The Gastropod Statolith Elemental Composition by EMPA

Abstract

Gastropods statoliths are biogenic structures of CaCO3, analogous to fish otoliths. Their application in sclerochronology is recent and may be advantageous comparing to the most traditional organisms used hitherto. Moreover, since elements bind within calcified structures, statoliths theoretically incorporate chemical information from the surrounding environment over time. So, besides being used for age estimation and growth studies, statoliths elemental fingerprinting is recognized as natural tag to track larval dispersal and may also be considered as a potential marker of environmental change and/or pollution. However there is a lack of information on the elemental composition of adult gastropod statoliths. Here we characterize Nassariusreticulatus (Gastropoda: Caenogastropoda) statoliths elemental composition by electron microprobe analysis (EMPA).One statolith from each of 3 adults was prepared for EMPA (Figure 1) and showed the same structural pattern: a nucleus circled by a metamorphic ring and, in addition to this core, 4 other rings delineating 5 increments (Figure 1F). General composition was assessed by Energy and Wavelength Dispersive Spectroscopy revealing C, Ca, O, Sr, Na, S and Mg as the most representative elements. These were then quantified in 3 points per increment. Carbon was not determined since surfaces were C-coated and the amount of oxides of Ca, Sr, Na, S and Mg was estimated from the valence. In addition, Ba – trace element in statoliths of some gastropods larvae – and Al, Cu, Hg, Sn – known as environmental pollutants – were also sought in one of the statoliths. For such, mean values for Ca, Sr, Na, S, Mg and O were set as the sample matrix. Standards and mean detection limits by element and statolith are indicated in Table 1.Concentrations of Ca, O, Na, Sr and S varied between 3.7x105-4.2x105, 1.5x105-1.7x105, 2.5x103-6.1x103, 1.3x103-8.9x103 and 4.7x102-1.3x103 ppm, respectively; Mg, which was not always detected, ranged from 2.0x102-8.6x102 ppm. No significant differences in these elements mean concentrations were found between increments, except Na in one statolith between increments 1 and 5 (higher at the edge; Dunn’s test s=2.014, p>0.05). Thus the general elemental composition is nearly constant throughout lifespan (i.e. along statoliths radiuses). Regarding trace elements: Sn was never detected; Al and Hg were at 22.2% of the punctual analyses conducted while Cu was at 66.7% and Ba at 88.9%. Concentrations of Al, Hg, Cu and Ba varied between 55-71, 176-180, 183-302 and 647-969 ppm, respectively.EMPA proved to be a useful technique in sclerochronology, allowing the characterization of the overall composition of gastropods statoliths and the quantification of elements in specific locations with high resolution.This work was supported by the Portuguese Foundation for Science and Technology (FCT) through the grant SFRH/BPD/70368/2010.