Better water quality has been linked to smaller cockles growing in Wales, new data has revealed.

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Cockles have been harvested along the south Wales coast for centuries with the Burry Inlet and Loughor Estuary, near Swansea, being the main habitats.

The Swansea University study, which has looked at 50 years of data, also found a higher mortality rate.

Marine biologist Dr Ruth Callaway said the findings were "baffling".

Following her study, she argued the change was likely to be a natural adjustment and said she believed smaller cockles may be just the price we have to pay for cleaner water in Loughor.

Between 1958 and 2009 water quality around the south Wales coast has been massively improved and before 1997 wastewater effluent was discharged into the estuary from seven sewage plants.

This was modernised with two new plants using treatment processes that disinfected the effluent and removed the nitrogen.

This meant cleaner, healthier water for humans. Although the estuary is still subjected to sewage outlets, there seem to be fewer nutrients to sustain cockles.

"Studies of cockles across south Wales show they're amazingly adaptable to their surroundings," she said.

"We've found healthy cockles in the Bristol Channel's most polluted waterways, such as Port Talbot harbour.