From Gills to Lungs
The coconut crab, Birgus latro, is an evolutionary leap from sea to land. Unlike its aquatic relatives, it breathes air using a "branchiostegal lung." This organ is not a true lung but a highly modified gill chamber with tissues adapted to absorb oxygen from the air. While they retain rudimentary gills, these are greatly reduced in size, and adult crabs will drown if submerged for more than an hour. To keep their breathing apparatus functional, the crabs must keep it moist, often using their smallest pair of legs to stroke water over the spongy tissues.
The crab's life cycle begins in the ocean, where larvae are released and live as plankton. As juveniles, they resemble other hermit crabs and use empty gastropod shells for protection. Upon reaching adulthood, they abandon this mobile home, having developed a hardened, calcified abdomen that protects them and reduces water loss on land. This independence from shells allows them to grow to their immense size, reaching a weight of up to 4.1 kg (9 lbs) with a leg span of 1 meter (3.3 ft). They dig burrows for shelter, which they may close off with a claw to maintain a moist microclimate inside.