You’re probably familiar with hermit crabs—maybe you kept one as a pet when you were a kid. Picture a shell-less hermit crab with a carapace wider than your average male’s hand and legs that span a full meter, and you’ll have a pretty accurate mental image of a coconut crab. These massive crustaceans, the largest land-dwelling invertebrates on the planet, live on islands across the Indian and Pacific Oceans, and, as their name suggests, coconuts are their primary food source.
But not their only food source.
Dartmouth College’s Mark Laidre traveled to the Chagos Archipelago in the Indian Ocean for a two-month research expedition in 2016 to learn more about hermit crabs’ giant cousins, which hadn’t been comprehensively studied since the days of Darwin. He spent nearly every waking hour collecting data—measuring the strength of their claws, studying their social behavior, taking genetic samples—but he wasn’t prepared for what he saw the crabs do one night.
Coconut crabs share their islands with a variety of seabirds, including Red-footed Boobies. Occasionally, Laidre found booby carcasses in crab burrows, though he assumed they must be scavenged quarry—perhaps the birds had died at sea and washed up on the shore, and the crabs had dragged them home. However, his perspective changed when he watched a coconut crab climb into a tree toward a booby sleeping with its head tucked into its feathers. The crab lunged at the bird and easily broke one of its wings in a large claw.
“The bird fell to the ground and wasn’t able to go anywhere,” Laidre says, “and the crab slowly made its way down the tree and approached the bird again and grabbed the other wing.” Within 20 minutes, the smell of the booby’s blood summoned several more crabs. “The rest of it was very gruesome, them fighting over it while it was still alive, tearing apart this Red-footed Booby and dragging it away . . . I had a lot of trouble sleeping that night.”
Shocked though he was, Laidre soon learned that this was not a one-time occurrence; someone else had seen a crab kill an unwary booby in daylight two years before.
Crab predation on boobies is probably rare, but Laidre believes it still has the potential to shape where birds choose to nest. While boobies sometimes spend time on the island where both attacks occurred, they don’t nest there. Laidre surveyed three neighboring islands that were similar in every way, except that one hosted no crabs, and that crab-free island was the only one where seabirds nest on the ground. Of course, this is only a correlation and not experimental proof that crabs were behind the birds’ choice of nesting sites. Yet it's a pattern intriguing enough that Laidre hopes to return to the Chagos Archipelago in the future to learn more about coconut crabs’ relationships with the vertebrates that share their islands. He has a plan to use dummy eggs, for example, to confirm that crabs prey on nests as well as adult birds.
As if seabirds didn’t already have enough problems to deal with—plastic trash, declining fish stocks, rising oceans—add giant, predatory crustaceans to the list. At least this threat originated with Mother Nature. Coconut crabs have been on these islands since long before humans arrived on the scene, so for once, we don’t have to shoulder the blame.