Ghost crabs are common inhabitants of sandy coastlines worldwide, often spotted darting across beaches with remarkable speed. Their elusive nature and ability to seemingly disappear make them a subject of curiosity. This article describes their physical appearance and unique adaptations to their environment.
Overall Physical Characteristics
Ghost crabs possess a distinctive body shape, characterized by a squarish, box-like carapace that appears deep from the side and squarish from above. Their size can vary, with the carapace typically measuring around 5 cm (2 inches) across at maturity, though some can reach up to 6.4 cm (2.5 inches). Males generally tend to be larger than females.
The most striking aspect of their appearance is their coloration, which predominantly ranges from pale, sandy, or grayish-white. This coloration provides exceptional camouflage against the sandy beaches they inhabit. The texture of their shell is generally smooth.
Unique Identifying Features
Among their most recognizable features are their prominent, stalked eyes, positioned high on their head. These club-shaped eyestalks allow for an impressive 360-degree field of vision, crucial for surveying their surroundings. Ghost crabs can also retract their eyes into grooves on the front of their shell for protection.
Their two claws, known as chelae, are another distinguishing characteristic. One claw is typically larger than the other, a trait known as heterochely. This size difference is present in both male and female ghost crabs. These claws are used for feeding, defense, and digging burrows. Ghost crabs possess eight long, slender walking legs, adapted for swift movement across sandy terrains. While they have five pairs of legs in total, the first pair consists of their claws, leaving four pairs of walking legs.
Ghost crabs are semi-terrestrial, meaning they spend most of their lives on land, but they still rely on specialized gills for respiration. These gills are adapted to absorb oxygen from the air. They must periodically wet their gills to maintain moisture, often by wicking water from damp sand using fine hairs near the base of their walking legs or by running into the surf.
Appearance and Survival
The pale coloration of ghost crabs serves as an effective camouflage, allowing them to blend seamlessly with the sandy environment of beaches. This ability to match their surroundings makes them remarkably difficult for predators to spot. Their color can also change based on a daily rhythm, becoming lighter during the day and darker at night, further enhancing their camouflage and survival chances.
Their long legs and streamlined body contribute to their remarkable speed and agility. The genus name Ocypode translates to “swift-footed,” aptly describing their ability to move quickly, reaching speeds of up to 3-4 meters per second (approximately 6.7 to 8.9 mph), with some observations suggesting speeds up to 10 mph. This speed allows them to vanish rapidly, earning them their “ghost” moniker. When running at top speeds, they often use only two pairs of their walking legs.
The elevated placement and 360-degree rotational ability of their eyes are crucial for their survival. This wide field of vision enables them to detect both predators and prey from a distance, even while remaining partially hidden in their burrows.