|Anyone who has walked on the beach at night along the East Coast may have spotted a quick flash of eyeshine dart away and disappear. These spooky occurrences are the work of a common, yet seldomly seen, mesocarnivore of the Atlantic sand dune community. The nocturnal behavior of the Atlantic ghost crab is one of the reasons it achieved such a name. Although their secretive lifestyle keeps them hidden from most beachgoers, this crab is an important link in the trophic cascade of sandy beach ecology and a valuable indicator of habitat health.
Cryptically colored to match their sandy surroundings, Atlantic ghost crabs are pale yellow to grayish with irregular darker spots scattered across the carapace. No matter the color of the carapace, every crab has bright white claws. Multiple studies have shown that Atlantic ghost crabs can change their coloration to match their surroundings, a camouflage adaptation seen in other marine species. However, this color alteration is not instant but rather takes about a month to complete.
Adult carapaces average around two inches in width but have been recorded as large as three inches across in healthy systems. Males are usually larger than females, as with many crab species. Related to the popular fiddler crabs, Atlantic ghost crabs also have unequal chelipeds, or claws, with one growing about double the size of the other. However, both male and female ghost crabs have one large and one small claw, unlike the exaggeratedly large claw of only male fiddler crabs.
Atlantic ghost crabs inhabit most of the beaches along the East Coast, spanning from Rhode Island in the North to Brazil in the South. Popular among the Chesapeake and Delaware Bay systems as well as the Outer Banks, ghost crabs can survive wherever enough sandy beach space exists. Although the terrestrial species cannot swim, it somehow made it to Bermuda and many of the Caribbean Islands.
Individual ghost crabs do not require much space, as territories usually average just over 4,000 square feet of beach and often overlap with other crabs. Burrows are dug between the high tide line and a quarter mile away from the water, with a caste system in place where younger crabs dig their burrow near the surf while the oldest crabs dig at the furthest distance from the water line. Needing saltwater to moisten their lungs every several hours, ghost crabs are bound to live near the ocean as they must visit the surf frequently to keep breathing.
Reproduction occurs year-round within most of the Atlantic ghost crab’s range, however, within the colder northern areas breeding is limited to the warmer months. A viable female visits a male’s burrow, where copulation occurs. After mating, the mother carries the developing eggs under her abdomen and must keep them wet by frequenting the surf, where she may flip upside down in the water to better aerate the egg mass. The eggs then hatch within the ocean and float away as planktonic zygotes, drifting wherever the sea takes them.
After six stages of development in the ocean, the metamorphosed immature crabs climb upon the shore where they will live for the remainder of their lives. Young ghost crabs build burrows near the water line as they require more frequent journeys into the surf to wet their small gills and to keep away from older crabs, who are known to prey upon the immatures. After one year, both males and females reach maturity and can reproduce. Most ghost crabs only live for about three years.
Atlantic ghost crabs are considered both omnivores and scavengers, consuming whatever the dynamic beach environment provides. When on the hunt, they seek out small clams and mole crabs at the intertidal zone but will also eat marine worms or insects along the way if found. When in sea turtle territory, the crabs will often raid the underground nests, stealing the eggs. This unfortunate behavior is under scrutiny where protections are established for endangered turtles and some eradication of local crab populations has been conducted to protect the nests.
When prey is not readily available, ghost crabs will graze upon dune grasses, seaweed, and tidal wrack (detritus) that washes ashore. Their nocturnal nature prevents many predators from having an opportunity to prey upon them, however, gulls and other shore birds lead the list of top predators. Burrowing owls and other raptors are known to dine upon the crabs, and recently raccoons have been documented feasting upon ghost crab communities. Much research has centered around the important link ghost crabs play in transferring energy from lower to higher trophic levels within the beach ecotone, a quintessential niche of mesocarnivores.
The translation of Ocypode, the Atlantic ghost crab genus, is “swift foot” and they live up to the name. Able to scurry across the uneven sand at an impressive 9 mph, ghost crabs can move side-to-side, front and back with equal ability and speed. This fleet of foot allows them to catch prey in all directions, grabbing it with their smaller, swifter claw. Then the large claw crushes the prey and holds it as the small claw picks off pieces to feed upon. Large, stalked eyes can rotate to see 360° around the crab to both locate prey and detect predators. The eyes are extremely sensitive to light, allowing the crabs to see accurately at night. It is well known that ghost crabs are more active during a full moon, as they can see better in the increased light. This trait also contributes to their namesake, as they appear ghostly while traversing the beach by moonlight.
As morning approaches, the crabs return to the safety of their burrows, escaping the heat of the day and the plethora of predators that follow. Each burrow can extend up to 4.5 feet deep and holds only one crab, as ghost crabs remain solitary except when breeding. The top of the tunnel is usually covered with sand during the day to conceal the location and insulate the subterranean home. In northern climates, ghost crabs can hibernate within their burrows from October until April.
Atlantic ghost crabs communicate through a series of sounds created by their legs and gills. They scrape their legs together to create a rubbing sound or tap their claws on the sand, and can make a bubbling sound by moving water through their gills. Perceptive sunbathers unknowingly lying above a burrow can hear this strange bubbling sound below. The meaning behind these communicative sounds is not currently understood.
INTERACTIONS WITH HUMANS
Due to the nocturnal habits of ghost crabs, most humans never directly interact with them. Even when trying to locate the crabs during nighttime adventures on the beach, ghost crabs are acutely aware of their surroundings and much faster than any person, so they avoid us. The shy, speedy crabs are near impossible to catch and typically will not fight back even when disturbed, so they pose absolutely no danger to humans. Ghost crabs are not considered edible either, so don’t waste your time attempting to catch any for dinner.
Multiple studies have shown that Atlantic ghost crabs are reliable indicators of ecosystem health. Their presence in a system means the trophic cascade is operating at optimum, while their absence is cause for concern. Unfortunately, increasing human activity along beach communities is having a profound negative impact on ghost crab populations. Compaction of the sand caused by foot or vehicle traffic can both crush individuals and their burrows. Destruction of sand dune habitat through commercial or residential development also challenges ghost crab survival.
Beach erosion is a significant problem in many East Coast communities, where tons of sand are washed away each year. To combat this issue, many locations import sand from other areas or build seawalls, both of which can destroy entire populations of ghost crabs. The bulldozing of new sand across the beach collapses burrows and seawalls direct wave energy downward which also destroys crab burrows. Without adequate subterranean habitat, ghost crabs will be eliminated from the community causing great disruption to the fragile beach food web.
Although Atlantic ghost crab populations are deemed stable across most of the Eastern Seaboard, local populations remain at risk from irresponsible human activities. As research continues to find beneficial ecological aspects of ghost crab presence, it is time for local beach managers to consider potential negative impacts on crab survival for every proposed project. Protection of sand dunes is critical to not only ghost crabs but a multitude of other species. Creating established human foot traffic pathways/boardwalks minimizes impacts upon burrows while making beach travel easier for humans. Some responsible beach communities are leading the way with such proactive projects and hopefully, more will follow this lead to protect the ecologically important Atlantic ghost crab.