Sept/Oct 2006For Wildlife Watchers: Atlantic ghost crabby Rob Simbeck, photography by Phillip Jones
It may be a magnet for vacationing humans, but the seashore can be a harsh, inhospitable place. The heat can be intense and drying, and occasionally devastating storms blow through. Perhaps most importantly for wildlife, the beach offers little or no cover from predators. Few creatures of any size can thrive there.
One that does is the Atlantic ghost crab, a creature marvelously suited to the task. It has, in fact, been called an evolutionary bridge, having adapted to the exposed, sandy strip at the ocean's edge.
One key element of the ghost crab's success is diet.
"They are generalists and opportunists," says Gary Sundin, a University of Georgia graduate student who has worked for several summers in loggerhead turtle conservation in South Carolina. "They are ideally suited to take advantage of the resources available on open beaches. In areas where coquina clams or mole crabs are abundant, ghost crabs prey heavily on them. They are scavengers, as well, and will eat most any dead thing the tide washes up. They also have the ability to 'deposit feed,' in which they pass sand through their mouthparts and extract the nutrients from the algae in the sand. And, of course, ghost crabs will tackle larger prey like sea turtle hatchlings when such prey is available."
Atlantic ghost crab - Ocypode quadrata "swift-footed square"
Description: Approximately 2 inches across, sandy white, periscopic black eyes.
Habitat and Range: Sandy beaches, Rhode Island to Brazil. Along most South Carolina beaches.
Reproduction: Stylized male fighting. Eggs laid in the water.
Viewing Tips: Night walks with flashlights along beaches, especially those away from developed areas. Be aware, though, that you should not use flashlights on the beach during turtle nesting season.
Escaping predators is the other prime survival challenge, and the aptly named ghost crab has a full arsenal. For a creature as common as it is, with its ubiquitous burrow holes and tracks, it keeps an amazingly low profile. Generally nocturnal, it forages from dusk until dawn and stays out of sight during the day. Its sandy coloration blends quite nicely with its surroundings - it is all but invisible until it moves. Then there is its speed - the Ocypode in its scientific name, Ocypode quadrata, means swift-footed. This is a creature that can zip off as quickly as the Road Runner, changing direction almost instantly, scooting into the safety of a burrow with amazing alacrity.
Although untrammeled beaches may hold the biggest numbers, even the most active North and South Carolina beaches are well-supplied with ghost crabs.
Elizabeth Wenner, senior marine scientist with the S.C. Department of Natural Resources' Marine Resources Research Institute in Charleston, recalls graduate-school fishing trips to North Carolina's Outer Banks with her husband, Charlie Wenner, also a marine biologist with the DNR.
"We would drive from our camp for about ten miles along a dune road," she says. "The headlights would illuminate hundreds of ghost crabs. My husband would drive slowly so that we wouldn't crush any, and I'll never forget how many crabs there were and how they were transfixed by our headlights."
The Atlantic ghost crab is one of twenty ghost crab species worldwide, part of a family that includes fiddler crabs. About two inches across, not counting its long legs, it has a pale, sandy color. Its eyes, held aloft on stalks, are capable of 360-degree vision, highly useful in spotting predators and, to a lesser extent, prey, as taste and smell are key in detecting food. It has both gills and simple lungs and must keep the former moist, which it does with several daily visits to the water's edge. Hairs near the base of the legs help in that effort, as does moisture in its damp burrow.
Younger crabs tend to dig closer to the water and generally excavate "short, vertical burrows," says Dr. Elizabeth Wenner. Other burrows, she says, slope downward at a 45-degree angle away from the shoreline to as much as 4 feet deep, with some having branches, one of which may extend upward almost to the surface. Burrows keep ghost crabs out of the hot sun and provide shelter both during the day and during cold spells - they become quiescent in temperatures under about 60° Fahrenheit. During the day they are likely to stay close by the burrow, cleaning and digging when not quietly resting inside.
As in most species, reproduction involves competition. Males square off for ritualized fighting, raising both their serrated claws and their bodies in threatening postures until one sinks into a submissive posture or until there is what Wenner calls a "pushing fight." Occasionally, the animals jump from their threat stances toward each other.
Copulation occurs on the sand or within burrows, and the female lays eggs in the water, with some researchers suggesting this occurs at high tide near the full moon. The larvae drift for four to six weeks, prey to small fish and other aquatic creatures, until those that survive return to the sand as apple-seed-size young.
The ghost crab has few predators, with raccoons being foremost among them, but humans' development is a prime hazard. Building and beach traffic can both displace ghost crabs and compact the sand, which can destroy burrows, force needed moisture from the sand, crush vegetation and enhance erosion from waves.
"Oil spills and industrial pollutants may also take their toll," says Wenner. "Natural phenomena such as erosion because of hurricanes, storms and diversion of sand by groins and jetties can also have an impact on ghost crab populations. Humans' attempts to rebuild eroding beaches can result in mortality and displacement of ghost crabs. Perhaps in the long run we are doing too much to preserve beach areas by unnatural means such as erosion-control measures that may actually cause harm to ghost crab populations and their prey."
Fortunately, the elusive, well-adapted ghost crab remains a vibrant part of the ecology of the state's beaches, well worth the extra effort it takes to track it down and see it in action.