Abstract
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.
Highlights
The continental margins, along with other parts of the deep oceans, still represent locations of discovery science [1], [2]
(Figure 1) we encountered a vertical wall extending from 631 m depth to 780 m depth
On this wall we identified the presence of large numbers of Acesta excavata on the vertical faces
Summary
The continental margins, along with other parts of the deep oceans, still represent locations of discovery science [1], [2]. The novelty of deep-sea habitats is reflected by a habitat-discovery curve that does not yet have a plateau [3]. This relative lack of knowledge of deep-sea habitats is coupled with incompletely characterized, but important, ecosystem functions and services including nutrient recycling, carbon sequestration and nursery areas [4]. Sampling constraints have certainly restricted the description of deep-sea habitats. The vertical faces of canyon walls and other related habitats have only become accessible to survey with the availability of deep-water ROVs [5], [6].
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