Adult coconut crabs breathe with branchiostegal lungs, chambers lined with tissue that absorbs oxygen from air rather than water. Vestigial gills remain in the same cavity but are too reduced to supply an animal of this size. The lung lining must be kept damp, which is why the crabs wet their breathing chambers with their hind legs and stay in humid burrows through the heat of the day.
The life cycle keeps one foot in the ocean. Females carry fertilised eggs to the shoreline and release them into the sea, where the larvae drift as plankton for several weeks before settling, taking up a gastropod shell like any other hermit crab. Only later does the juvenile leave the shell behind and calcify its own abdomen, at which point the return to water becomes fatal.
The popular image of the crab climbing a palm to cut down coconuts is only partly right. Coconut crabs do climb and do open fallen coconuts, but most of the diet is fallen fruit, nuts, seeds and carrion, and the amount of active coconut harvesting is far smaller than the name suggests.