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

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.

Ecological Role of Submarine Canyons and Need for Canyon Conservation: A Review

“…In Whittard Canyon (Celtic Margin of the NE Atlantic), accelerated currents increase the organic matter influx and therefore the availability of food compared to less active areas on the continental slope (Morris et al, 2013;Palmas et al, 2015). In the upper region of Whittard Canyon (∼700 m depth), a very dense assemblage of corals and large bivalves was observed associated with a nepheloid layer that might provide a significant amount of food (Johnson et al, 2013). Furthermore, the scleractinian coral Lophelia pertusa was observed at great depth and higher densities in this canyon, than usually recorded in the NE Atlantic .…”

The Whittard Canyon – A case study of submarine canyon processes

“…Neopycnodonte zibrowii (Johnson et al 2013). Other commonly observed species found within this assemblage included the cup coral Desmophyllum dianthus, unidentified feather stars and unidentified pink cerianthids.…”

“…At the foot of these walls, high concentrations of SPM and the presence of bottom nepheloid layers have been recorded (Huvenne et al, 2011, Johnson et al, 2013. The potential increased mixing following the occurrence of down-canyon sediment gravity flows or the presence of internal waves (Wilson et al, 2015a) cliff habitats may act as refuges from fishing activities, play a role in providing nursery habitats and protection against predation, and add complexity beneficial for other filter feeders (Huvenne et al 2011, Johnson et al 2013). …”

Morphological and genetic variation in North Atlantic giant file clams, Acesta spp. (Bivalvia: Limidae), with description of a new cryptic species in the northwest Atlantic

“…A frequent associate of these deep-water communities is the giant file clam, genus Acesta H. Adams & A. Adams, 1858 (Johnson et al 2013;Kenchington et al 2014). In the northeast Atlantic, the European giant file clam, A. excavata (J.C. Fabricius, 1779), was the first species of this group to be described (López Correa et al 2005) and, out of ca.…”

“…30 recent Acesta species, it probably has the broadest geographic distribution. Since its original description based on Norwegian specimens (Fabricius 1779), A. excavata has been recorded from a wide range of depths (33 to 3200 m) and localities in the northeast Atlantic (Vokes 1963;López Correa et al 2005;Järnegren & Altin 2006), including the North Sea, Iceland and Greenland, the Mediterranean, the continental margin south of Ireland (Johnson et al 2013), the Canary Islands, and along the Mid-Atlantic Ridge, between the Azores and Iceland (Dautzenberg 1927;Copley et al 1996). It also occurs in Pliocene-Pleistocene deposits in the Mediterranean region (López Correa et al 2005).…”