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.
A Crushing Grip and a Keen Nose
The coconut crab's claws are its most formidable tools. The pinching force of a large crab can reach 3,300 newtons, a force that exceeds the bite of most terrestrial predators and is comparable to a lion's bite. This strength allows them to crack open coconuts, but their diet is highly varied. They are opportunistic omnivores, feeding on fleshy fruits, nuts, and seeds, as well as scavenging on carrion and even preying on other crabs.
To locate food, the coconut crab relies on an exceptionally well-developed sense of smell. Its antennae are covered in specialized organs called aesthetascs, which are shorter and blunter than those of aquatic crabs and more similar to those of insects, showing convergent evolution. By flicking their antennae, they can detect the scent of rotting fruit or meat from considerable distances. This powerful sense of smell is their primary method for finding food across the island's forested interior, sometimes miles from the sea.