quay," said Mr Stone.
"But it is rare to find them so far north although they may be adapting to the colder water temperature conditions in the same way a person who drinks six pints a night can the cope with drinking ten pints."