The first whale fall on the Mid-Atlantic Ridge: Monitoring a year of succession

Abstract

While the Mid-Atlantic Region is a frequented migration route for multiple cetacean species, to date no whale falls have been encountered or studied in the area. In 2015, a juvenile sperm whale was sunk south of Faial (Azores, Portugal), implanted at 760m depth and its decomposition was monitored for one year with seven submersible dives. Based on imagery, two different successional stages were observed: the mobile/scavenging stage characterised by sharks and 3 different species of decapods and the enrichment/opportunistic stage, dominated by polychaetes. While all scavengers were known background fauna for the region, whale fall specialist fauna was observed during the enrichment-opportunistic stage, including bone-eating worms (Osedax spp.). After one year at the bottom, a vertebra and a rib were sampled to assess the species composition. While all Polychaeta, Arthropoda and Mollusca families were known from whale falls, few could be identified to species level, indicating species new to science. Based on the presence of sulphide tolerant organisms (Mytilidae aff. Bathymodiolinae and Polycladida, Platyhelminthes) in the samples, a transition to the chemosynthetic-based stage appeared to have been initiated. Taxon overlap at genus and possible species level of some Polychaeta was observed with the cow fall off the coast of Portugal (Ophryotrocha cf. lusa and O. cf. sadina) and with the whale fall off the coast of Brazil (Pleijelius, Sirsoe syn. Vrijenhoekia), thus connecting both sides of the Atlantic Ocean.