About 100 killifish genetically modified to emit light from their bodies have been imported from Taiwan and sold in pet shops in Japan, according to an Environment Ministry official.

The official said it is the first known case in which genetically modified pets have been distributed in Japan and voiced concern that the fluorescent fish, if released into rivers, could upset the ecosystem.

Starting next year, new legislation will require imported genetically modified organisms to undergo prior screening by the government, but current laws make no provision for formal procedures for such imports to go through.

Azoo Japan, which imported the killifish, or "medaka," said there is no danger of the fish interbreeding with native species, but it will stop importing them for now at the ministry's request.

The company, based in Yamato, Kanagawa Prefecture, said it imported about 100 of the fluorescent fish in May and delivered them to pet shops and other outlets, where they were sold for about 6,000 yen apiece. Azoo said it later collected for screening those fish not sold by the stores.

The fluorescent killifish were developed two years ago by Taikong Corp., a Taiwan manufacturer and distributor of aquarium products, in a gene-splicing project that involved extracting luminous genes from light-emitting jellyfish and injecting them into killifish embryos.

The fish, whose bodies glow green, are said to have been made sterile through chromosome manipulation during the course of embryo development to ensure they do not disturb the ecological balance in the event they are released into rivers.

The ministry official said the government cannot determine whether the release of the fish would affect ecosystems unless experts thoroughly study them.