The "Ankle-Breaker Crab" (Coenobita iversonii) is a species of carnivorous land crab found in the Caribbean Islands. It closely resembles the ordinary Caribbean hermit crab (Coenobita clypeatus), but is distinguished from its more common cousin by its habit of living in special titanium-reinforced shells, and its diet, which consists primarily of hikers.
Coenobita iversonii are most commonly found at altitudes of 150 m or more above sea level, living in colonies of 100 or more in the underbrush near hiking trails. They are the only arthropod known to feed primarily on humans, and they hunt through a highly developed behavior that has been the subject of much controversy over the years.
In order to obtain prey, a colony of Coenobita iversonii will lurk in low ground cover along the sides of a hiking trail on the upper part of a steep hill. The colony detects the approach of a human hiker through vibrations in the ground (rather like a trapdoor spider), and when the human draws close, a single member of the colony on the uphill side of the trail will retract into its titanium-reinforced shell, and roll into the middle of the path, where an unwary hiker can step on it. When the hiker slips on the round and highly durable shell, and falls to the ground with a broken ankle, the rest of the colony will swarm from the bushes, and devour their prey. A colony of Coenobita iversonii can skeletonize an adult human in five minutes or less. The bones will either be buried beside the trail, or hauled back to the colony's underground lair for use in creating more reinforced shells.
This hunting behavior is often cited as an example of "natural altruism" or group selection, as even with the titanium reinforcing struts inside the shell, the crabs who roll into the path suffer frightful levels of death and destruction, particularly along hiking trails popular with fat American tourists. Other scientists claim that the hunting method of Coenobita iversonii is so sophisticated and peculiar that it could only be the work of an intelligent designer (a sub-group of these scientists believe that it is the work of the Devil, while the majority say "Shhhh! We're not supposed to talk about that!"). Mainstream biologists scoff at the idea, though they admit that the exact mechanism by which such a behavior might evolve is not clear, nor do they have an explanation for where crabs get titanium and machine tools. Both sides agree that more research is needed.
In recent years, Coenobita iversonii populations have dropped preciptously. Long the beneficiaries of the near-total indifference of the Caribbean population, increased tourism and development have pushed it into decline, through a combination of habitat destruction, and the Westin Hotel Group's tendency to eradicate colonies with flamethrowers. They continue to survive in small enclaves in national parks and nature preserves. Some ecologists have suggested saving the species by seeding some colonies in the American Rockies, where they would help "thin the herd" of Californian tourists, but this suggestion is not taken seriously by reputable scientists, who prefer to move them to Australia, where "all the other freaky and venomous critters live."
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(This bit of fanciful naturalism is brought to you by a hike up the Caneel Hill- Margaret Hill trail on St. John, on which I saw lots of hermit crabs several hundred feet above sea level. The little guy shown above was fairly typical-- I was walking along the path, and heard a bit of rustling in the leaves on the uphill side, then a small shell came rolling down into the middle of the trail, where I took a picture of it.
(This happened a dozen times or more as I hiked up and down the hills. I have no idea what they thought they were accomplishing. Nor, for that matter, do I have any idea how or why these little crabs lugged those shells up to 800 feet above sea level, but there were tons of the things up there. Nature is weird.)
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Were they rolling onto the trail only when people were coming, or do they just fall all the time? How can you observe them without changing their behavior? Another application of Heisenberg!
That is amazing that they have those shells so far up from the ocean. I wonder does one crab carry a shell that far? Or did it take several generations of crabs discarding shells progressively farther inland?
There's a pretty simple explaination. You're on a hill, so a lot of the ground is going to be at an incline, which the crabs are able to traverse by using their 4 walking legs to get a grip. When something large and potentially dangerous comes by (like a human), they react by retracting their limbs into their shells, losing their grip on the hill. They then roll down the hill until an obstruction or a sufficiently level piece of ground (like a walking path) causes them to come to a stop.
IIRC in Florida some hermit crabs get huge; they used to crawl across roads sometimes and, when startled by an approaching car, retracted into their shells. The shells often punctured tires.
You recognize the irony here, I'm sure, in wondering what crabs that you observed on a hike might be accomplishing in their natural habitat.
Am I really the first one to say it?
"If we pull this off, we'll eat like kings!"
And natural habitat for hermits are off the beaten path? Um..., never mind.
Though I wonder where the close relative to these and the Coconut Crab were, the Skull-Crusher Crab? It is rumored to drop onto unaware victims and exercise them to death without using a proper weight bench.
Not bad for a puny crab.