I walked confidently on the yellow sand, staring at the shells deposited on the beach by the powerful waves of the Atlantic Ocean. Movements of the water displaced tiny pieces of sand along the shore, exposing sand dwellers quickly snatched by the seashore birds.
As we were walking along the water, my sister screamed suddenly, “Watch it!” Confused, I froze in place looking around. She pointed at the sand right in front of me. A ghost crab (Ocypode quadrata). I was inches away from its body and it took me several seconds to realize he was actually there. Great camouflage!
His closely spaced, black eyes were staring directly at me, unsure about my intentions. Looking at crustaceans is like looking into the very distant past. The ghost crab evolved over 100 million years ago, during the Cretaceous period. Compare that to modern humans, which are thought to have evolved from Africa about 200,000-300,000 years ago. Crab evolutionary history is not fully understood, but over the evolutionary timeline many marine species tried to imitate the crabs.
Male ghost crabs have a very unique anatomical feature absent in females. One of their claws is much bigger than the other. Such differences between males and females are called sexual dimorphism.
Ghost crabs blend very well with their environment, and their body color can adjust to their environment over longer periods of time. For that purpose, they have special cells called chromatophores, that are responsible for making pigment.
Contrary to popular belief, ghost crabs cannot swim, and if submerged in water will drown. However, they require moisture for their gills in order to breathe. Thus, you can sometimes spot ghost crabs near water, but always immediately to the beach.
Ghost crabs are semi-terrestrial. They burrow right underneath the intertidal zone (the area between high and low tides, where the ocean meets the sand), and prefer to come out at night, although they can be spotted during the day.
Unfortunately for the crabs, they are a treat for the gulls. I have seen many seagulls consuming crabs, always making sure to avoid their sharp claws.
Although quite common, ghost crabs are not as present on crowded beaches, as human foot traffic destroys their burrows. This significantly alters their distribution, and skewes their presence to the far sides of the beach. Research shows that some ghost crabs will dig deeper, steeper, and smaller burrows on heavily populated beaches. Specifically, car traffic on the beach and rapid urbanization of the adjacent marine neighborhoods were found to be associated with a high incidence of crab mortality. Thus, even if we do not litter on the beach, we will always leave our mark.
Ghost crabs developed interesting ways of communicating. They can make sounds with their claws when the enemy is further away, and another with their gut, specifically their gastric mill used to grind food, when the enemy is closer. Precise reason for this is not known, but it is believed that it allows the crab to free its claws, and strike back if necessary.
I hope next time you see those amazing creatures, you won’t see it primarily as dinner, but you think of its history and importance to the marine ecosystem.
Marta Grońska-Pęski 