Honolulu Star-Advertiser © Susan Scott
April 19, 2010
Information on Hawaii’s ghost crabs is obscure When fourth-graders Julia and Alyssa of Pearl City’s Waiau Elementary School e-mailed to thank me for my column on crabs, I wasn’t sure which column they meant. After a few exchanges, I learned that Alyssa is writing a report on ghost crabs and needed more information.
Usually I encourage students to look up marine animal facts on their own, but Hawaii’s ghost crab details are nearly as elusive as the crabs themselves. Besides that, I like these racing recyclers and wanted to learn more, too.
Alyssa wanted to know whether ghost crabs can really walk 50 mph. As with most aspects of biology, the answer is complicated.
Several Internet sources state that ghost crabs run about 10 mph, but those sites refer to the ghost crab species of the U.S. East Coast. Ours are different.
Two kinds of ghost crabs grace Hawaii’s beaches. The largest one is called the horn-eyed ghost crab, a reference to its elevated eyes, and its shell grows to about 3 inches wide. A narrow hornlike shoot rises from the center of each of this crab’s oblong eyes, which themselves stand on stalks. Because eyes so exposed need to dodge obstacles, ghost crabs can lay their long eyes sideways in grooves of their shells.
It’s nearly impossible to sneak up on a healthy ghost crab, because those supersize eyes give the crab 360-degree vision. The crabs are also world-class sprinters. Australian researchers clocked their horn-eyed crabs (the same species as Hawaii’s) running on a beach at 4.1 mph.
I don’t know whether the nonskid surface on the deck of a boat provides better traction than sand or if the crabs were just more alarmed, but when the Aussie biologists timed the crabs’ sprints on their boat deck, the crabs ran even faster at 5.2 mph.
That might not sound anywhere near cheetah records of 60 to 70 mph, but in one way ghost crabs are even speedier. When you calculate speed as the number of its own body lengths an animal can run in a given time, the ghost crab beats the cheetah.
A ghost crab can also do something no mammal could accomplish. During an all-out run a ghost crab can change the leading side of its body. The crab races sideways on two or three pairs of legs at a time. When those legs need a rest, the animal pivots 180 degrees, without slowing down, and continues running on an alternate set of legs. And when a route change is prudent, ghost crabs can alter their direction almost instantaneously.
Hawaii’s other ghost crab is the pallid ghost crab and grows only about an inch wide.
In Hawaiian both species are known as ‘ohiki.
You can tell which ‘ohiki is which without even seeing the crab. To dig a burrow, the horn-eyed crab scoops sand in a front pincer, carries it to the opening and piles it up in the shape of cone. The pallid crab stays inside its hole and heaves the sand out, leaving a fan-shaped pattern.
Ghost crabs keep our beaches clean, eating anything and everything they find. Periodically, these crabs must dip their gills in the ocean to keep them wet and functional. At those times the crabs are prey to moi (Pacific threadfin), shoreline fish with a taste for ghost crabs.
Julia and Alyssa thoughtfully asked how they could help with this ghost crab column. Here’s one way: In your report, remind people to give aloha to these remarkable native animals by not letting dogs dig them up and kill them. Crabs need love, too.