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Sclerochronology of oyster shells – differences in trace and minor element composition between native Ostrea edulis and invasive Magallana gigas

In the northern Adriatic Sea, European flat oyster Ostrea edulis and invasive Pacific oyster Magallana gigas coexist in the same area. Growth performance of oysters is one of the most important parameters in terms of O. edulis aquaculture and in terms of M. gigas invasion success. As both species do not have clear periodic growth lines on their outer shells or in shell cross sections they are both a challenging species for ontogenetic age determination. Objectives of this study were: (i) to estimate the potential of LA-ICMP-MS analysis for ontogenetic age determination of oyster shells, (ii) to compare trace and minor element composition within and between shells of two species, (iii) to compare results obtained for oysters with geochemical results previously obtained for other bivalve species in the Adriatic Sea, (iv) to evaluate the potential of oyster shells as environmental archives. Oysters shells were collected alive in November 2017 from the aquaculture farm in the Lim channel, in the North Adriatic Sea. Three similar sized specimens of each species were prepared for the analysis. Analyses were performed at the Institute of Geosciences, JGU, Mainz, Germany, using LA-ICP- MS. Analyses yielded high resolution time-series of Na/Cashell, Mg/Cashell, Mn/Cashell, Sr/Cashell, Zn/Cashell and Ba/Cashell. Since oysters do not have clear periodic growth lines in shell cross sections of their hinge area, obtained chemical data could not directly associate to certain calendar years. Furthermore, time series of trace and minor element composition to calcium ratios had low reproducibility among different individuals and between different species. Evidence of yearly cyclicity in Na/Cashell, Mg/Cashell, and Sr/Cashell ratios, previously found for shells of Glycymeris pilosa, Callista chione, and Venus verrucosa from the Adriatic Sea (Markulin et al., unpublished data), were only hinted for Mg/Cashell time series obtained from oysters indicating its potential use in estimating age and growth. Na/Cashelltime-series showed higher values in M. gigas than in O. edulis. Oyster species showed higher values of Zn/Cashell and Mn/Cashell and lower values of Ba/Cashell in comparison to other analyzed bivalve species from the Adriatic Sea (Markulin et al., unpublished data). Uneven and irregular growth of oysters could be the reason of low reproducibility among different individuals, and future data should aim to find and study shell parts that are comparable among different individuals.

oysters ; sclerochronology ; element composition ; invasive species ; Adriatic sea

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