Adaptations to hypoxia in hydrothermal-vent and cold-seep invertebrates

The deep sea harbors very unusual environments, such as hydrothermal vents and cold seeps, that illustrate an apparent paradox: the environmental conditions are very challenging and yet they display a high biomass when compared to the surrounding environment at similar depth. Hypoxia is one of the challenges that these species face to live there. Here, we review specific adaptations of their respiratory system that the species have developed to cope with hypoxia, at the morphological, physiological, and biochemical levels. Most studies to date deal with annelids and crustaceans, and trends can be drawn: development of ventilation and branchial surfaces to help with oxygen extraction, and an increase in finely tuned oxygen binding proteins to help with oxygen storage and transport. Beside these respiratory adaptations most animals have developed enhanced anaerobic capacities and specific ways to deal with sulfide.