Last month, four marine biologists were accused of exporting state secrets. The Security Bureau of Ukraine raided the Institute of Biology of the Southern Seas in Sevastopol. They interrogated four researchers and searched their homes.

The scientists were told they could be charged with transferring secret information to foreign governments. However, their Western colleagues remain convinced that the biologists are innocent of any wrongdoing.

Submarine spotting

Now a report in the magazine New Scientist says the probable reason for the action is that the scientists are all study bioluminescent plankton which can reveal the whereabouts of submarines.

Many species of plankton can generate light, so measurements of bioluminescence can be used to study their global distribution. But in releasing the results of bioluminescence studies done over the past three decades, the biologists found themselves in trouble.

The shining plankton often glow when they are physically disturbed. The wake of a submerged submarine could cause this. This flash of light can be spotted from surface vessels, aircraft or even orbiting satellites.

Very sensitive

Peter Herring of the Southampton Oceanography Centre and president of the International Society for Bioluminescence and Chemiluminescence, says that the plankton are very sensitive to disturbance. "Even swimming mackerel produce a nice effect," he says.

Glowing plankton have a long history in naval warfare. On 9 November, 1918, the German submarine U-34 was destroyed in the Mediterranean after bioluminescent plankton betrayed its position.

And last year, Captain Michael McHugh, former manager of the US Navy's programme for assessing strategic threats to ballistic missile submarines, admitted in the magazine Undersea Warfare that the US military had developed a way of using bioluminescence to detect submarines.

UK research

Britain appears to be doing similar research. "I know that one or two colleagues work on bioluminescence for the detection of subs," says Simon Boxall, a remote sensing expert at the University of Southampton.

The UK Ministry of Defence has declined to comment.

Developing a system that can reliably detect submarine wakes means knowing a lot about the normal distribution and activity of the plankton.

And it is this information that Ukraine seems so determined to protect. In June, for instance, border guards confiscated a poster presentation describing the mapping of plankton by bioluminescence as it was transported to a symposium in Lithuania.