Ecological and environmental insights from hard parts of organisms

 

It's amazing what information can be obtained from the biomineralised hard parts of organisms. Just as tree growth rings aid in putting the present into historical context, provide information on current environmental conditions and allow future conditions to be predicted, similar information can be gleaned from the hard parts of invertebrates, fish and marine mammals. The ear bones (otoliths and statoliths) of fish and cephalopods, vertebrae of sharks, shells of bivalves, and teeth of marine mammals are all being used in the Gillanders Lab to answer ecological and fisheries related questions.

Population structure

 

Knowledge of how populations are structured is fundamental not only to determining the spatial scale at which stock replenishment occurs, but also for determining the scale of spatial management units. We use morphology (e.g. otolith shape), trace elements and stable isotopes to investigate population structure. As part of this research we seek to determine the spatial scales of variation in population structure. These approaches are complemented by use of molecular-based approaches including use of microsatellite DNA and genomics. We have worked on a number of fish species (snapper, garfish, Australian salmon, sardines, mulloway), cephalopods (giant Australian cuttlefish) and elasmobranchs (whaler sharks).

 
 
Connectivity & migration

 

An understanding of recruitment and movement of organisms underpins population sustainability. We use the chemistry of ear bones of fish to determine origins and track movements (e.g. between juvenile and adult habitats). We investigate spatial and temporal variation in juvenile elemental signatures prior to retrospective determination of adult origins. Thus we are able to match adult elemental signatures to elemental signatures of juveniles from the corresponding year class. This research has focused around determining links between estuaries and open coastal populations. We are also assessing partial migration in fish – where part of the population is migratory and the other part remains resident – and determining potential drivers and benefits of moving versus remaining resident.

Environmental reconstructions

 

Quantifying the impact of environmental disturbance on populations is difficult because historic baselines of data are often lacking. There is also a paucity of long-term ecological datasets in aquatic systems especially in the Southern Hemisphere, but these are vital for teasing apart ecological responses to a changing environment. The annual growth increments within calcified structures record the life history of the organism. This calcified material also reflects the chemical and isotopic composition of the ambient water. These structures can preserve a continuous and finely resolved record of the fish’s environment throughout its life. We are developing chemical tracers that record environmental data as changes in chemical concentration.

 

Growth-environment relationships

 

Analysis of growth increment patterns or biochronologies in calcified tissues can be used to reconstruct growth histories and determine possible drivers of growth variation. Understanding what drives growth variation can help us predict how fish populations will change in the future. We have applied traditional dendrochronology (tree ring analysis) modeling techniques to fish otoliths, and are also using mixed modeling approaches to simultaneously analyse multiple intrinsic (within individual) and extrinsic (environmental) drivers of growth variation in bivalves, fish, elasmobranchs and marine mammals. These biochronologies are providing long-term information that spans decades.

Historical ecology

 

Comparing data collected at different time scales (e.g. ecologists collect data at the human life span or shorter scale, whereas archaeologists collect data at a macro scale, generally >10,000 years) can delineate the influence of anthropogenic and environmental influences on ecosystem processes and biota. Hard parts of organisms (e.g. otoliths) provide an excellent source of samples from earlier time periods because they occur in sedimentary deposits and archaeological middens. These samples can be compared to contemporary collections and more recent archived samples to provide long-term records of the biology of the species but also environmental information. Thus the bony parts of organisms may provide a means to judge the condition of ecosystems and how they may have changed. We are using otoliths of fish from archaeological middens to address questions related to shifting baselines.

Development & validation of geochemical tracers

 

To use chemical tracers in fish ear bones (and other structures) to reconstruct aspects of fish life history and changes in their environment it is necessary to have an understanding of how different tracers change in response to environmental conditions. We use laboratory experiments and natural environmental gradients to better understand these relationships (e.g. salinity, temperature, pH, hypoxia). In addition, our experimental work has also investigated the contribution of water versus food to ear bone chemistry. We are investigating new chemical tracers, different analtyical methods and statistical approaches to data analysis.

Chemical structure of biomineralised hard parts 

 

Understanding the chemical structure of biominerals, such as otolith aragonite is also important for interpreting the geochemical information locked within these structures. For example, strontium is the most widely used element in otolith chemistry studies to reconstruct the movement and environmental histories of fish. It has been assumed that strontium is bound within the aragonitic calcium carbonate lattice of otoliths via random chemical replacement of calcium, although other alternatives are possible, which would influence how otolith strontium reflects the environment. We have recently validated, using X-ray absorption spectroscopy (at the Australian Synchrotron in Melbourne), that strontium does indeed randomly replace calcium within the aragonite lattice of otoliths and statoliths. We are also using X-ray diffraction techniques to investigate structural differences in otoliths of fish from different taxa and environments. Our otolith work is comparable to research on other aragonitic structures.

 

Photo credits: Morgan Disspain, Zoe Doubleday, Chris Izzo, Talia Wittmann

We utilise a range of equipment to answer many of these ecological and fisheries-related questions including:

  • Isotope ratio mass spectrometers (funded via ARC LIEF scheme)

  • Thermal ionisation mass spectrometer (funded via ARC LIEF scheme)

  • Laser ablation (LA) and solution inductively coupled plasma-mass spectrometers (ICP-MS)

  • Multi collector LA ICP-MS (funded via ARC LIEF scheme)

  • Australian Synchrotron in Melbourne

Gillanderslab.org