While breeding colonies are well known in seabirds, they remain exceptional for marine fishes. Here, we report on fifteen massive breeding colonies of picarels (Spicara smaris), a small benthic zooplanktivorous fish, observed by chance during video transects in spring 2021 along the East coast of Corsica (French Mediterranean). In total, these colonies cover more than 134.6 hectares (ha) within a surveyed area of 712.1 ha, a single colony covering from 2.2 to 28 ha between 37 and 50 meters deep. The seabed, including the lower limit of Posidonia oceanica seagrass meadows, soft bottoms, and the predominant rhodolith beds, has been completely rebuilt in circular jointed nests measuring 55 cm in diameter on average. With a density of 2.6 nests per m2, the estimated number of nests in the colony exceeds 18 million. Each nest is guarded by a male. Females swim in groups above the nests and sometimes lay eggs. A rich macrofauna including threatened species can be observed around the nests, eating eggs or adults. This finding highlights the exceptional ecological role of this small fish as an ecosystem engineer creating oases of marine life. This warrants further studies and better protection of the area, at least during the breeding season.