Abstract
Extensive beds of the deep-sea mussel Bathymodiolus mauritanicus (currently also known as Gigantidas mauritanicus) linked to active cold seeps related to fissure-like activity on Al Gacel mud volcano, Gulf of Cádiz, were filmed and sampled for the first time during the oceanographic expedition SUBVENT-2 aboard R/V Sarmiento de Gamboa. Al Gacel mud volcano is one of up to 80 fluid venting submarine structures (mud volcanoes and mud volcano/diapir complexes) identified in the Gulf of Cádiz as result of explosive venting of hydrocarbon-enriched fluids sourced from deep seated reservoirs. This mud volcano is a cone-shaped edifice, 107 m high, 944 m in diameter constituted by mud breccias and, partially covered by pavements of seep carbonates. Extensive beds of this deep-sea mussel were detected at the northern flank at 810–815 m water depth associated with bacterial mats around intermittent buoyant vertical bubble methane plumes. High methane concentrations were measured in the water column above living mussel beds. Other chemosymbiotic species (Siboglinum sp., Solemya elarraichensis, Isorropodon sp., Thyasira vulcolutre and Lucinoma asapheus) were also found in different parts of Al Gacel mud volcano. Al Gacel mud volcano may currently represent one of the most active mud volcanoes in the Gulf of Cádiz, delivering significant amounts of thermogenic hydrocarbon fluids which contribute to foster the extensive chemosynthesis-based communities detected. This finding is of paramount importance for linking extremophile bivalve populations along the North Atlantic, including cold seeps of the Gulf of México, hydrothermal vents of the Mid-Atlantic Ridge and now, detailed documented at the Gulf of Cádiz.
Highlights
Bathymodioline deep-sea mussels (Mytilidae, Bathymodiolinae) are one of the dominant macroorganisms of chemosynthesis-based communities in hydrothermal vents on spreading ridges and back-arc basins and in cold-water seeps along subduction zones across the globe (Miyazaki et al 2010; Laming et al 2018)
This study provides, for the first time, images and data of dense beds of living deep-mussels B. mauritanicus associated with cold seeps sourced from one of the most active mud volcano, Al Gacel Mud volcanoes (MVs), of the Gulf of Cádiz (GoC), NE Atlantic
This study suggests that the GoC was an area of formerly widespread populations of Bathymodiolus mauritanicus associated with a past higher activity of cold seeps, which has waned, persisting in some isolated areas, such as Al Gacel MV that currently represent a relict oasis for this deep-mussel species (Fig. 1)
Summary
Bathymodioline deep-sea mussels (Mytilidae, Bathymodiolinae) are one of the dominant macroorganisms of chemosynthesis-based communities in hydrothermal vents on spreading ridges and back-arc basins and in cold-water seeps along subduction zones across the globe (Miyazaki et al 2010; Laming et al 2018). According to the ‘‘Evolutionary stepping stone hypothesis’’, the ancestors of bathymodioline deep-sea mussels exploited resources from organic-fall habitats (e.g. sunken wood, whale carcasses) in their progressive adaptation to deep-sea reducing environments (Distel et al 2000; Jones et al 2006). These changes of habitat preferences resulted in adaptive trends in shell lengths (related to the availability of space and energy, and physiological trade-offs) and in the successive colonization of greater water depths (Lorion et al 2013). Fueled by energy derived from chemosynthetic symbioses, their contribution to ecosystem productivity is conspicuous, with many bathymodioline species forming dense and extensive aggregates and beds (Petersen and Dubilier 2009)
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