They may not have evolved language or consciousness but freshwater mussels are talking to scientists about the state of their environment.
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| Chemical receptor cells in the molluscs send signals commanding the muscles controlling the valves to clam up. Credit: Wanderin Weeta |
Scientists at ANSTO are getting the lowdown on pollution levels in rivers, lakes and estuaries by measuring the frequency with which mussels open and close their shells.
The scientists are also using the shellfish as archives of environmental data to monitor changes in water pollution.
Mussels open their valves to feed. "They filter algae and the organic particles they need from the water," says Dr Scott Markich, of ANSTO's Environment Division. "They also open up to expire, excrete and reproduce."
But if the water is toxic, chemical receptor cells in the molluscs send signals commanding the muscles controlling the valves to clam up. "We have exploited this behavioural response to develop a living chemical sensor," he says, adding that the sensor was designed originally to detect uranium in mine waste water.
Dr Markich has developed a device for recording the patterns of opening and closing in mussel shells. When the shell moves, electronic signals are sent in real time to a personal computer which records them.
"But to make sense of what the mussels are saying, I have to calibrate them. This means recording electronically their natural patterns of opening and closing before exposing them to sub-lethal levels of polutants and recording their response," he said.
"If they are exposed to an environmental insult in the field, they flex their muscles, and I can interpret
the response."
Dr Markich has used the mussels to detect uranium, copper, cadmium and manganese in both natural and waste waters. The living sensors can detect some metals at levels as low as 10 thousandths of a gram per litre. The molluscs are not quite as sensitive as hi-tech chemical instruments but the information they convey is much more valuable.
"There has been a move away from measuring exactly what is in the water towards measuring the biological impact of it," Dr Markich says.
ANSTO is in talks with conservation agencies aimed at putting the new sensors into action. Meanwhile, Dr Markich and colleague Dr Ross Jeffree have worked out how to read records of past pollution levels written in mussel shells.
Mussel shells grow by about a millionth of a metre in diameter each day. Toxins are incorporated in the shell so chemical analysis of the growth bands reveals the pollution history of the environment. Scientists can detect changes over weeks, months, years or even decades. Mussels have very long natural lifespans - up to 60 years in the southern hemisphere.
Dr Kathryn Prince, also of ANSTO's Environment Division, analyses the shells with a sophisticated technique called secondary ion mass spectrometry (SIMS). In ANSTO's SIMS instrument, oxygen ions (charged particles) bombard the sample, dislodging secondary ions from it.
Ultimately, these secondary ions pass through a mass spectrometer which sorts them according to their mass, enabling the identification of the elements in the sample.
"Our instrument can detect copper concentrations in mussel shells at concentrations between one and 10 parts per million," says Dr Prince, who heads the SIMS facility.
The scientists have used the technique to build up a pollution history of the water around the abandoned uranium and copper minesite at Rum Jungle in the Northern Territory. The Rum Jungle project ran in the 1950s and early 1960s. The site was remediated in the early 1980s.
There were no mussels older than 10 years, pointing to the toxic effects of pollutants that lingered in the environment before remediation. Copper, which is one of the most toxic substances to aquatic life, decreased in concentration over the lifespan of the mussels.
"We scanned for copper from the oldest to the youngest parts of the shells and found an exponential decrease in levels in the ten years since remediation," Dr Markich says. Preliminary tests suggest that uranium levels are low.
Published: 03/08/1999