Atlantic Ghost Crab: A fascinating decapod

A common but always interesting sight along the beaches of Cape May, New Jersey - and far beyond - is the Atlantic Ghost Crab (Ocypode quadrata). They're well named, as the crab's sandy coloration and fleeting movements, combined with general wariness, makes them wraithlike. People new to them might wonder if they actually saw something and if so, what it was. But with minor perseverance it isn't hard to get good looks at these fascinating decapods (ten-legs).

A crab lurks at the entrance to its burrow. They are accomplished diggers and always seem to maintain burrows to which they can retreat. Some tunnels can apparently be four feet deep. But the excavation is easy in soft beach sands and it does not take them long to mine out a new hole.

A crab in the act of construction. It uses its first two legs and the claw on that side (the claw, or cheliped, is actually part of a highly modified leg) to create a basket in which it scoops sand from the excavation.

Once the subterranean dwelling is complete, it becomes the crab’s home base. They retreat to it in the blink of an eye if threatened or disturbed. When exiting, I noticed that a crab would sometimes pause below the opening, and use its long-stalked periscope-like eyes to check the surroundings before emerging. They can rotate the eye stalks 360 degrees, increasing their efficiency at predator detection. I would imagine that gulls are one of the major threats. Great Black-backed, Herring, Laughing, and Ring-billed gulls are constantly patrolling the shoreline and would snap up a crab in a heartbeat if given a chance.

A crab dines on what appear to be the remains of another crab. They are omnivorous opportunistic scavengers, cleaning the beaches of various edible detritus. The chelipeds are used like hands, and the crabs quite dexterously handle their meals. They also will thump the claws on the sand, apparently to send signals to other crabs, and may use them as semaphores in mating rituals as do some other crabs, but I’m not sure about the latter.

There is also another use for the claws: jousting with rivals. I saw this several times, and finally got an opportunity to photograph a joust. Occasionally a foraging crab would stray far from its burrow, or venture to the sea to wet its gills. On these trips, if it would trespass too closely on another occupied burrow, the offended crab would sometimes pop out and do battle with the interloper. That’s what happened here. The guy on the left owned the burrow, and the intruder on the right was quickly vanquished after a brief locking of the horns. He sidled speedily back to his own burrow.

Atlantic Ghost Crabs take about a year to reach sexual maturity. After mating, the female deposits eggs in the ocean. The larvae develop there, and then head ashore as young crabs. I saw many juveniles on the beaches, but only because I was looking. They can be really tough to spot. The crablets are small, and their camouflage is incredible. If immobile or partially buried in the sand, they are essentially invisible. At this stage, I imagine various shorebirds – plovers, sandpipers, etc. – are major threats. The coloration and patterning of the carapace and upper legs is amazingly sand-like – a textbook example of crypsis.

After my crustacean hiatus, it was back to the business at hand: bird photography, and there was no lack of subjects there.