Crustacean "Swarm" Destroying Small Hiroshima Island

Julian Ryall in Tokyo, Japan
for National Geographic News
January 3, 2008

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.

Continued on Next Page >>




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