|dc.description.abstract||The density, distribution and dynamics of benthic bivalve populations are believed to be largely determined by the planktonic larval phase of the life history. As the hard parts of larvae, such as the prodissoconch (larval shell), develop and grow, ambient environmental conditions are recorded as chemical signatures (elemental fingerprints). If the chemical signals of reference sites are known, the larval signatures can be matched to reference sites, hereby reconstructing the origin of the larvae. The collection and identification of larval bivalves is extremely difficult, however, previous studies have shown that the prodissoconch is retained into the juvenile phase, thus enabling juveniles to maintain a record of the larval movement. Before elemental fingerprinting can be used as a larval tracking tool, site specific signatures must be evident in the shells. As a precursor study to test the application of elemental fingerprinting to track bivalve larvae, the presence of spatial variability in shell signatures as well as the scale at which these variations occurred, were established for New Zealand conditions. Furthermore, temporal stability of the shell signatures was explored, as temporal stability is crucial if the signals of shells collected at one time are to be used as predictors of unknown samples collected at a different time.
The venerid bivalve Austrovenus stutchburyi is a common and widespread constituent of New Zealand estuarine communities and were therefore selected as the study species. The chemical signatures of entire Austrovenus stutchburyi shells were examined to determine the inter-site spatial differences in elemental fingerprints of shells and also to characterise the temporal stability of the signatures. Furthermore, shells were ablated at two reference points (the prodissoconch and most recently formed shell material) to determine the intra-shell variation in the chemical signatures. Juvenile individuals were collected from 19 sites in the North Island of New Zealand as part of the whole shell spatial study. One site (in Tauranga Harbour) was examined for the temporal study, whilst four sites were used to compare intra-shell chemical variation. Whole shells were digested and analysed as solution based samples using inductively coupled
plasma-mass spectrometry (ICP-MS) for the spatial and temporal studies, and by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) for the point ablation intra-shell variation study.
Results showed that shell concentrations were sufficiently different to yield a classification success of 68% over 19 sites, however the classification success markedly increased as the number of sites included in the analysis decreased (e.g. 12 sites = 75%, 10 sites = 84%, 5 sites = 90%, 3 sites = 95%). Shells were successfully classified across all of the spatial scales that were tested (approximately 10 km to 1150 km). Temporal stability in chemical signatures was observed over a 44 day period. The chemical signatures were not correlated with ambient temperature or salinity, however more vigorous sampling is needed to accurately assess the relationship between shell elemental incorporation and environmental conditions. Intra-shell variation was also observed for some of the shells analysed from two of the four sites. These results were promising and indicated that there may be chemical variations between the larval shells and the more recently formed shell material, thus suggesting the possibility of external recruitment.
The results from this study emphasised the potential for the application of elemental fingerprinting techniques to track and better understand the larval transport and population connectivity for New Zealand invertebrate species, however more research is required before elemental fingerprinting can effectively be applied to New Zealand invertebrate species. With a fundamental understanding of the origin of bivalve recruits, restoration efforts following estuarine disturbance events can be effectively employed.||en_NZ