Mussels warn of pollution

Measurement the growth of mussels in the sea from hour to hour by means of laser technology not only shows the daily rhythm of these shellfish, but provides a "real-time" report on the condition of the marine environment where the mussels grow. Slower mussel growth is a sign of imbalance in the condition of the sea.

SINTEF Metrology has developed equipment that is capable of making highly accurate measurements of mussel growth. A laser beam allows changes in growth rate to be rapidly and simply measured and stored on a computer. The laser beam passes through a narrow slit, one side of which consists of the edge of the mussel shell. The light is spread in a particular pattern as a result of diffraction and tiny changes in the light, caused by the growth of the shell, can be measured.

The mussel is glued to a sheet of plexiglass and installed in a measurement rig equipped with a laser and measurement equipment. The rigs come in different designs and sizes. This technique can also be used to measure the growth rates of other organisms such as algae.

"We have developed this test equipment in collaboration with Professor Tor Strømgren of the Royal Norwegian Society of Sciences and Letters. The basic optical measurement methodology was introduced during the 70s by Strømgren and Tor Berge, who is now a senior engineer in NTH's Faculty of Chemistry and Chemical Engineering. At that time they had to record the changes in light that reflected the growth of the mussels by means of photography. Photographing and developing hundreds of light patterns and then manually measuring them on the film was time-consuming and expensive, says Bernt M. Førre, a SINTEF Metrology scientist and the man behind the new technique.

"Now we can pick up the signals using a sensor similar to a simple TV camera, and read the data right into an ordinary PC for storage and calculation purposes. We are using a measurement resolution of a thousandth of a millimetre. Since a mussel normally grows by about 0.3 mm a day, we can record how much it grows in less than five minutes."