1. Brittany has the majority of French maerl beds and some of the most extensive maerl grounds in Europe. 2. Breton maerl forms ancient (>5500 yr) but threatened habitats; several beds have been destroyed in the past 30 years. 3. Maerl extraction, eutrophication, mariculture and fishing are all major threats to Breton maerl beds, as is a continuing uncontrolled spread of the invasive gastropod Crepidula fornicata. 4. Maerl beds occur in 14 of the 28 Natura 2000 sites proposed along the Breton coast. 5. Maerl habitats are currently impacted by human activities in nearly all of the Breton proposed Natura 2000 sites which were mainly selected for terrestrial nature conservation features.