Functional respiratory anatomy of a deep-sea orbiniid polychaete from the Brine Pool NR-1 in the gulf of Mexico

An undescribed species of Orbiniidae (Annelida; Polychaeta) is found in large numbers associated with communities of the mussel, Bathymodiolus childressi at hydrocarbon seeps on the Louisiana slope (Gulf of Mexico). Their microhabitat is often hypoxic and sulfidic, which poses serious respiratory challenges for an aerobic metazoan. They display several anatomical features that are quite unusual for this family, which likely allow them to live in their food-rich, but oxygen-limited, habitat. The anterior gills are hypertrophied whereas the posterior gills are not. These anterior gills provide the worms with a large gill surface area (≤9.9 cm²/g wet weight), which represents 90-95% of the total gill surface area. The gills contain two blood vessels: a central blood vessel, delimited by a coelomic epithelium, and an intraepidermal vessel. The diffusion distance between this latter and the environment is only 3 μm in the anterior gills, which facilitates gas diffusion. Only the anterior gills are ciliated, which may also facilitate gas exchange across this respiratory surface. The gill cells also contain numerous mitochondria and other electron-dense organelles that might be involved in sulfide detoxification.