Mercury and selenium concentrations in the oysters Saccostrea palmula and Crassostrea corteziensis from four coastal lagoons of NW Mexico: health risk assessment

Mercury (Hg) and selenium (Se) concentrations in the soft tissue of the oysters Saccostrea palmula and Crassostrea corteziensis from four coastal lagoons (Altata, Macapule, Navachiste, El Colorado) of Northwest Mexico were determined. A total of 2520 specimens of S. palmula and 3780 of C. corteziensis (72.15 ± 4.95 and 73.57 ± 5.31 mm, respectively) were sampled seasonally from summer 2019 to spring 2020. The higher Hg concentration (wet weight) for S. palmula (0.13 ± 0.03–0.11 ± 0.02 µg g−1) and C. corteziensis (0.12 ± 0.02–0.11 ± 0.03 µg g−1) were obtained in summer−autumn 2019; Hg level was similar among the coastal lagoons and did not exceed the limit established by Mexican legislation and by the Food and Drug Agency. The higher Se concentration occurred in spring 2020 for both oyster species in El Colorado lagoon (4.55 ± 0.02 and 4.08 ± 0.05 µg g−1, respectively). The Se/Hg molar ratio of S. palmula and C. corteziensis ranged from 48.76–149.59 and 31.72–155.37, respectively; while the Se health benefit value was 19.23–42.28 and 17.82–35.30, respectively. The hazard quotient for Hg -estimated as methylmercury- and Se in both species of oyster was below 1. The high molar ratio obtained (Se/Hg > 1) indicates that the concentration of Se was sufficient to neutralize the possible toxicity of the Hg, therefore, the consumption of S. palmula and C. corteziensis from the four coastal lagoons studied does not represent a risk.

Amazonia Seasons Have an Influence in the Composition of Bacterial Gut Microbiota of Mangrove Oysters (Crassostrea gasar)

The mangrove oysters (Crassostrea gasar) are molluscs native to the Amazonia region and their exploration and farming has increased considerably in recent years. These animals are farmed on beds built in the rivers of the Amazonia estuaries and, therefore, the composition of their microbiome should be directly influenced by environmental conditions. Our work aimed to evaluate the changes in bacterial composition of oyster's microbiota at two different seasons (rainy and dry). For this purpose, we amplified and sequenced the V3-V4 regions of the 16S rRNA gene. Sequencing was performed on the Illumina MiSeq platform. According to the rarefaction curve, the sampling effort was sufficient to describe the bacterial diversity in the samples. Alpha-diversity indexes showed that the bacterial microbiota of oysters is richer during the rainy season. This richness is possibly associated with the diversity at lower taxonomic levels, since the relative abundance of bacterial phyla in the two seasons remained relatively constant. The main phyla found include Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria. Similar results were found for the species Crassostrea gigas, Crassostrea sikamea, and Crassostrea corteziensis. Beta-diversity analysis showed that the bacterial composition of oyster's gut microbiota was quite different in the two seasons. Our data demonstrate the close relationship between the environment and the microbiome of these molluscs, reinforcing the need for conservation and sustainable management of estuaries in the Amazonia.

PARÁSITOS METAZOARIOS Y OTROS EPIBIONTES EN EL OSTIÓN DEL PLACER CRASSOSTREA CORTEZIENSIS (HERTLEIN, 1951) EN DOS ZONAS COSTERAS DEL PACÍFICO MEXICANO

El objetivo de este estudio fue determinar la presencia de parásitos metazoarios y epibiontes en el ostión del placer <em>Crassostrea corteziensis</em> (Hertlein, 1951) en dos localidades costeras y pesqueras del estado de Sinaloa, en el noroeste del Pacífico mexicano. Se colectaron 30 ostiones en la cooperativa pesquera de “Playa norte” en la bahía de Mazatlán, Sinaloa y se realizó una comparación con la presencia de parásitos en ostiones en la laguna costera de “Ceuta”. La inspección de ostiones en la laguna costera reveló la presencia de 36 metacercarias encapsuladas de la familia Acanthocolpidae en la gónada del ostión <em>C. corteziensis</em>. En la bahía de Mazatlán, en la zona de Playa norte, se encontraron seis metacercarias: tres de la sp. 1 (pertenece a la familia Hemiuridae) en el manto, un espécimen de sp.2 (Familia Fellodistomidae) en la glándula digestiva y una metacercaria sp3. no identificada en el manto, una metacercaria de (Familia Zoogonidae) en la glándula digestiva, dos nemátodos de vida libre del género <em>Theristus </em>sp. Como organismos epibiontes se encontraron siete ejemplares de poliquetos de la familia Nereididae en la concha de los ostiones. Este trabajo reporta por primera vez la invasión del digeneo <em>Stephanostomum</em> sp. Looss, 1899 de la familia Acanthocolpidae parásito de la gónada del ostión <em>C. corteziensis</em> en el Pacífico mexicano. Así como, la presencia de digeneos pertenecientes a la familia Fellodistomidae, Hemiuridae y Zoogonidae. Los resultados de este trabajo contribuyen al conocimiento en la biología, la biodiversidad y la preferencia del hospedero de estos parásitos, con posibles implicaciones en los riesgos para la salud que plantea el consumo humano del ostión del placer, con gran regionalización en el consumo de mariscos crudos que presentan amplia identidad social gastronómica en estas zonas costeras en el Pacífico mexicano.

Perkinsus marinus in the pleasure oyster Crassostrea corteziensis cultivated on the southeast coast of the Gulf of California, Mexico

The protozoan Perkinsus marinus has been associated with high mortality episodes of the eastern oyster Crassostrea virginica in the USA. The presence of P. marinus on the pleasure oyster Crassostrea corteziensis cultivated in two estuaries on the southeast coast of the Gulf of California was evaluated. Oysters were collected monthly (September 2016 to September 2017) and analyzed using Ray's fluid thioglycollate medium (RFTM) and polymerase chain reaction (PCR). Water parameters and oyster biometrics were also recorded. Pathogen prevalence increased over time from 0 to 100% in oysters from La Pitahaya, and from 0 to 83.33% in those from Bacorehuis. At both oyster farms, infection intensity was light (<1×104 parasites g-1 wet tissue), pathogen prevalence and infection intensity were correlated with oyster size and weight, and there was a strong correlation between P. marinus prevalence and intensity (La Pitahaya r = 0.91; Bacorehuis r = 0.82). The oysters that resulted positive for P. marinus by RFTM also were assayed using PCR. P. marinus presence was confirmed in 98.27% (114/116) of the oysters from La Pitahaya, and 95.83% (46/48) of those from Bacorehuis. The detection of P. marinus confirms that this pathogen is well established in the area with high expression during the warmer season. Despite the light infection intensity of this parasite at both sites, health surveillance of this bivalve in the region is highly advisable.

Gonadal maturity of Crassostrea corteziensis cultivated in the Gulf of California

The gonadal maturation of the pleasure oyster Crassostrea corteziensis during four cultivation cycles (November 2004-June 2005, N4J5; August 2014-February 2016, A14F16; September 2015-December 2016, S15D16; September 2016-August 2017, S16A17) was studied in the southeastern Gulf of California, Mexico. Although C. corteziensis exhibited the typical reproductive phases (undifferentiated or resting, initial gametogenesis, advanced gametogenesis, maturity, spawning and post-spawning), the intensity of reproduction and the timing of the onset of the different phases of gonadal development varied in each cultivation cycle. Reproductive peaks were observed in different months (N4J5 in May, A14F16 in July, S15D16 in May and S16A17 in June) during spring and summer. Spawning correlated with temperature in the N4J5 and A14F16 cultivation cycles but it showed no correlation with chlorophyll-a concentration in any of the cultivation cycles. C. corteziensis presented an opportunistic continuous reproductive strategy with spawning occurring during several months of the year. The modification of the species' reproduction pattern could contribute to the natural settlement of larvae, increasing the natural production of this native oyster resource in the southeast of the Gulf of California, where it has been fished intensively.

Growth and antioxidant response of juvenile oysters Crassostrea sikamea and Crassostrea corteziensis treated with Streptomyces strains

ABSTRACT The effect of three Streptomyces strains (N7, RL8 and V4) and a mixture of Bacillus (BMix) on the growth (Weight, Size) and superoxide dismutase activity (SOD) in hatchery-reared juvenile oysters Crassostrea corteziensis and Crassostrea sikamea was investigated to determine their probiotic potential. Microorganisms were added to culture water at 1×106 CFU/ml once a day during 30 days and all oysters fed daily a microalgae mix. Juveniles of C. sikamea treated with strains N7, RL8 and V4 had a significant weight gain compared to the control group. C. corteziensis juveniles treated with strains RL8 and BMix showed a significantly higher weight gain than the control group. No significant size increase was observed in any treated group for both oyster species. SOD activity significantly increased in C. sikamea treated with RL8 and with RL8, N7 and BMix in C. corteziensis. Streptomyces strains RL8 and N7 emerge as promising probiotic agents to cultivate C. sikamea and C. corteziensis and may also be useful to other molluscs and marine invertebrates .

Occurrence and Abundance of Pathogenic Vibrio Species in Raw Oysters at Retail Seafood Markets in Northwestern Mexico

ABSTRACT Seafood has frequently been associated with foodborne illness because pathogens are easily introduced during seafood cultivation, handling, and processing. Vibrio parahaemolyticus and Vibrio cholerae are human pathogens that cause gastroenteritis and cholera, respectively, and Vibrio vulnificus can cause fatal wound infections and septicemia. However, information about the occurrence of these pathogens in oysters from the Pacific coast of Mexico is limited to V. parahaemolyticus. In the present study, we evaluated the presence and abundance of these three Vibrio species in 68 raw oysters (Crassostrea corteziensis) obtained from retail seafood markets in Sinaloa, Mexico. The most probable number (MPN)–PCR assay was used for amplification of the tlh (thermolabile hemolysin), ompW (outer membrane protein), and vvhA (hemolytic cytolysin) genes that are specific to V. parahaemolyticus, V. cholerae, and V. vulnificus, respectively. All oyster samples were positive for at least one Vibrio species. V. parahaemolyticus, V. cholerae, and V. vulnificus prevalences were 77.9, 8.8, and 32.3% overall, respectively, and most species were present in all sample periods with increased prevalence in period 3. The tdh (thermostable direct hemolysin) gene was detected in 30.1%, trh (TDH-related hemolysin) was detected in 3.7%, and tdh/trh was detected in 7.5% of the total tlh-positive samples (53 of 68), whereas the pandemic serotype O3:K6 (orf8 positive) was detected in only 1 sample (1.8%). The total prevalence of tdh and/or trh was 41.5%. In none of the samples positive for V. cholerae were the cholera toxin (ctxA) and cholix (chxA) toxigenic genes or the rfb gene encoding the O1 and O139 antigens amplified, suggesting the presence of non-O1 non-O139 V. cholerae strains. Our results clearly indicated a high prevalence of pathogenic Vibrio species in raw oysters from retail seafood markets in Mexico. Consumption of these raw oysters carries the potential risk of foodborne illness, which can be limited by cooking. HIGHLIGHTS

First record of Stephanostomum sp. Looss, 1899 (Digenea: Acanthocolpidae) metacercariae parasitising the pleasure oyster Crassostrea corteziensis (Hertlein) from the Mexican Pacific coast

SummaryThe aim of this investigation was to identify the parasites present in the largely understudied pleasure oyster Crassostrea corteziensis in Sinaloa state in the northwestern Mexican Pacific coast. Inspection of twenty-eight oysters collected on “Ceuta” lagoon revealed the presence of the digenean Stephanostomum sp. (Digenea: Acanthocolpidae) cysts. Metacercariae were found encapsulated and embedded in the digestive gland and mantle tissue of oysters. The prevalence of infection revealed that 84.6 % were infected, the abundance was 13.62, with a mean intensity of 16.09 per host. The members of this genus are characterized by a double crown of spines in the cephalic region surrounding the buccal opening of the worm. Significantly, we report the first incidence of the digenean Stephanostomum sp of the family Acanthocolpidae parasitizing Crassostrea corteziensis. Further we report that this bivalve is now considered a new intermediate host, and the northwestern Mexican Pacific coast is a new geographical distribution area for this digenean. The findings contribute to our understanding of the biology, biodiversity and host preference of these parasites, with implications for health risks posed by human consumption of the pleasure oyster.