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Tuesday, April 15, 2014         

OCEAN WATCH


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Land crabs scuttled in Hawaii until people brought predators

By Susan Scott

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Last month, Florida researchers reported that fossil remains found in Hawaii are that of a crab that lived farther inland (three miles) and at higher elevations (3,000 feet) than any crab in the Pacific today. The scientists speculate that about 1,000 years ago, after humans arrived in the islands with rats, pigs, dogs and chickens, the crabs couldn't compete and the species soon went extinct.

Palmyra Atoll, however, about 1,000 miles south of Hawaii, is crawling with land crabs: two kinds of fist-size hermit crabs, two species of burrowing crabs called "tupas" by Pacific islanders, and the Godzilla of all crabs, the coconut crab. But those islands have also been invaded by rats, cats, dogs and people. What type of crab, I wondered, went extinct in Hawaii?

While working in Palmyra, I came across the above five crab species almost daily. The literature, however, lists six.

The sixth is a land crab so rare that in four months of living on the atoll, I never saw one. That scarce little crab happens to be the closest relative of Hawaii's vanished species.

This sister crab is known on Pacific and Indian Ocean islands as the little nipper crab, scientific name Geograpsus grayi. And they mean little. The little nipper's shell is only 1 inches across. Hawaii's extinct nipper was larger but still no giant. Its shell was calculated to be about 2 ½ inches across.

Photos of little nippers remind me of Palmyra's tupas, common in Tahiti and other island groups. The largest tupas measure about 5 inches across and come in a variety of colors and patterns so stunning that they look fake.

My first encounter with a tupa on Palmyra startled us both. The crab ran from beneath some philodendron leaves into my path and froze.

The creature looked like an enchilada with legs, but no Mexican dish could compete with this tupa's shell colors. Its pinks and greens were as exquisite as a Monet watercolor.

One of the tupa's pincers was larger than the other and had serrated edges, a Vise-Grips with teeth.

As I got out my camera, the tupa made a run for it, holding its large claw like a shield. I followed the crab to a tree trunk, and when I squatted down to take the picture, the bold creature thrust its claw up at me. En garde! The challenge worked. I stepped back.

Tupas, nippers and all other Pacific island land crabs evolved from marine ancestors and must return to the ocean to spawn. When the females make their long migrations to the shoreline to drop their fertilized eggs in the sea, they are particularly vulnerable. Hawaii's land crabs, which evolved with no predators, must have been extra-easy pickings.

Most land crab eggs become fish food, but the ones that survive molt several times and then crawl ashore as tiny crabs.

Tupa babies spend their first three years of life inside tributaries of adult burrows eating leftovers. Researchers believe it takes this long for the youngsters' gills to develop the capacity to breathe air.

We will never know what the Hawaiian Islands were like when nippers roamed the mountains and valleys, but we can thank the little crabs for this: Their fast demise highlights how susceptible our native species are to alien species, and in that, they can guide conservation methods today.

———

Susan Scott can be reached at www.susanscott.net.






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