CHIBA - The recent heat wave has killed huge quantities of baby-neck clams and helmet crabs in Tokyo Bay's Yatsu tidal flat, according to researchers.

They are believed to have died from lack of oxygen after the extended period of high temperatures accelerated the decomposition of marine alga, an ecology research group of Toho University said.

According to the researchers, it is very rare for baby-neck clams and helmet crabs to die en mass in the Yatsu tidal flat, which is located in Narashino, Chiba Prefecture.

The group launched an urgent study, examining the area section by section, after a graduate student from the university discovered the phenomenon on July 28.

The death rate among baby-neck clams was found to be more than 80 percent in some sections.

The researchers also found remains of many razor clams, soft-shelled clams and green crabs.

Many quahog hard-shell clams, a nonnative species said to be relatively resilient in unfavorable conditions, were also found dead.

Large amounts of sea lettuce, an edible green algae, develop every year in the Yatsu tidal flat.

Researchers believe the sea lettuce dissolved abruptly when water temperatures increased due to the heat wave, resulting in less oxygen and more sulphide in the water and making it uninhabitable for certain species.

In the Sanbanze tidal flat, which is near Yatsu but where little sea lettuce is propagated, large-scale deaths of sea creatures could not be confirmed.

According to the Meteorological Agency, the average temperature in Chiba in July from 1966 to 2009 was 24.7 C, but this year it was three degrees higher at 27.7 C.