Annelids of the genus Ophryotrocha are globally distributed in a wide variety of organically enriched habitats. Although mostly known from shallow water, the expansion of deep-sea research revealed several new Ophryotrocha species mainly associated with organic-falls, in particular with cetacean carcasses, or whale-falls. Despite the great deal of interest that fauna associated with whale-falls have attracted, very little is known about this fauna in the deep-Atlantic Ocean. Hereby we analyse, for the first time, the Ophryotrocha assemblage associated with a mammal-fall in this region of the world's ocean. Specimens were collected from the bones of degrading cow carcasses deployed in the Set ubal canyon at approximately 1000 m depth. Using morphological characters and phylogenetic analyses based on the nuclear gene H3 performed on 31 Ophryotrocha species, we report the presence of five different species, we describe three new species and extend the distribution of O. scutella. Ophryotrocha scutella was previously known from a shallow-water whale-fall and organically enriched sediments beneath a fish farm off Scandinavia and is the first species to be reported from organic-falls in both shallow and deep waters. The presence of five congeners emphasize the importance of the genus Ophryotrocha in the degradation of organic matter in the deep-sea. Our results substantially increase the number of deep Atlantic species in the genus Ophryotrocha and highlight the remarkable unknown diversity and biogeographic patterns hidden behind unstudied deep-sea habitats.http://zoobank.org/urn:lsid:zoobank.org:pub:A12699DA-E580-4782-941A-48D7B31E4DAE

Sunken whale carcasses, known as 'whale falls', deliver large pulses of organic material to the seafloor and serve as habitat islands for unique assemblages of deep-sea fauna that include generalist scavenging species, chemosynthetic fauna related to those from hydrothermal vents and cold seeps, and bone-specialist species. Despite the growing interest in this peculiar habitat in the last decade, all long-term studies of deep-sea whale-falls have been restricted to the Pacific and the ecology of these habitats in the Atlantic Ocean has been overlooked. Here we report the first study on large mammal carcasses in the deep Atlantic Ocean. Cow carcasses amounting to 570 kg were deployed at 1000 m in the Setubal canyon, where cetaceans naturally occur. Eighteen months after deployment, all soft tissues had been consumed and the visible remains of the carcasses consisted solely of skeletal material that supported a diverse macrofaunal assemblage distinct from that of the background, including organic-enrichment respondents, sulphophiles and bone-specialists. The trophic structure of this assemblage denotes an overlap of successional stages previously described in whale falls. Our results provide important insights into the contribution of mammal falls to deep-sea biodiversity in the NE Atlantic, species distribution and biogeography of bone specialists, and the potential role of these habitats as dispersal stepping-stones for vent and seep species. Additionally we show that cow carcasses may serve as an accessible model system for the study of ecological processes at deep-sea whale-falls.