Saving an evolutionary icon
How atmospheric nuclear weapons testing may help conservation of the lungfish
As it’s name suggests, the lungfish has primitive lungs enabling it to breathe in air. Lungfish hold a special place in evolutionary biology being a relative of the first air breathing amphibious land animals. Today living lungfish can be found in Africa, South America, and Australia. This distribution probably relates to their presence in the early super continent Gondwana that broke up about 150 million years ago and this gives a measure of just how ancient these creatures are. However the Australian lungfish is under pressure as Queensland’s water resources become increasingly stretched to meet a variety of human needs. A number of dams have been constructed across the state in the past few decades and this has had inevitable repercussions on the lungfish’s habitat. But establishing just how much of a threat is posed to this iconic species is complicated by the fact that scientists aren’t sure how long they typically live in the wild or what the age distribution of the current population is.
With many fish, establishing the age of an individual is a relatively simple thing to do. Fish have structures called otoliths in their inner ears onto which layers of calcium carbonate and a gelatinous matrix are deposited each year. Seasonal variations create rings much like the growth rings on a tree and by counting these rings, it’s possible to work out how old a particular fish is. However with lungfish this method doesn’t work because their otoliths are too gelatinous. So instead scientists try to look for seasonal changes in the Horny scales that cover their backs. However this is a very tricky thing to do because most of the growth is concentrated in the first, rapidly growing portion, compressing the later years into a very small area.
Ironically the solution to this conservation problem may come from the pollution created by the numerous atmospheric nuclear weapons tests conducted through the 1950s and 60s. As part of her Honours Degree, Kelly James has been working with Dr Stewart Fallon on analysing atomic bomb C14 residue in lungfish scales as a method of determining their age. The process involves removing some sample scales from a specimen and isolating tiny quantities of material from different points along the length of each scale. The samples can then be subjected isotopic analysis to establish the amount of C14 in them.
Although naturally present in the environment in microscopic quantities, the concentration of C14 increased significantly but temporarily, during the atmospheric nuclear weapons testing of the mid twentieth century. This means that any organism alive during that period would have incorporated some of this c14 into its tissues. And with tissue that continually grows such as a horny scale, that means that there is a C14 peak in the tissue corresponding to 1962 - the peak of bomb testing.
“On each of the scales from our sample fish we were able to detect the signature of the start of testing and the peak.” Kelly explains, “We combined that with the date of capture of the fish giving us the three points we need to fit the von Bertalanffy growth curve and establish the age of the fish which in this case turned out to be about 75 years.”
However although the technique sounds relatively straight forward, there is a huge amount of work associated with gathering this data. Firstly the material must be cleaned and isolated to ensure that it’s all from the right tissue type and isn’t contaminated with other material of different age. Next the carbon needs to be extracted in the form of CO2 gas. With organic samples like the lungfish scales this is done by a heating process. The CO2 is then cleaned of contaminants using multiple stages of freezing and exposure to vacuum. Once the pure CO2 sample is created it is mixed with hydrogen and heated in the presence of an iron catalyst, which generates water and graphite. This graphite isolates the carbon from the original sample in a solid mass that’s easy to handle and analyse.
The next step is to determine the amount of radioactive C14 each of these graphite samples contain. To do this scientists use what is known as a mass spectrometer. In this case that’s the dedicated SSAMS spectrometer at the Research School of Earth Sciences. The basic principle of mass spectrometry is that charged ions of any element will follow a curved trajectory when passing through a magnetic filed. The acceleration depends on the charge and the field but how much deviation that acceleration produces is dependent on the mass. The upshot of this is that a heavier isotope like carbon 14 will be deflected slightly less than a lighter one like carbon 12. By placing two detectors slightly apart, scientists can then count the ratio of C12 to C14 atoms.
“We were delighted with our initial results” Kelly says, “One of the key things in managing the conservation of a species is establishing the basic population dynamics. How long does a particular animal live, how old are the individuals in the current population and things like that. And this technique enables us to do this reliably in lungfish for the first time. The other really great thing about this technique is unlike counting otolith rings, we don’t need to kill the fish. Simply remove a few scales and return it to the water. This is especially important when we’re dealing with endangered and protected species.”