INTRODUCTION

Mercury is naturally found in very low concentrations in aquatic ecosystems. Natural and anthropogenic sources contribute notable amounts of Hg to these ecosystems, which contaminate the water, sediments, and biota. The main industrial sources of atmospheric Hg, such as impurities in fuels and raw materials, are coal burning, mining, and industrial activities that process ores and other raw materials to produce metals and cement. Other notable sources of Hg stem from the intentional use of this element. The largest of these sources are artisanal and small-scale gold mining activities, followed by the wastes generated from consumer products, including those derived from metal recycling, the chlor-alkali industry, and the production of vinyl-chloride monomers (^{UNEP 2019}).

At elevated concentrations, Hg is dangerous to human health and damages the central nervous system (^{Davidson et al. 2004}). This damage can result in a loss of balance, weakness or difficulty coordinating the muscles used to speak, numbness in extremities, loss of hearing, blurred vision, loss of consciousness, and, in extreme cases, death. Among the organic forms of Hg, methyl-Hg (MeHg) is the most dangerous chemical species (^{Honda et al. 2006}). In most organisms, the main pathway of Hg uptake is ingestion (^{Luoma and Rainbow 2005}), thus the products that are consumed by humans that may contain Hg must be regularly monitored.

Mollusks are a diverse group of organisms that may be consumed either fresh or canned and are highly valued by consumers despite their potential Hg content. As such, it is imperative to monitor the Hg content of fresh and canned mollusks given the potential risk to human health. Indeed, the high market demand for mollusks necessitates special consideration and monitoring efforts for these food items. The most commonly purchased and consumed mollusks are clams, oysters, snails, slugs, octopuses, and squids, although this varies among regions. In Mexico, fresh mollusk consumption is low. For example, the per capita consumption of fresh oysters, squids, octopuses, and clams is less than 1 kg per year (^{CONAPESCA 2013}). Currently, there is no consumption information available for canned shellfish.

To address this lack of much needed information, the content of Hg and MeHg were evaluated in domestic and imported canned mollusks sold in northwestern Mexico. This study assesses the health risk to consumers based on the Hg and MeHg concentrations in mollusks and the average consumption of fishery products in Mexico. In addition, this study compares the Hg concentrations in canned mollusks and fish in northwestern Mexico with those from other regions.

MATERIALS AND METHODS

Canned mollusks were purchased in January 2014 in different supermarkets in Mazatlán, Sinaloa, Mexico. We evaluated 5 types of mollusks in this study: oysters, clams, octopuses, mussels, and squids. The oysters and clams were produced in Mexico, whereas the octopuses, mussels, and squids were imported from the United States of America (USA), South Korea, and Spain, respectively. For each mollusk type, 10 cans were analyzed as individual samples. The cans were opened and drained, and one half of the product in each can was considered a sample.

The duplicate samples were placed in plastic containers, weighed, tagged, and frozen at -20 °C. The laboratory materials and plastic utensils used to store, manipulate, and process the samples were washed and rinsed with distilled water and then acid washed with 2 M HCl and 2 M HNO₃, followed by a final rinse with Milli-Q water (MilleporeSigma, Burlington, MA, USA; ^{Moody and Lindstrom 1977}). The samples were lyophilized (-49 °C and 133 × 10^-3 mbar) for 3 days, and then manually ground in an agate mortar. The moisture percentage was determined by the difference in weight between the wet and dry samples. Aliquots (0.25 ± 0.02 g) of dried and homogenized tissues were digested according to the methods of ^{Taylor et al. (2008)}, with modifications. The samples were placed in Teflon containers with 5 mL of concentrated nitric acid (trace metal grade; JT Baker, Phillipsburg, NJ, USA). The samples were allowed to pre-digest for 12 to 18 h at room temperature in an extraction hood, after which they were placed on a hot plate (120 °C) for 3 h and allowed to cool. The digested samples were placed in plastic containers with screw-tops and diluted to 25 mL with Milli-Q water (18.2 MΩ cm resistivity).

The Hg content was analyzed by cold vapor atomic absorption spectrophotometry (CV-AAS) in a 410 Mercury Analyzer (Buck Scientific, East Norwalk, CN, USA). To evaluate the quality of the analyses, blanks were run with every batch of 20 samples, and a certified reference material (NIST 2976, mussel tissue) was used (98.7 ± 3.1% recovery). The limit of detection of Hg (2 standard deviations of a blank) was 0.002 mg·kg^-1.

The MeHg:Hg ratio in mollusks ranges from 38-48% (^{Hight and Chen 2006}). In bivalves, the MeHg:Hg ratio was estimated considering a mean MeHg:Hg ratio of 0.3915, according to the published information of these organisms (^{Claisse et al. 2001}, ^{Pan and Wang 2011}, ^{Apeti et al. 2012}). The mean MeHg:Hg ratio used for octopuses and squids was 0.81 (^{Annual et al. 2018}) and 0.92 (^{Miklavčič et al. 2011}), respectively. The health risk to consumers was assessed with the hazard quotient (HQ) obtained with the following equation (^{Newman 2009}):

where E is the level of exposure and RfD is the reference dose (Hg: 0.500 µg·kg^-1 body weight per day; MeHg: 0.100 µg·kg^-1 body weight per day) according to the ^{Environmental Protection Agency (2001)}. The level of exposure was determined with the following equation:

where C is the concentration (mg·kg^-1 wet weight) of Hg or MeHg in the product of interest, I is the rate of ingestion of mollusks per day in Mexico (1.07 grams per person per day; ^{CONAPESCA 2013}), and W is the average weight of an adult in Mexico (70.00 kg; ^{Delgado-Álvarez et al. 2015}).

The differences in Hg concentrations among the mollusks sampled in this study were evaluated by Kruskall-Wallis and post hoc Dunn tests. The differences in Hg concentrations between bivalves (mussels, oysters, and clams) and cephalopods (squids and octopuses) were assessed by Mann-Whitney tests in GraphPad Prism v. 4.0 (GraphPad Software, San Diego, CA, USA) at a confidence level of 95%.

RESULTS

The mean Hg concentrations ranged from 0.020 mg·kg^-1 dry weight in octopuses to 0.008 mg·kg^-1 dry weight in mussels (Table 1). The average Hg concentrations in mollusks followed the order of octopuses > squids = clams > oysters > mussels. The Hg concentrations in mussels were significantly (P < 0.05) lower than those in squids, oysters, clams, and octopuses. Given that bivalves and cephalopods belong to different trophic levels, the Hg concentrations in mussels, oysters, and clams were averaged and compared with the average values of squids and octopuses. The mean Hg concentration of bivalves (0.013 mg·kg^-1 dry weight) was significantly (P < 0.05) lower than that of cephalopods (0.018 mg·kg^-1 dry weight).

The mean concentrations of Hg and MeHg (µg·g^-1 wet weight) and their corresponding HQ values are presented in Table 2. Hg and MeHg concentrations in the mollusks in this study followed the order of cephalopods (octopuses and squids) > bivalves (mussels, oysters, and clams). The Hg concentrations varied among mollusks, although none of the products contained Hg concentrations that were above the maximum permissible limits for Hg (1.000 mg·kg^-1 wet weight) or MeHg (0.500 mg·kg^-1 wet weight) established by the Mexican legislation for cephalopods (NOM-129 1993; ^{SSA 1997}) and bivalves (NOM-032 1993; ^{SSA 1994}). HQ values were below 1.0 for Hg and MeHg, thus the consumption of the canned mollusks evaluated in this study does not carry a health risk. As was observed in the Hg and MeHg concentrations, both HQ_Hg and HQ_MeHg were higher in cephalopods than in bivalves. Although the potential exposure to Hg and MeHg by consuming canned mollusks is higher in cephalopods than in bivalves, it is unlikely to result in adverse health effects, even for vulnerable groups within the population.

From our results and the studies presented in Table 3, it can be seen that the Hg concentrations were generally low and variable. The highest Hg concentrations (0.140 mg·kg^-1 wet weight) were reported in octopuses canned in Portugal, whereas the lowest concentration (0.003 mg·kg^-1 wet weight) corresponded to the canned mussels evaluated in our study. The canned mollusks sold in Mexico have Hg concentrations that are not currently of concern; however, information related to other toxic elements, consumption rates, and the co-occurrence of selenium is necessary for precise assessments of consumer health risks.

Among fishery products, finfish constitute the main source of Hg exposure to humans in tropical and subtropical regions (^{Costa et al. 2012}). To provide additional context for the Hg concentrations in canned bivalves sold in northwestern Mexico, we compared these concentrations to those of canned fish. The Hg concentrations in our study (0.003-0.007 mg·kg^-1 wet weight) were 2 orders of magnitude lower than those of canned tuna (Thunnus germo; canned in water: 0.431 mg·kg^-1 wet weight, canned in oil: 0.419 mg·kg^-1 wet weight) from the USA (^{Burger and Gochfeld 2004}) and Thunnus albacares canned in water (0.362 mg·kg^-1 wet weight) from Mexico (^{Ruelas-Inzunza et al. 2011}).

DISCUSSION

Bivalve mollusks have been extensively used as biomonitors of marine pollution because of their sedentary nature and ability to concentrate metals such as Hg. Indeed, more information is available for bivalve mollusks than cephalopods, which may reflect the heightened popularity and accessibility of bivalve products among diverse sectors of the population. In Mexico, detailed information related to the consumption of fishery products is nonexistent (^{Ruelas-Inzunza et al. 2011}). Thus, it is difficult to accurately estimate the health risks associated with the consumption of mollusk products.

The Hg concentrations in this study were similar to those reported by ^{Gutiérrez et al. (2006)}. Those authors found that mussels exhibited lower Hg concentrations than those of other canned bivalves from Spain, which may have been due to their short life cycles and low trophic positions. In a study that evaluated diverse components of a marine food web in Bohai Bay, China, the trophic positions (determined by stable C and N isotopes) of bivalves ranged from 2.15 in the bay scallop (Argopecten irradians) to 2.17 in the short-necked clam (Ruditapes philippinarum), with bivalves occupying lower trophic positions than those of either fish or seabirds (^{Yi et al. 2005}). Notably, fish that occupy elevated trophic positions (e.g., tuna) exhibit elevated Hg concentrations compared to those that occupy lower trophic positions. Similar patterns were found by ^{Blanco et al. (2008)} and ^{Olmedo et al. (2013)} when comparing Hg concentrations in canned fish and mollusks consumed in Spain. These authors found elevated Hg concentrations in tuna when compared to those of cephalopods or bivalves.

Other authors have reported variable Hg concentrations in various mollusk and fish species. ^{Karimi et al. (2013)} found elevated intraspecific variability in Hg concentrations among diverse bivalve species from Long Island, New York. In a study with various canned mollusks from Portugal (^{Lourenço et al. 2004}), the Hg concentrations were higher (0.110 mg·kg^-1 wet weight) in cephalopods (squids and octopuses) than in blue mussels (0.030 mg·kg^-1 wet weight). Interestingly, in Oman, ^{Al-Mughairi et al. (2013)} found that Hg concentrations were not significantly different between cephalopods and bivalves. These authors argued that the high variability in metal concentrations may have been responsible for the lack of differences. In a study with canned tuna in oil matrices, salads, sauces, and pastes, no statistically significant differences were found among presentations, and it was concluded that Hg content was not affected by the packaging medium (^{Pawlaczyk et al. 2020}). High variability in metal content among canned fisheries products makes it difficult to establish patterns of Hg accumulation. This variability may be due to mixing differently sized specimens during canning and the environmental conditions of the sites in which the mollusks and fish were collected.

From the information provided on the labels of fisheries products, we determined that the oysters and clams evaluated in this study were caught and processed in Mexico, whereas the mussels, squids, and octopuses were imported from South Korea, Spain, and the United States, respectively. The origin of fishery products is important in terms of the potential degree of metal accumulation in the collection site. Unfortunately, accurate information on this topic is nonexistent in Mexico. In addition to the origin, other factors that may account for the Hg concentrations in canned mollusks are the pH of the canned products, oxygen concentration in the headspace, storage time, and humidity of the storage location (^{Oduoza 1992}).

The variation in metal concentrations among canned products can also be the result of thermal processing prior to canning. Indeed, cooking has been found to affect the concentrations of metals in seafood. Although ^{Morgan et al. (1997)} found that Hg concentrations in raw seafood remained the same after cooking, ^{Torres-Escribano et al. (2010)} found that the Hg concentrations increased in cooked seafood due to water loss. Similarly, cooked bivalve mollusk (Lucina pectinata and Anomalocardia brasiliana) samples were found to exhibit significantly higher Hg concentrations than those of raw samples (^{Costa et al. 2016}), which may have been due to the influence of heat on the formation of complexes between Hg chemical species and sulfhydryl groups, such as methylmercury-cysteine, in muscle tissues (^{Clarkson and Magos 2006}).

To compare our results with those of studies that reported their results on the basis of dry and wet weights, we estimated a moisture percentage for our study (67%). This value is similar to those reported by ^{Gutiérrez et al. (2007)} for canned variegated scallops (Chlamys varia) of 7 commercial brands sold in Spain (67.34-71.89%). Variations in humidity may be due to the thermal processes used during canning, as ^{Gutiérrez et al. (2007)} reported for mussels. Furthermore, ^{McCarron et al. (2008)} reported that the moisture content ranged from 79.1-79.2% in fresh mussels (Mytilus edulis) from Ireland, whereas moisture content ranged from 67.4-67.8% in autoclaved specimens. Thus, heat processes may decrease the moisture content, which results in higher elemental concentrations than those of fresh products.

The Hg concentrations in this study varied by an order of magnitude. Nevertheless, the cephalopods commonly used to produce canned goods showed higher Hg concentrations than those of canned bivalves. Thus, Hg concentrations in this study followed the order of octopuses > squids = clams > oysters > mussels. The high variability in Hg concentrations in this study might be related to the conditions of the sites in which the mollusks were collected, the thermal processes used to prepare products, and the practice of mixing differently sized specimens during canning. When comparing our results to those of canned mollusks from different regions, the Hg concentrations were low and variable. Indeed, none of the Hg concentrations in the mollusks were above the maximum permissible limit of 1.0 mg·kg^-1 wet weight established by Mexican legislation. When compared to those of canned fish, the Hg concentrations in the mollusks in this study were lower, especially when compared to those of fish occupying high trophic levels.