The Italian coastguard arrested 18 people on Wednesday for illegal fishing of a rare mollusc, breaking up what police said was a criminal organisation that had been destroying a stretch of protected coastline south of Naples.

The arrests followed a three-year investigation into the group which had allegedly been harvesting date mussels, a protected species, using hammers to get them out of the rocks near the seaside beauty spot of Sorrento.

Fishing for date mussels has been illegal in Italy since 1998, because they are an endangered species and the invasive methods used to get them out of the rocks they bore into are destructive for the marine ecosystem.

The valuable shellfish, considered a delicacy, are longer than common mussels and have a browner shell. They sell at up to 200 euros ($235) per kilo on the black market.

The people arrested are accused of numerous crimes including illegal fishing, destroying the marine habitat and selling unsafe foods, said a statement from the prosecutors' office of Torre Annunziata which led the investigation.

The "criminal organisation," which had allegedly operated since 2016, was also responsible for collecting and selling clams from a "highly polluted" area near the mouth of a river carrying hydrocarbons and heavy metals, the statement said.

Less than three months ago on the nearby island of Capri, a glamorous tourist destination, police broke up two other organisations for date mussel fishing.

A police video showed the holes in the three "Faraglioni" rock formations, a symbol of Capri, caused by the drills and hammers the fishermen had used to extract the molluscs.