Vampire squid
In order to cope with life in this habitat, vampire squid have developed several radical adaptations. Of all deep-sea cephalopods, their metabolic rate is the lowest. Their blue blood hemocyanin binds and transports oxygen most efficiently, aided by gills with especially large surface area. The animals have weak musculature but maintain agility and buoyancy with little effort thanks to sophisticated statocysts (balancing organs akin to a human's inner ear) and ammonium-rich gelatinous tissues closely matching the density of the surrounding seawater.