The group allegedly destroyed limestone formations with hammers to extract the molluscs, which can sell for as much as €200 per kilo on the black market.
Italy's coast guard has arrested 18 people for illegally harvesting date mussels off the coast of Naples.
Authorities say a criminal network is responsible for an "environmental disaster" across a six-kilometre stretch of coastline by fishing for the endangered shellfish over the last five years.
The group has allegedly destroyed rocks and reefs in a systematic way, crushing limestone formations with hammers to extract the molluscs, which can sell for as much as €200 per kilo on the black market.
Fishing for date mussels has been illegal in Italy since 1998 because of their endangered status and the invasive method used to catch them, which can be destructive for the marine ecosystem.
This isn't the first time a gang has been targeted for this practice.
Less than three months ago, on the nearby island of Capri, police broke up two other organisations for date mussel fishing.
If convicted, they could face several years behind bars.