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We describe a novel biotope at 633 to 762 m depth on a vertical wall in the Whittard Canyon, an extensive canyon system reaching from the shelf to the deep sea on Ireland’s continental margin. We explored this wall with an ROV and compiled a photomosaic of the habitat. The assemblage contributing to the biotope was dominated by large limid bivalves,  Acesta excavata  (mean shell height 10.4 cm), and deep-sea oysters,  Neopycnodonte zibrowii,  at high densities, particularly at overhangs. Mean density of  N. zibrowii  increased with depth, with densities of the most closely packed areas of  A. excavata  also increasing with depth. Other taxa associated with the assemblage included the solitary coral  Desmophyllum dianthus , cerianthid anemones, comatulid crinoids, the trochid gastropod  Margarites  sp., the portunid crab  Bathynectes longispina  and small fish of the family Bythitidae. The scleractinian coral  Madrepora oculata,  the pencil urchin  Cidaris cidaris  and a species of  Epizoanthus  were also common. Prominent but less abundant species included the flytrap anemone  Actinoscyphia saginata,  the carrier crab  Paramola cuvieri,  and the fishes  Lepidion eques  and  Conger conger . Observations of the hydrography of the canyon system identified that the upper 500 m was dominated by Eastern North Atlantic Water, with Mediterranean Outflow Water beneath it. The permanent thermocline is found between 600 and 1000 m depth, i.e., in the depth range of the vertical wall and the dense assemblage of filter feeders. Beam attenuation indicated nepheloid layers present in the canyon system with the greatest amounts of suspended material at the ROV dive site between 500 and 750 m. A cross-canyon CTD transect indicated the presence of internal waves between these depths. We hypothesise that internal waves concentrate suspended sediment at high concentrations at the foot of the vertical wall, possibly explaining the large size and high density of filter-feeding molluscs.

A Vertical Wall Dominated by Acesta excavata and Neopycnodonte zibrowii, Part of an Undersampled Group of Deep-Sea Habitats