Ocean Watch
By Susan Scott
Friday, July 13, 2001
Kailua Beach has always been one of my favorite places to walk. The beach is wide and flat, the firm sand makes good footing, and the tradewinds blow in the most amazing stuff. Ghost crabs haunt
unlucky sea creaturesLast week, I found three hammerhead shark pups there, about 2 feet long, lying dead on the sand. When you see hammerheads like that, they usually were caught in a net and released too late to survive.
Young hammerheads often swim in small groups when they're young, making it common for anglers to net several of these small sharks at a time.
As I stared at my Kailua Beach hammerheads, I noticed they were missing small chunks of flesh from their bodies and that messy piles of wet sand surrounded them. It appeared a dog had been digging around the fish. But a flash of movement told the real story.
A closer look revealed that the tossed sand came from holes in the ground. Ghost crabs had found the dead sharks and moved in for the feast.
How does a little crab eat a hammerhead shark? Very efficiently.
Ghost crabs, often called sand crabs in Hawaii, live on the beach between the ocean and the land and are therefore part land crab, part marine.
Like most marine crabs, ghost crabs are both predators and scavengers, eating whatever they can catch or whatever drifts their way. When a scavenging crab comes upon a meal, it uses an assembly line of claws, jaws and teeth to get the food through its body.
First, the crab uses its front claws to rip off a piece of flesh. The claws pass the piece to two outer jaws, which meet together over the crab's mouth like a pair of double doors. These jaws open wide and push the food further inside. There, three more pairs of jaws go to work. One pair holds the morsel while the two others tear it up. When the pieces are small enough to fit in the mouth, down they go through the esophagus.
Once the food lands in the stomach, the crab chews it up with three teeth. This toothy stomach, called the gastric mill, is common in most crabs.
Ghost crabs can be efficient predators. Once, at the first light of morning, I went for a walk on the beach at Tern Island. This island is part of Hawaii's northwest wildlife refuge and is one of the few spots left where Hawaii's sea turtles nest.
As I walked along recording monk seal tag numbers, I saw something wiggle in the sand. I crossed the beach to investigate and found a newly hatched sea turtle, head down, flippers flailing.
I picked up the struggling hatchling. It was not head down in the sand, but rather in a ghost crab hole. The large crab inside stood boldly in its doorway, looking for its prize.
Evidently the crab dragged the turtle home, or reached up and snatched it as it passed by, and was trying to tug it inside.
The little turtle survived the ordeal without apparent injury, but not all are so lucky. Later that morning, I bent to rescue another upside-down turtle and saw that it had that distinct feature of chocolate bunnies the day after Easter: The crab had eaten the turtle's head right off.
Ghost crabs don't have a lot of admirers because of their voracious appetites, but these little creatures are the recycling champions of the beach. Because of these skills, ghost crabs can also turn an average morning beach walk into an unforgettable experience.
Marine science writer Susan Scott's Ocean Watch column
appears weekly in the Star-Bulletin. Contact her at susanscott@hawaii.rr.com.
