Introduction

The global production of bivalves (e.g. cockles, clams, mussels and oysters) has notably increased since the 1990s, reaching over 16 million tons in 20121. Bivalves are filter-feeders and are able to retain, accumulate and concentrate pathogens (e.g. Salmonella and Vibrio) known to cause foodborne infections worldwide2. Consequently, this high trade volume of bivalves, combined with its raw or lightly cooked consumption, represents a potential risk to global human health3. It is, therefore, acknowledged that supply chains trading live bivalves presently rank food safety issues at the top of their priorities4.

Council Regulation 853/2004 and 854/20045,6 set the microbiological safety standards for bivalves destined for human consumption in the European Union (EU). Briefly, EU fishing/production areas for bivalves are ranked as A, B or C according to the levels of Escherichia coli present in the flesh and intra-valvular liquid of live specimens, with this feature determining whether they are suitable for human consumption immediately after harvesting or need to undergo depuration procedures5,6,7,8. In most EU countries, the fishing/production of bivalves is centred in estuaries and coastal lagoons, being common to have different classifications within the same aquatic system. Therefore, tracing the origin of traded bivalves to their specific fishing/production area, even within the same aquatic system, is paramount to ensure public food safety9. As a consequence, the EU developed specific requirements for seafood traceability. Particularly, article 58 of EC 1224/2009 requires that “all lots of fisheries and aquaculture products shall be traceable at all stages of production, processing and distribution, from catching or harvesting to retail stage”. More recently, the European regulation (EC) No 1379/2013 “on the common organization of the markets in fishery and aquaculture products” further contributes to the implementation of seafood traceability and requires that the category of fishing gear or production method (i.e. caught or farmed) is provided together with geographic detail of the catch area. However, this information is not always available to end consumers and is prone to fraudulent use (e.g. mislabelling of place of origin). Consequently, even conscientious buyers aware of the potential hazards associated with the consumption of bivalves may not be able to securely purchase this highly-prized seafood. It is critical to develop and validate reliable techniques that allow competent authorities to trace the origin of traded bivalves to ultimately fight fraud and prevent notable risks to public health10.

The taxonomic identification of bivalves and the traceability of their fishing location are often difficult to achieve, as bivalves are commonly processed after collection (e.g. precooked, canned). Most studies have relied on molecular techniques, including PCR11,12, FINS13,14 and DNA barcoding15,16, for species identification. Molecular tools, particularly microsatellites17,18, as well as biochemical methods, such as fatty acids19,20,21 and stable isotopes21,22,23, have also been used to assess geographical origin of bivalves. Trace element fingerprinting (TEF) of bivalve mineral structures may also be useful to distinguish populations or stocks24,25,26. Trace elements are influenced by environmental features of each ecosystem27 and are recorded in hard structures (e.g. shells, statolith and otoliths) of marine organisms28. Several trace elements are found in a wide range of marine species29, with the most common being aluminium (Al), barium (Ba), calcium (Ca), cobalt (Co), chromium (Cr), copper (Cu), magnesium (Mg), manganese (Mn), lead (Pb), zinc (Zn) strontium (Sr) and uranium (U). TEF of sea snails larvae28 and shells from bivalve larvae30 and adults26 have been successfully used to distinguish specimens from geographically close populations (20–50 km). However, it remains unclear whether this geochemical approach has enough resolution to discriminate specimens from adjacent areas (<1 km apart) within the same aquatic system.

The present study aimed to validate TEF of shells from fresh bivalves as a proxy to discriminate the origin of specimens collected from adjacent areas of the same estuarine system. It is important to highlight, that unlike previous studies on TEF that use laser ablation of a small part of the larval or early juvenile shells of bivalves28,30, the present study uses the whole shell of adult bivalves. The rationale for using this approach was to somehow minimize the temporal variability of TEF in the shells of adult specimens. We used cockle (Cerastoderma edule) as a model species due to its economic importance as a fishery resource31, with the coastal lagoon Ria de Aveiro (Portugal) being selected as the collection site due to its diverse tidal system and important role in Portuguese bivalve fisheries31. Once cockles are usually fished by hand or hand-raking, this study also aimed to test if the use of metal rakes could induce some type of metal contamination and be a source of bias for TEF. The following hypotheses were tested i) TEF of C. edule shell does not differ with fishing method (i.e. hand-raking vs. by hand) and ii) TEF of C. edule shell is similar among different locations within the same coastal lagoon.

Material and Methods

Study area and cockle collection

Cerastoderma edule with a shell length > 25 mm (i.e. commercial size) (likely displaying an age of 3+ years; the species lifespan may be up to 6 years32) were collected during June 2013 in five different locations of Ria de Aveiro distributed among Mira (M1 and M2), Espinheiro (E1 and E2) and Ílhavo (I) Channels (Fig. 1). All locations play an important role on the fishery of C. edule in Ria de Aveiro, which usually exceeds 1000 tons per year in this region31. Two fishing methods were used to collect twenty specimens of C. edule at M1: ten by hand-raking and ten by hand (n = 10 *2). Subsequently, ten specimens were collected by hand on the other locations: M2, E1, E2 and I (Fig. 1). All samples were stored in aseptic bags kept refrigerated during sampling and brought to the laboratory and frozen at −20 °C for later processing.

Figure 1
figure 1

Sampling locations of Cerastoderma edule in Ria de Aveiro, Portugal:

Mira Channel (M1/M1A: 40°38'26.30"N, 8°43'58.90"W and M2: 40°35'58.30"N, 8°44'47.80"W), Ílhavo Channel (I: 40°38'35.40"N, 8°41'35.40"W) and Espinheiro Channel (E1: 40°39'48.50"N, 8°41'45.03"W and E2: 40°40'2.72"N, 8°41'26.08"W). The map was created using the software ArcGIS v9.2.

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Shell preparation

Volumetric polyethylene material and micropipettes with plastic tips were used to prepare collected shells for trace elements analysis33. Plastic bottles, ceramic coated blades and tweezers kept in 2–5% solution of DECON 90 over 2 h were washed with running water, immersed in 10% of HNO3 for 24 h, washed with Milli - Q (Millipore) water and dried in a laminar flow hood. The preparation for ICP-MS analysis was performed in a class 100 (ISO class 5) clean room. The valves were separated and the organic tissues were removed using ceramic coated blades and tweezers. The right valve was transferred to a previously acid-washed plastic bottle and the left valve discarded.

Samples were soaked in 20 mL high-purity H2O2 (30% w/v) (AnalaR NORMAPUR, VWR Scientific Products) overnight (14–16 h) to remove organic matter from the shell including the periostracum. After organic matter removal, the valve was rinsed in Milli – Q (Millipore) water three times. Digestion of entire valves was performed with addition of 20 mL of high-purity concentrated (70% w/v) HNO3 (Trace metals; Sigma-Aldrich). To avoid having Ca masking the concentrations of the remaining elements34,35, the resulting solution was diluted with Milli – Q (Millipore) water to a final acid concentration of 2% HNO3.

ICP-MS analysis

Samples were analysed for total aluminium (Al), barium (Ba), calcium (Ca), cadmium (Cd), copper (Cu), magnesium (Mg), manganese (Mn), lead (Pb), strontium (Sr) and zinc (Zn) by an accredited laboratory at the University of Aveiro (Portugal). The concentrations of 27Al, 137Ba, 111Cd, 65Cu, 55Mn and 66Zn were determined through inductively coupled plasma mass spectrometry (ICP-MS), on a Thermo ICP-MS X-Series equipped with a auto sampler CETAX ASX-510, Peltier Nebulizing Camera Burgener nebulizer, nickel cones and the CeO+/Ce+ ratio was optimized at <2%. The concentrations of 48Ca, 24Mg and 88Sr were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) on a ICP-OES Jobin Yvon Activa M equipped with auto sampler JY-AS500 and Burgener Mira Mist nebulizer.

Statistical analysis

Concentrations of trace elements of the shells were standardized to Ca and all data analyses were carried out on the element ratios (X: 48Ca)25,30,36. To assess if fishing method significantly affected TEF, a resemblance matrix based on the normalized Euclidean distance was calculated37 for a one-way analysis of similarity (ANOSIM)38, which calculates a global R statistic that assesses the differences in variability between groups when compared to within groups and checks for the significance of R using permutation tests38. Differences among fishing locations for each elemental ratio were assessed using a one-way analysis of variance (ANOVA) and Tukey’s HSD pairwise comparisons when significant differences were observed (p < 0.05). Similarity percentages (SIMPER) were calculated to quantify the contribution of each trace element to the dissimilarities recorded among locations. Only trace elements that cumulatively contributed up to 80% of the dissimilarities recorded were selected39. A Canonical Analysis of Principal Coordinates (CAP)39 was performed to test if TEF could be used to predict the fishing location of collected specimens. CAP is a constrained ordination tool that discriminates locations defined a priori and determines the level of misclassification among sampling locations. Appropriate axis (m) was applied by maximizing the leave-one-out allocation success (m = 5)40. This approach tests how well locations were discriminated using CAP. To quantify the effect of each trace element to potential differences recorded among locations, Spearman correlation were calculated for all trace elements and the CAP axes. Only the trace elements with a correlation coefficient (r) > 0.30 were considered. ANOVAs were performed using GraphPad Prism 6 (GraphPad Software. Inc., San Diego, CA, USA) and multivariate analyses were performed using PRIMER v6 with the add-on PERMANOVA + .

Results

Five trace elements (137Ba, 24Mg, 55Mn, 207Pb and 88Sr) were detected in C. edule shells from Ria de Aveiro, with Mg and Sr denoting the highest ratios to Ca (Fig. 2). While no differences between specimens collected by hand or by hand-raking were detected (ANOSIM, p = 0.268, R = 0.025), significant differences among locations were observed for each trace element ratio (Fig. 2; one-way ANOVA, p < 0.05 for all trace elements; Table 1, summarizes ANOSIM results). The ratios of Mn and Ba were significantly higher at location M2 (p = 0.0001 and 0.0001, respectively). In contrast, the Mg ratio was lowest for C. edule shells from M2 (p = 0.0004). The Pb ratio was only significantly higher (p = 0.0001) at location I, whereas the Sr ratio was also higher at this location but only significantly different from shells collected at E2.

Table 1 Similarity (ANOSIM) between trace elements fingerprinting of Cerastoderma edule shell from five locations within Mira (M1 and M2), Espinheiro (E1 and E2) and Ílhavo (I) Channels in Ria de Aveiro (Portugal).
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Figure 2
figure 2

Ratios of trace elements to Calcium (Ca) concentrations (mmol to mol) (average ± SD; n = 10) of Cerastoderma edule shells from five locations within Mira (M1 and M2), Espinheiro (E1 and E2) and Ílhavo (I) Channels in Ria de Aveiro (Portugal).

Significant differences (p < 0.05) among different locations are noted with different letters.

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Pairwise comparisons revealed significant differences among locations, apart from those within the Espinheiro Channel, i.e. E1 and E2 (ANOSIM, p = 0.059, R = 0.123). SIMPER analysis showed that the dissimilarity among locations was associated to five elemental ratios: Mg, Sr, Pb, Ba and Mn. (Table 2). Mg and Sr were always among the elements that most contributed for the variability between location M1 and locations from Espinheiro Channel (E1 and E2). Mg and Sr varied significantly between M1 and Espinheiro (p = 0.0001 and 0.021, respectively) and SIMPER revealed that these elements explained more than 55% of the differences recorded between these locations (Table 2). Specimens from location I were significantly different from other areas due to their concentrations of Pb (Fig. 2). SIMPER analysis revealed that Pb alone accounted for 28 to 53% of all differences recorded between location I and all other locations (Table 2). Ba and Mn together contributed for more than 43% of the differences recorded among specimens collected in M2 and other locations.

Table 2 Similarity percentage analysis (SIMPER) identifying the elements contributing to the differences recorded in the shell of Cerastoderma edule from five locations within Mira (M1 and M2), Espinheiro (E1 and E2) and Ílhavo (I) channels in Ria de Aveiro, Portugal (Ind – individual; Cum – cumulative).
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TEF differences among locations were strong enough to accurately assign collected specimens to their fishing location. The leave-one-out procedure resulted in an average CAP classification of 92% (Table 3), i.e. 92% of the specimens were correctly assigned to their origin. Locations M1, M2 and I had the highest percentage of correct classification (100%), whereas two replicates from E1 and E2 were misclassified, which led to 80% correct classifications. Vector overlay of Spearman correlations of TEF with CAP axes are shown in Fig. 3. Vectors of Ba and Mn ratios were positively correlated with samples from location M2, Mg ratio with areas E1 and E2 and Pb and Sr ratios associated with samples from location I (Fig. 3). C. edule from area M1 were not associated with a particular trace element.

Table 3 Classification success of cross-validation for cockle Cerastoderma edule based on trace elemental composition in the shell from five locations within Mira (M1 and M2), Espinheiro (E1 and E2) and Ílhavo (I) Channels in Ria de Aveiro, Portugal.
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Figure 3
figure 3

Canonical analysis of principal coordinates (CAP) based on Euclidean distances matrices of normalized elemental ratios, with axes drawn to maximize discrimination among assemblage types.

Vector overlay Spearman correlations of trace elements composition with canonical axes are shown if |r| > 0.30.

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Discussion

In general, adult bivalves display a reduced locomotor ability, being their aragonitic shells potential biogenic archives of marine ecosystems environmental fingerprints41. This feature prompted the use of trace elements of bivalve shells to assess their geographic origin. TEF has been successfully used to geographically distinguish populations of blue mussel M. edulis26, black mussel M. galloprovincialis and California sea mussel M. californianus29,30, soft shell clam Mya arenaria36 and Olympia oyster Ostrea lurida42. This geochemical tool also allowed to distinguish juveniles of green-lipped mussel (Perna canaliculus) ~13 km apart43 and to record differences in scallop shells (Argopecten irradians) within a small bay (~10 km2)44. The present study shows, for the first time, that TEF of bivalve shells can be used to assign the origin location of bivalves with a resolution <1 km.

While Mg and Sr ratios were relatively higher than Ba and Mn (Fig. 2), the latter ratios were among the most important to differentiate locations (Table 2). The presence of Ba and Mn with elevated concentration, as observed in M2, have been already reported for Isognomon ephippium45, Mercenaria mercenaria, Spisula solidissima46 and M. edulis47. Such high concentration in Ba and Mn are usually associated with freshwater inputs and nutrient runoff to estuarine systems, which ultimately causes phytoplankton blooms, particularly diatoms46,47,48,49. It is possible that the environmental conditions at M2, which is located more upstream and has stronger riverine input, causes diatom blooms more often than the conditions observed at others locations such as M1, which is located near the inlet50. Ba and Mn end up in bivalves’ tissue and shell as a consequence of the ingestion of Ba and Mn-rich particles associated with such diatom blooms. Heavy metals are also incorporated in calcite and aragonite shells of bivalves51,52. The high levels of Pb in the shells from location I are likely associated with anthropogenic impacts, particularly acute pollution from boats using leaded gasoline. Note that location I is relatively close to the commercial harbour of Aveiro.

Cockle shells from each location displayed a different TEF, with the exception of stations E1 and E2 that showed no statistical differences between each other (Table 1). Nevertheless, CAP results showed a success of 80–100% to identify the origin of cockles collected from Ria de Aveiro (Table 3). The important to highlight that the misclassifications were solely associated with locations E1 and E2. It is the potential of TEF for geographical traceability purposes as we were able to identify the origin of cockles using this statistical tool (CAP) in, at least, 80% of the cases. Nevertheless, the average 92% correct classification is still higher than results by Sorte et al.26 for the blue mussel M. edulis in the Gulf of Maine (68%) and by Becker et al.30 for the congeners mussels M. californianus and M. galloprovinciallis in Southern California, USA (56%). The latter and other studies26,29,30,42,43 have also shown that Ba, Mn, Mg, Pb and Sr play an important role discriminating specimens among areas, as observed here through the magnitude of the vectors of the standardized discriminant functions (Fig. 3).

The chemical nature of the trace elements deposited over time in bivalves is determined by metabolic efficiency and environmental conditions53. As this study was conducted in Ria de Aveiro, which is a highly dynamic tidal-system with notable spatial variability in environmental conditions54, it is likely that different fingerprints are associated with contrasting environmental conditions recorded at each channel (Fig. 1). The spatial variability here recorded for TEF of cockle shells thus stresses the potential of this method to validate screening for fraudulent use of origin certification. However, temporal variability in environmental conditions may also change TEF and interfere with this traceability tool. Indeed, it has already been shown that seasonal and annual variation may change the TEF of bivalves and other biogenic carbonate structures such as fish otoliths29,30,55. In opposite, Carré et al.56 showed that environmental changes have minor influence on Sr, Ba, Mg and Mn concentration in shell aragonite of the marine bivalve species Mesodesma donacium and Chione subrugosa. This study aimed to validate a tool for origin certification of bivalves and not to study the temporal variability of TEF. Consequently, we used cockles with similar size and, therefore, similar age, in order to minimize any bias associated with potential differences in the age of selected specimens. The analyses performed in this study used the whole shell and, consequently, averaged the present and the past elemental fingerprints of cockles. While this approach may have the TEF over multiple years, notable differences among sites were still recorded (Table 1), which emphasizes the robustness of this method for geographical traceability purposes. However, if legal authorities aiming to fight the fraudulent mislabelling of origin location want to minimize this potential temporal bias associated with the analysis of the whole shell, they may rather monitor the elemental fingerprint of the outer margins of bivalve shells from each fishing location or, those more prone to fraud, as these will reflect the most recent elemental fingerprints from the location where they were collected. By comparing the fingerprint of the investigated shells with monitoring data and/or samples from the different sites in the same season, legal authorities may minimize the effect of temporal variability and, ultimately, use of this tool to expose fraudulent situations.

Although TEF fails to detect differences associated with fishing method, this information would be potentially relevant for legal authorities to manage bivalve trade, from fishing to the end consumer, as fishermen using hand-raking usually collect larger volumes of bivalves. The effect of environmental conditions on the TEF of bivalves occurs within a relatively long time frame, i.e. within weeks or months57, which likely explains the lack of differences associated with fishing methods. The effect of fishing method on TEF, if any, would probably occur within a very small time frame as fishing duration usually takes less than one hour.

Most traceability tools have been focused on issues associated with species mislabeling13,58,59 or with identification of geographical origin of specimens separated by distances higher than 20 km26,30. However this study shows, for the first time, that TEF can be a fast, reliable and accurate method that may be used for origin certification of bivalves collected from locations less than 1 km apart. While this is probably associated with the high environmental variability observed within Ria de Aveiro, it is still unknown if TEF is a reliable tool to accurately identify the origin of bivalves collected from different ecosystems with similarly high variability. Follow-up studies are already being developed to clarify if TEF can be used to discriminate between bivalve shells from specimens originating from distinct ecosystems (from tens to hundreds of km apart). An additional benefit of TEF is that there is no post-harvesting shift and/or degradation associated with bacterial action as recorded for biochemical and molecular methods. The present approach may also play a relevant role on the conservation and management of cockle populations being exploited, namely in the fight against illegal/unreported fishing.

Additional Information

How to cite this article: Ricardo, F. et al. Trace element fingerprinting of cockle (Cerastoderma edule) shells can reveal harvesting location in adjacent areas. Sci. Rep. 5, 11932; doi: 10.1038/srep11932 (2015).