Biotope characterisation and compiled geographical distribution of the deepwater oyster Neopycnodonte zibrowii in the Atlantic Ocean and Mediterranean Sea

It may seem surprising that probably one of the largest and most long-lived deep-water bivalves, the gryphaeid Neopycnodonte zibrowii Gofas, Salas & Taviani in Wisshak et al. (2009) [1], experienced a taxonomic description as late as in the early 21 century. Previously N. zibrowii oyster findings wereinformally treated as ‘large oyster’, Ostrea sp. or they were admixed with the cosmopolitan gryphaeid Neopycnodonte cochlear (Poli, 1795). Thus, a taxonomic re-evaluation of some N. cochlear and Ostrea sp. samples from unusual depths and/or sizes led to additional records for N. zibrowii. A large number of dead N. zibrowii records were accidental findings deriving from dredging and contributing to the fact that just some records of the data compilation have previously been published in the literature. The everincreasing number of in situ recordings and direct sampling of N. zibrowii in its specific biotopes over the last 2-3 decades is related to the systematic reconnaissance of cold-water coral habitats as biodiversity hotspots and their environmental controls along the continental margins of Europe and Africa, and jointly with the increase in applying advanced marine technologies, such as manned submersibles and remotely-operated vehicles (ROVs). This study is based on geographical N. zibrowii records retrievedfrom cruises, non-shipbased submersible expeditions and terrestrial field trips, carried out between June 1882 and January 2016.The revealed geographical distribution of live N. zibrowii occurrences spans latitudinally from 48° N (Whittard Canyon, Celtic Sea) to 9° S (Anna Ridge off Angola, South Atlantic Ocean), and longitudinally from 28° W (Faial Channel, Azores, North Atlantic Ocean) to 13° E (Urania Bank, Sicilian Channel, central Mediterranean Sea). Late Pleistocene submerged occurrences further extent this distribution to 26° E (southeast off Crete, eastern Mediterranean Sea). Bathymetrically, live N. zibrowii are found between 234 m (off Mauritania, North Atlantic Ocean) and 983.5 m water depth (Emile Baudot Escarpment, western Mediterranean Sea), usually between 500 and 700 m. Within suitable physical limits and nutritional settings, N. zibrowii grows on hard substrates and shows the highest colonisation densities in sedimentprotected and current-exposed zones, such as rocky overhangs and (sub-) vertical rock exposures on seamounts, escarpments and in canyons. Nowadays, also anthropogenic items, like shipwrecks, serve as substrate. The lifespan of the thick-shelled N. zibrowii can exceed 500 years, thusbelonging to the longest-living non-colonial animals; the largest valves with up to 30 cm length were reported from the Azores. Live N. zibrowii and their post-mortem remains serve as substrate for their own larvae, generating over several generations a built-up, formed as chaotic clusters or stacks. Also, they provide a suitable ecospace for a diverse associated community of attached/excavating sclerobionts, mobile organisms and for demersal benthopelagic fishes, and therefore configure a well-defined deep-sea habitat.Neopycnodonte zibrowii often co-occurs with scleractinians, such as Lophelia pertusa, Madrepora oculata and Desmophyllum dianthus. Based on video and image material from ROV and submersible dives, we characterise the N. zibrowii biotopes, following the “Coastal and Marine Ecological Classification Standard”.Fossil N. zibrowii can be preserved in situ (double valved) and contribute to a lithified oyster-Desmophyllum framestone, as reported off Crete, but more commonly, the non-cemented right valve falls off and is accumulated at or near the base of the steeply inclined substrate. The oldest geological record dates back to the Early to Mid-Pleistocene and is based upon shells putatively attributable to N. zibrowii occurring on outcrops at Capo Milazzo (Sicily, Italy) and St. Paul’s Bay Limestone (Rhodes, Greece).The present database contains 261 entries and provides valuable sitespecific metadata, including standardised describers for the variety of observed deep-sea oyster biotopes.