Julian Ryall, National Geographic News 3 Jan 08;
Millions of tiny crustaceans are causing a small island in southern Japan to disappear at an alarming rate, according to a local expert.
The animals, tiny relatives of crabs and shrimp that are known in the region as nanatsuba-kotsubumushi, are boring into the uninhabited island of Hoboro.
Recent rises in ocean temperature have led to an increase in the amount of plankton in the protected waters around the island.Plankton are a staple of the nanatsuba-kotsubumushi, and the abundance of food has led to a surge in the crustaceans.
"I first went out to the island two years ago, and I was shocked to see the number of crustaceans on the island and what they had done to it," said Yuji Okimura, an emeritus professor at Hiroshima University.
"The creatures make holes in the rock as they make nesting areas," Okimura said, "which makes it weaker and very susceptible to weathering from the ocean and the wind."
Dissolving Island
Hoboro lies about 1,650 feet (500 meters) off the coast of Japan's Hiroshima Prefecture, in the narrow sea that separates the main islands of Honshu, Kyushu, and Shikoku (see map).
The island was originally named for its shape, which resembled an upside-down version of a type of bamboo basket used by the local people.
In a survey carried out in 1928, Hoboro was recorded as being 390 feet (120 meters) long and standing nearly 72 feet (22 meters) above sea level at its highest point.
Photographs from the 1960s show that the island was getting smaller but still had two distinct rocky peaks partially covered by vegetation.
All that remains today, however, is a rocky promontory about 20 feet (6 meters) high at the western end—and that is almost covered at high tide.
Hiroshima residents living nearby first noted that the island seemed to be getting smaller after every storm or typhoon.
The island's soft rock, a material called tuff that is primarily composed of densely compacted volcanic ash, is an ideal habitat for the booming population of nanatsuba-kotsubumushi, Okimura said.
Through normal weathering, it would usually take thousands of years for the elements to reduce an island the size of Hoboro to debris, he noted.
Geological Oddity
But some experts have estimated that, at the current rate, the island may be gone within a century.
"This is a quite fascinating bio-erosion phenomenon that I've never seen anywhere else," said Akihiro Kano, an associate professor of historical geology at Hiroshima University.
"From Professor Okimura's evidence, it is obvious that these creatures are promoting the erosion of this island at a very rapid pace.
"By looking at the older images of the island and comparing them with the way it looks today, the scale of the changes is incredible," he added.
"Also, I don't think there has been another case of such a high density of these creatures, so there must be some interesting reasons for that we can learn more about."
What's more, Hoboro appears to be something of a geological oddity, Okimura said, with no other islands in the immediate area made of the same material.
Although he prefers not to put a time scale on the island's destruction, Okimura added that it's unclear where the burrowing creatures will go when Hoboro does ultimately disappear beneath the waves.
