Description
Fish bodies contain numerous sclerochronological structures that have been used for routine age determination (from annual increment deposition) and increasingly, to study elemental and isotopic composition for life history interpretation. The most widely used structures are otoliths (literally, ear-stones), made of aragonite (CaCO3) precipitated on a protein framework, that are part of the hearing/balance system in modern fishes. We have been studying these with synchrotron Scanning X-ray Fluorescent Microscopy (SXFM) since 2003, beginning at CHESS. Otoliths take up trace elements throughout life, and the visual annual zonations put a time-stamp on key life history events. Thus, we can use strontium to track migration histories and manganese to document hypoxia exposure events. We can combine this information with biological data such as fish size and body condition to document environmental impacts.
In addition to otoliths, the community is now studying trace elemental chemistry of eye lenses of fishes as a complement to otoliths. At CHESS we discovered that mercury is readily taken up in lenses as the fish grow, providing lifetime chronologies of this toxic substance. Moreover, because lenses are made of crystallin proteins, we can also analyze light stable isotopic composition over time, to interpret such things as provenance and trophic status through time. It is now possible, for example to take the lens of an adult predatory fish and determine its size and age when it became piscivorous (preyed on other fish).
We provide examples here of trace elemental SXFM mapping conducted at CHESS and the Australian Synchrotron, and also mappings made with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), which can quantify elements not possible with SXFM due to interferences. Both methods have pro’s and con’s. A possible forefront is in-situ characterization of key proteins, such as reproductive and stress proteins, in both lenses and otoliths.