Country diary: march of the molluscs

Seawater held within the square, sheltered confines of Plymouth’s King Point marina moves smoothly up and down with the tides, like a lift between two floors. As the surface descends, pale shapes can be seen scattered among the swags of seaweed, like handfuls of fresh plaster pressed against the exposed wet stone. Some are slim and oval, others plump as a pasty, crimped along one side with a sharp scalloped edge. I count almost 200 on a quick circuit of the harbour.

These intriguing creatures are Pacific oysters – a non-native shellfish that is making itself at home in Britain’s coastal waters. Originally from Japan, Magallana gigas has been farmed for decades in the UK and across the globe. It was believed our seas were too cold for them to breed. However, wild populations have recently become established and are spreading rapidly in muddy estuaries and rocky bays around the West Country and beyond.

The population at this boatyard in Millbay is relatively small compared with the thousands-strong congregations that have smothered stretches of shoreline in southern England with dense reefs of jagged shells. Fragile marine habitats, including areas at the mouth of the Yealm in Devon and Fowey in Cornwall, have become encrusted with serrated carpets, too hazardous to walk on without stout footwear. A gastronomic delicacy for the taking, one might think – but only those in approved waters are fit for consumption. It is also far from easy to prise wild Pacific oysters alive and intact from their tightly packed clusters in order to carry out the process of depuration – cleansing them of impurities.

Unlike the declining native oyster – a round bivalve that lives beneath the tidal zone – this fast-growing mollusc thrives in brackish water and reproduces at extraordinary rates, with an individual capable of producing 200m eggs in a single spawning.

It will be a challenge to slow their onward march, and eradication is now considered unfeasible. For this coastal empire-builder, the world is their oyster. And, strange as it may sound, these scruffy-looking trespassers flaunting themselves on the harbour walls in Plymouth look like they know it.