Johnson MP et al. (2013), A vertical wall dominated by Acesta excavata an...

ECB-ART-43132

PLoS One 2013 Jan 01;811:e79917. doi: 10.1371/journal.pone.0079917.

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A vertical wall dominated by Acesta excavata and Neopycnodonte zibrowii, part of an undersampled group of deep-sea habitats.

Johnson MP , White M , Wilson A , Würzberg L , Schwabe E , Folch H , Allcock AL .

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.

PubMed ID: 24260319
PMC ID: PMC3832389
Article link: PLoS One

Genes referenced: LOC100887844 LOC100893907 LOC115925415

Article Images: [+] show captions

	Figure 1. Location of the A. excavata -N. zibrowii biotope on the southern side of the surveyed canyon (green circle on inlay marked as ‘ROV’).Deeper waters have cooler colours.
	Figure 2. Photomosaic of A. excavata -N. zibrowii habitat at a depth of 666 m.Total area approximately 52. A prominent N. zibrowii is visible in the top left of the image, with other oyster individuals embedded in the matrix of A. excavata and other species.
	Figure 3. Images from different depths on the wall showing qualitative differences with depth, including more sponge cover at the shallowest depth, increased A. excavata and N. zibrowii with depth until the deepest, low biomass, section is reached.
	Figure 4. Variations in mean A. excavata shell height, A. excavata density and N. zibrowii density as a function of depth.The range of depths where A. excavata shells were observed on the wall was from 633 to 762 m (open square symbols). Lines are fitted linear regressions except for the panel displaying A. excavata densities where the line is a quantile regression estimating the position of the third quartile (0.75). Quantile regression coefficients were significantly different from zero when tested using bootstrap estimates of SE.
	Figure 5. Water mass properties in the Whittard Canyon.A. Temperature-salinity plots (CTD casts at 750 m, 1000 m, 1820 m, 3100 m). Isopycnals indicate potential density, σt (kg m−3) and the colourbar indicates depth (m). B. Temperature (°C) profiles at 750 m (dash line) and 1000 m (solid line). C. Salinity profiles at 750 m and 1000 m. D. Density,σt (kg m−3) profiles at 750 m and 1000 m.
	Figure 6. Light attenuation profiles (m−1) in the eastern branch of the Whittard canyon at 375 m (green), 750 m (red), 1000 m (blue) and 1320 m (black) water depth.Dashed lines (620–740 m) indicate depth of wall where high densities of suspension feeders were found.
	Figure 7. Transect of isopycnal displacement (absolute value in m) calculated from a 6–7 hr repeat transect across the canyon channel 4 km downstream of the ROV dive site.The southern side of the canyon (where the A. excavata-N. pycnodonte biotope was found) is on the right side of the figure. A scale is shown to the right, CTD locations by ‘x’ and the seabed by the black line. The parallel white lines across the transect indicate a possible beam of high isopycnal displacement associated with an internal wave emanating from the northern canyon wall.

References [+] :

Davies, Global habitat suitability for framework-forming cold-water corals. 2011, Pubmed