The polychaetous annelid Diopatra biscayensis Fauchald et al., 2012 was recently described from the Atlantic coast of France. It has been the subject of a plethora of publications dealing with its importance in the field of ecology: from its role as an ecosystem engineer, being an indicative species of climate change in western Europe to questions of whether it was native or introduced to the old continent, spawning theories about its hypothetical routes of introduction and spreading. We have redescribed D. biscayensis, traced its biogeographical history in the Bay of Biscay through examination of old museum holdings and studied its ecology and reproductive biology throughout a one year period from a northern Spain estuary, showing for the first time that the species is a protandric simultaneous hermaphrodite. The annual spawning season is from early August to late September, when large numbers of oocytes of 260 μm diameter were deposited in gelatinous egg masses attached to the parental tubes. Early trochophores developed in the jelly mass by 4–6 h, 3-chaetiger metatrochophores after 48 h, and the jelly mass had totally disintegrated by 72 h, releasing the lecithotrophic larvae. Our studies have clarified the morphology and reproductive pattern of D. biscayensis, documented that the species has inhabited the European waters for more than a century and we hope that these findings will serve as a basis to robust ecological studies and hypotheses concerning this and the related species.