Sunday, 11 March 2012

Ghost Crab

 Ghost crabs are rather cute crustaceans which are difficult to spot because they are sand-colored and somewhat diaphanuous. Their stalked black eyes give them an extraterrestrial appearance, and they scurry about in a rather purposeful manner — but you will probably only see them at dusk or at night. Ghost Crabs scavenge the beach, looking for tasty things to eat. Once a year, when baby sea turtles are hatching out, they enjoy special feasts. They drag the three-inch-long hatchlings down into their underground burrows, and devour them.

You might notice the golfball-sized entrance holes of the Ghost Crab burrows in the dry sand of the upper beach, or in the sand dunes. The burrows extend down 3-4 feet. To watch some species of Ghost Crabs built (or repair) their homes is particularly fascinating. The “Ghostie” brings up clawfuls of sand and tosses them 6-12 inches away from the burrow opening. Later on, the Ghost Crab tromps down the strewn-about sand, and, using its claws, smooths out the surface. (In contrast, other species bring up the sand in the form of little balls and leave them scattered about the entrance.) Crab tracks also clearly mark the burrow entrance. Yet another entrance style is represented by a dome of sand which covers the burrow hole. Obviously some Ghosties are more inclined than others to camouflage their home. 
.
A large Ghost Crab, seen at Marsa Honkorab - Marsa Alam - Red Sea - Photo by/ Ahmed F.Gad - Copyright © 2009
The burrow may slant down at a 45° angle, and has a “turn-about” chamber at the end. The tunnel home is constructed with wet grains of sand so that it will not collapse. In the winter Ghost Crabs hibernate in their burrows, “holding their breath” for six weeks by storing oxygen in specialized sacs near their gills.

When not hibernating, the Ghost Crab has to wet its gills periodically for purposes of both respiration and reproduction. The creature maintains a little seawater in the bronchial chambers. When this supply of water needs to be replenished, the Ghost Crab approaches the shoreline sideways, standing there until a wave washes in far enough to wet him. Then he scampers back to the upper beach.

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.
 
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 Red Sea’s beaches, well worth the extra effort it takes to track it down and see it in action.


_____________________________________________________________________

Notes

  • The specimens pictured here were found at The Red Sea.
  • For more information on the Ghost Crab see:
    • Todd Ballantine, Tideland Treasure.
    • Peter Meyer, Nature Guide to the Carolina Coast.
    • Carol M. Williams, Beach Bountiful: Southeast.
  • Classification: Genus Ocypode quadrata; Family Ocypodidae.

No comments:

Post a Comment