Mangrove Oyster Research Articles

The mangrove oyster Saccostrea palmula is an option for artisanal aquaculture development in the Mexican Central Pacific

The mangrove oyster Saccostrea palmula is a relatively small but emerging species in aquaculture, and it is necessary to resume and increase studies on its cultivation. Therefore, we carried out a study that includes three main components for developing S. palmula culture in the Mexican Central Pacific: spat collection, pre-growth phase (eight weeks), and grow-out phase (six months). We tested natural (shell strings) and artificial (coupelles) collectors placed in contrasting environmental conditions: marine (La Boquita) and estuarine (Cuyutlan). The spat was pre-grown in suspended Australian trays at the two collection sites and under in-land pond conditions. Then, it was pooled under an estuarine environment and in a single lot for the final culture phase in Juluapan lagoon (all sites in Colima, Mexico). Spat collection was higher in coupelles (858.3 ± 175.6 spat coupelle-1) than in shell strings. The number of spat collected (13,062 spat), final pre-growth size (26.5 ± 0.6 mm shell height), and pre-growth survival (94.7%) were higher in La Boquita (marine environment). The final culture phase in Juluapan (estuarine conditions) was successful, recording an average height of 54.2 ± 4.9 mm and average total weight of 30.8 ± 6.9 g after six months of culture, after spat collection and pre-growth periods. We found the largest oyster, S. palmula, ever recorded (shell height 83.3 mm and total weight 59.4 g), and we confirmed that S. palmula has potential for artisanal aquaculture and socioeconomic regional development in the Mexican Central Pacific.

Spatial and Seasonal Variations in the Size-Weight Relationship and Condition Index of the Mangrove Oyster Crassostrea tulipa (Lamarck, 1819) in the Saloum Delta, Senegal

In the Saloum delta in Senegal, the mangrove oyster Crassostrea tulipa is under increasing human pressure, leading to a decline in wild stocks, a decrease in the quality of the marketed oysters and the degradation of mangrove tree roots. Previous studies have pinpointed some of these disturbances and yet, there is little information available on the structure of the population and the condition index of the species. The aim of this study was to document these aspects in order to support a sustainable management of the resource. From January 2021 to December 2022, monthly sampling was carried out at five stations to study the size-weight relationship and the condition index (CI) of oysters. A total of 26,790 individuals ranging in size from 0.23 to 93.19 mm with an average of 33.86±16.38 mm shell height were collected. The species was found to have a minority allometry (b = 2.54 <3), with a strong correlation between individual weight and oyster shell height (r = 0.88). CI showed seasonal and inter-site variations over the two years, with an overall mean value of 11.17±2.72%. The highest CI values were observed during the hot season and the lowest during the hot-cold transition period.

Quantification and characterization of microplastics ingested by mangrove oysters across West Africa

Microplastic ingestion by marine organisms presents a challenge to both ecosystem functioning and human health. We characterized microplastic abundance, shape, size, and polymer types ingested by the West African mangrove oyster, Crassostrea tulipa (Lamarck, 1819) sampled from estuaries and lagoons from the Gambia, Sierra Leone, Ghana, Benin, and Nigeria using optical microscopy and Fourier transform infrared (FTIR) techniques. A total of 780 microplastics were isolated in the whole tissues of the 250 oysters (n = 50 oysters per country). The abundance and distribution of microplastics in the oysters followed the pattern: the Gambia > Ghana > Sierra Leone > Nigeria > Benin. The Tanbi wetlands in the Gambia recorded the highest average of 10.50 ± 6.69 per oyster while the Ouidah lagoon in Benin recorded the lowest average of 1.80 ± 1.90 per oyster. Overall, microplastic numbers varied significantly (p < 0.05) among the five countries. Microfibers, particularly those within 1001–5000 μm size, dominated the total microplastic count with a few fragments and films. No spherical microplastics were isolated in the oysters. In the Sierra Leone and Benin oysters, fragments and films were absent in the samples. Microplastic between the 1001 and 5000 μm size class dominated the counts, followed by 501–1000 μm, 101–500 μm, and 51–100 μm. Five polymer groups namely polyethylene, polyester, nylon, polypropylene, and polyamide were identified across the five countries, with polyethylene occurring in oysters from all five countries and polyester occurring in all but the oysters from Nigeria. This diversity of polymers suggests varied sources of microplastics ingested by the studied oysters. The absence of microspheres across the five supports findings from other studies that they are the least ingested and highly egested by the oysters.Graphical

Histological study of the sex ratio and reproductive cycle of the mangrove oyster Crassostrea tulipa (Lamarck, 1819) in the Saloum Delta (Senegal)

The mangrove oyster Crassostrea tulipa has considerable socio-economic value in Senegal, as in many other West African countries. To better understand its biology, the objective of this study was to determine the sex ratio and the evolution of gonadal development stages of oyster populations in the Saloum Delta. To this end, a monthly sampling of 30 oysters between 34.19 mm and 96.68 mm height was carried out between January 2021 and January 2022. The part of the flesh that encloses the gonads was removed and fixed using Davidson's solution. After dehydration, paraffining and inclusion of the materials, 7 μm thick sections were made using a microtome. These sections were stained with Haris’s hematoxylin and eosin and the slides were observed under an optical microscope (Olympus BX41 microscope).The results showed a predominance of females over males, with a sex ratio of 1 in 2.85. The results also revealed that reproduction was continuous over the year and that spawning was asynchronous: Individuals at the spawning stage were more frequent in the warm season. These results will contribute to a better management of natural stocks and to improve oyster farming.

A record of change in oyster environment through high-resolution geochemical analysis of Late-Holocene sediments from Coastal Ghana

The near-coast environments where oysters occur are among the most impacted by humans globally, especially during the Late-Holocene. Yet, in West Africa, there is no documented historical record of change in these environments. We provide insight into the changing geochemical conditions of two oyster environments through high-resolution analysis of total organic carbon (C), total nitrogen (N), carbon and nitrogen isotope ratios (δ13C, δ15N), and trace elements, in two cores retrieved from the Densu estuary and the Anyanui (Keta) Creek in Ghana. Drastic shifts in sedimentation rate occurred in the Keta and Densu cores around 1996 CE and 960 CE respectively. At these times, comparatively, low levels of C and N were found in the Densu core. Increasing C and N levels and decreasing δ13C upcore aligned with the observed shift in sedimentation rate in the Keta core. The C/N ratios in the Keta core suggest allochthonous organic matter (OM) dominance in the creek. The Densu core showed periodic changes in C/N ratios from very high values (&gt;20) between 1918 BCE and 1321 BCE, to values between 20 and 11 between 1321 BCE and 1977 CE and below 10 from the late 1970s CE to the present day, suggesting a varying degree of transformation in the catchment basin. Extremely high Sulfur (S) and moderate to significant Iron (Fe) increases suggest reducing conditions in the Keta sediments. Moderate Calcium (Ca), Zinc (Zn), and Strontium (Sr) concentrations in the upper part of the Densu core suggest a stronger influence of marine processes in the Densu in recent times. The findings reflect the impacts of catchment basin modification on the health of the two coastal environments, likely to impact the growth, productivity, and sustainability of the fishery of the West African mangrove Oyster.

Isolation and evaluation of local microalgal isolates as feed for larval rearing of Crassostrea tulipa towards optimisation of mangrove oyster seed production in Ghana

AbstractThis study explored the potential of three local microalgal isolates as feed for oyster larvae in laboratory‐rearing experiments, towards the optimisation of seed production of Crassostrea tulipa to support its large‐scale farming along the West African coast. Three species of local microalgae—Rhodomonas sp., Nannochloropsis sp. and Pseudanabaena sp.—were isolated from waters off the coast of Ghana, West Africa, following a serial dilution technique. The growth performance of the isolates was assessed in the laboratory through daily estimation of cell density until the stationary phase was observed. Characterisation of the microalgal isolates was carried out by estimation of their biovolume, carbon content and energy content. Biovolumes of the microalgae were calculated from the estimated equivalent spherical diameters using proposed geometric shapes and formulae. Carbon weight and carbon energy content were subsequently calculated using derived conversions. The three microalgal isolates showed potential for large‐scale cultivation in the laboratory with marked differences in daily increases in cell densities. Nannochloropsis sp. and Rhodomonas sp. recorded the highest and the lowest peak densities of 2.4 × 105 and 1.5 × 105 cell mL−1, respectively, from an initial inoculating cell density of 1.05 × 105 cell mL−1. The estimated mean biovolumes of Rhodomonas sp., Nannochloropsis sp. and Pseudanabaena sp. were 238.9, 8.182 and 42.42 µm3, respectively, and the corresponding derived carbon energy contents were 1.7 × 10−6, 7.13 × 10−8 and 1.05 × 10−7 J, respectively. Results from a laboratory rearing experiment indicated that the individual microalgal isolates supported the growth and survival of oyster larvae at different scales, but a mixed diet of the three promoted superior growth and survival of C. tulipa larvae. The three local microalgal isolates‐ Rhodomonas sp., Nannochloropsis sp., and Pseudanabaena sp.‐ were well adapted to laboratory culture conditions, and the observed differences in growth and survival of the oyster larvae fed on these algal diets could be due to the differences in diet properties and biochemical compositions of the different species. A combination of the three algal diets, however, provided complementaery nutrients for the optimal growth and survival of C. tulipa larvae. The outcome of this study shows that local microalgal isolates have the potential to support hatchery rearing of C. tulipa, which is essential for the development of commercial mangrove oyster aquaculture in West Africa.

Population aspects of Crassostrea sp. in a protected mangrove estuary: perspectives for artisanal fishing management

ABSTRACT The mangrove oyster (Crassostrea) is one of the most common bivalves found on the Brazilian coast, widely used in artisanal fisheries. Oysters are dioecious with external reproduction, typically found in estuarine environments. The present study aimed to identify the population aspects of Crassostrea in different fishing locations in northeastern Brazil. The study area was the Acaú-Goiana Extractive Reserve, a protected area focused on the development of artisanal fisheries. Oysters were collected during low tide at three sampling points, where their removal from mangrove roots was carried out, during May, July, September, November 2021, and January and March 2022. In the laboratory, biometric measurements were conducted, and for the months of May and November 2021 histological preparations to study the reproductive cells. Biometric measurements were significantly higher in the dry season at all sampling points. The sex ratio was M = 0.38, F = 0.62, for May, and M = 0.30, F = 0.69, for November. The biometric and reproductive data obtained can support the development of management strategies, including a rotation plan between collection sites and the establishment of a minimum size for fishing. Here we recommend a minimum size of 20 mm shell length (SL) or larger for fishing.

Prevalence of bacterial pathogens in bivalve shellfish harvested from the brackish waters of Niger Delta, Nigeria

The presence of bacterial pathogens in bivalve shellfish harvested from the marine ecosystem globally has continued to raise serious food safety concerns, thereby undermining the nutritional and health benefits derived from their consumption. Four species of bivalve shellfish: Bloody cockle (Anadara senilis), Donax clam (Donax rugosus), Knife clam (Tagelus adansonaii) and Mangrove oyster (Crassosstra gasar) harvested from the brackish waters of the Niger Delta were assessed for their bacterial pathogens. Standard methods of analysis were employed to assess microbiological hazards accumulated by shellfish species. The results of the microbiological analysis revealed that the total viable count (TVC) was lower than the recommended limit (5.7 log cfu/g) for fresh bivalve shellfish while Vibrio cholerae, Vibrio parahaemolyticus, Salmonellaspp, Shigellaspp, Listeria monocytogenes and E. coli were far above the stipulated FDA standard for molluscian shellfish. The loads of microbiological hazards in bivalve species indicated that in all the locations, Bloody cockle and Donax clam accumulated more bacterial pathogens while the samples harvested from Ibeno and Iko Town brackish waters were lower in pathogenic loads when compared to shellfish harvested from Andoni and Bonny location. The result obtained in this study is of public health importance since, it will serve as a guide towards improved processing techniques and stimulate actions towards the development of novel surveillance as well as, prevention and control strategies, that will help to reduce foodborne disease outbreaks associated with shellfish.

The nomenclature of the amphi-Atlantic mangrove oysters

An article published last year in this journal (Ferreira et al., 2023) advocated the use of the name Crassostrea gasar (Dillwyn, 1817) for an amphi-Atlantic mangrove oyster. We challenge this terminology, which is not supported by the International Code of Zoological Nomenclature, and defend the status quo accepting the name Crassostrea tulipa (Lamarck, 1819).

Reproductive cycle of the mangrove oyster Crassostrea rhizophorae (Guilding, 1828) in Delta do Parnaíba, Brazil

This study describes some aspects of the reproductive biology of the native oyster Crassostrea rhizophorae in the Delta do Parnaíba estuary, located between the states of Maranhão and Piauí, Brazil. The oysters were acquired from landings at Porto dos Tatus (Ilha Grande - PI) between June 2017 and May 2018. Approximately 50 individuals per month (n=611) with an average total shell length of 74.44 mm (±14.31 SD) were collected for analysis of gonadal development and condition index and correlated with temporal variations in local rainfall. The C. rhizophorae population in the Parnaíba Delta estuary showed a sex ratio of 2.18 females to 1 male and continuous reproduction. The condition index (CI) values in the dry period were higher, coinciding with a greater proportion of individuals in the gametogenesis and pre-spawning stages. In the rainy season, there was a reduction in the CI as a result of animals in the gamete release stage.

Evaluation of Heavy Metal Pollution in Commonly Consumed Mollusc (Crassostrea gasar) from Elechi Creek, River State, Nigeria and the Health Risk Implications

Marine biotas are used to assess potential adverse human health risks associated with consuming protein-rich aquatic organisms. Heavy metal content of Mangrove oysters (Crassostrea gasar) was evaluated between January and June 2022 in 3 stations. Six heavy metals (copper, cadmium, zinc, lead, arsenic and iron) were determined using standard methods. Target Hazard Quotient (THQ) and Hazard Index (HI) were used for the non-carcinogenic assessment while Target Cancer Risk (TR) was used for the carcinogenic assessment of the potential human health risk of consuming the oysters. The heavy metal values recorded were Cu (473.2 – 596.7 mg/kg), Cd (2.33 – 3.84 mg/kg), 209.02 – 246.41 mg/kg), Pb (6.16 – 12.07 mg/kg), As (0.012 – 0.016 mg/kg) and Fe (1609.0 – 1846.0 mg/kg). All the heavy metals were above the acceptable limits except arsenic. Stations 2 and 3 had relatively higher values; attributed to anthropogenic activities. The THQ and HI values were less than 1 in all the metals and stations while TR for Pb and arsenic were within the negligible range in all the stations. However, Cd was unacceptable among the children in station 2. Station 3 had relatively higher values while the children were more vulnerable to both non-carcinogenic and carcinogenic risks. In conclusion, the consumption of oysters from Elechi Creek is considered safe based on acceptable levels of the THQ, HI and TR; though Cd-TR for children (Station 2) was unacceptable.

Growth rate of farmed Mangrove oysters (Magallana bilineata) at Laucala Bay, Suva, Fiji

The cultivation of oysters has a significant historical background in the South Pacific region. The cultivation of pearls has been a significant source of livelihood for numerous island nations. Despite the significant cultural, dietary, and subsistence value of bivalve molluscs among Pacific Islanders, there has been limited advancement in the aquaculture of edible oysters in To enable a comprehensive assessment of the viability of cultivating edible oysters in Fiji, an estimation was conducted pertaining to the growth rate of mangrove oysters Magallana bilineata. A total of 105 oyster spat were cultured in Laucala Bay, Suva, Fiji, over the course of one year, from January 25th, 2021 to January 26th, 2022. The investigation documented exponential growth in oysters, with measurements of 87.3 ± 1.0 mm in length, 67.4 ± 0.7 mm in width, 49.8 ± 0.6 mm in depth, and 104.90 ± 22.6 g in weight. The findings of the study indicated the occurrence of positive isometric growth in relations to weight, length, width, and depth. The observed survival rate was 94%. The growth rates observed in oyster farming in Fiji exhibit similarities to those documented in other geographical locations, thereby providing a favourable indication for the progress of oyster aquaculture in the region.

Genetic evidence of the unique identity of the West African Mangrove Oyster (Crassostrea tulipa) from the Gulf of Guinea

This research enabled the genetic identification of the West African mangrove oysters, Crassostrea tulipa, as well as establishing the evolutionary relationship between it and other Crassostrea species. Essentially, the study assisted in clearing up a long-standing confusion over this species’ synonymy with C. gasar. Also, the population structure of 60 C. tulipa individuals, from three different ecotypes, was analyzed using the mitochondrial cytochrome oxidase I (COI) genes as a marker. Results provided the first genetic sequences for C. tulipa and deposited in the GeneBank. Optimal and consensus bootstrap Neighbor-Joining trees distinctively differentiated C. tulipa from other Crassostrea species and consistently formed a different clade with C. gasar, with no bootstrap value from either NJ, MPT, or UPG trees supporting their similarity. C. tulipa sequences occurred as different haplotypes from other Crassostrea sp, with a mutation value as high as 288 and a haplotype diversity of 0.893 between C. tulipa and C. gasar sequences. High estimates of genetic distance (1.40–1.55) and patristic divergence were recorded between C. tulipa and C. gasar, in the same range as with seven other species. The study thus reveals the unique identity of C. tulipa as genetically distinct from C. gasar and other Crassostrea species. Based on the population structure analyses from the neutrality test, a low to high haplotype diversity h (0.000–0.963) and low nucleotide diversity π (0.00–0.378) were obtained. A negative mean Tajima’s D (−0.65247), and a positive Fu’s Fs (3.194), suggest rare variations or low-frequency polymorphisms. In addition to serving as the basis for phylogeny, the identification of C. tulipa and the recent data on its population structure also serve as the basis for conservation efforts and hatchery-based aquaculture.

Assessment of genetic resources of the mangrove oyster Crassostrea gasar along the south and southeast coast of Brazil: baseline for its conservation and sustainable use

Assessment of genetic resources of the mangrove oyster Crassostrea gasar along the south and southeast coast of Brazil: baseline for its conservation and sustainable use

Polychlorinated biphenyls residues in bivalve shellfish from Niger Delta: distribution pattern, tissue burden and food safety assessment

ABSTRACT This study investigated the distribution pattern, tissue burden, and safety of bivalve shellfish for human consumption. Edible species of bivalve shellfish harvested from Niger delta were screened for polychlorinated biphenyl (PCB) contaminants. Instrumental analysis and quantification of PCB congeners were performed using gas chromatography fitted with electron capture detector (GC-ECD). Results indicated an accumulation of PCB concentration of 0.36 ng/g, 0.89 ng/g, 0.98 ng/g, and 1.73 ng/g at Andoni, Iko Town, Ibeno, and Bonny, respectively. The PCB burden showed bloody cockle at Bonny location recorded the highest concentration of PCB congeners (2.37 ng/g), closely followed by mangrove oyster (2.13 ng/g) at Iko Town location, while Donax clam at Ibeno (0.09 ng/g) has the least PCB burden. The results obtained were lower in relation to food regulatory threshold. However, PCB tissue burden of bivalve shellfish harvested at the study locations indicated an environment impacted by anthropogenic activities that could negatively affect human health which depends on these shellfish for food.

Genetic Structure and Diversity in Wild and Cultivated Populations of the Mangrove Oyster Crassostrea gasar from Southern Brazil.

The mangrove oyster (Crassostrea gasar) is Brazil's second most cultured species and presents a high potential for aquaculture. However, artificial selection in a highly fecund species and significant variance in reproductive success can result in the loss of genetic diversity and increases the inbreedingrate, especially in cultivated populations. In this study, we investigated the genetic structure and diversity of C. gasar in wild and cultivated populations using 14 microsatellites. Spatial genetic comparisons revealed the existence of two main genetic groups of C. gasar, one comprising the population in cultivation and the other formed by wild populations along the southern and southeastern Braziliancoasts. Although no common genetic structure exists among wild populations, it is possible to observe a distribution gradient based on discriminant analysis of principal components consistent with their geographic distribution. However, it is insufficient to differentiate them genetically. Despite artificial reproduction, thegenetic diversity values of the cultivated population remained relatively high and did not show areduction. Therefore, monitoring the cultivated population and establishing reference values for genetic diversity will allow the adoption of strategies both for the viability of the cultivated population and the management of wild populations.

A historical and integrative taxonomic account of mangrove oyster species native to the Atlantic American coast: A re-evaluation of Brazilian Crassostrea species

The identification of native mangrove oyster species of the genus Crassostrea in Brazil is a recurring issue for specialists. Studies have recognized the presence of two species in the littoral zone: C. rhizophorae and C. gasar. However, other names may be used, such as C. mangle for C. rhizophorae and C. brasiliana or C. tulipa for C.gasar. As in most oyster taxonomic studies, their identification requires genetic evaluation for the precise determination of taxa. Moreover, previous revisions have proposed some morphological traits by which each species could be precisely identified, leading to a series of debates about the best model for species identity. The aim of this study was to apply an integrative taxonomic approach to native Crassostrea mangrove oyster species on the Brazilian coast to assert the correct species names, delimitations, and concise descriptions. Molecular tracing of cultivated specimens, along with wild specimens of Crassostrea collected from the Brazilian coast, was used to confirm the morphological identity of the taxa. The wild specimens were tentatively identified as either C. gasar or C. rhizophorae based on previous morphological revisions. PCR-RFLP and DNA analyses of the mitochondrial 16S gene mitochondrial rDNA from wild specimens were used to confirm taxa and re-evaluate morphological boundaries and descriptions of the species. We confirmed two patterns in the RFLP and sequence analyses: C. gasar and C. rhizophorae. Conchological and anatomical evaluations of 1006 identified Crassostrea specimens confirmed the inherent polymorphism between the native species, preventing reliable identification of the species by morphology alone. A clear exception, however, is found for specimens whose shell height is greater than 110 mm with internal view of white-colored valves and without an umbonal cavity, fitting the description of C. paraibanensis, which belongs to C. gasar. C. brasiliana and C. mangle are synonyms of C. gasar and C. rhizophorae, respectively. C. tulipa represents a species disconnected from any described mangrove oyster species from Atlantic seas. Conchological and anatomical descriptions of both species were improved to include the variability encountered.

Fatty acid contribution of microalgal diets on spat performance of the tropical mangrove oyster Crassostrea rhizophorae, in Mochima Bay, Venezuela

Fatty acids play a fundamental role in the ecology and growth of many invertebrates, such as filter-feeder bivalve. To guarantee adequate nutrition during transfer from the nursery to the natural environment, we studied the survival, growth and fatty acid (FA) profile of Crassostrea rhizophorae oyster spat fed with seven microalgae combinations of Chaetoceros sp. Araya strain (Ch-A), Tisochrysis lutea (Tl) and Tetraselmis chuii (Tc). The diets included three monoalgal diets (Ch-A, Tl and Tc), three binary combinations (Tl+Ch-A, Tl+Tc and Ch-A+Tc) and one ternary mixture (Tc+Ch-A+Tl). Groups of triplicated carboys each containing 21 oyster spat were used to test the experimental diets, while an additional group was deployed directly in the environment as a control. After 36 days, total lipids, FA profiles and biometric parameters of the spat were analyzed.​ The spat were then transferred to the outdoor environment, and cultivated under suspension conditions for 30 additional days. At the end of this period, FA profiles and growth were again determined. Environmental variables were sampled weekly. Highest shell height growth and soft tissues mass were observed in the spat fed with Ch-A in any combination: the mono-algae Ch-A diet, the binary Ch-A+Tc and Ch-A+Tl diets, a tendency that persisted in the outdoor environment. Our results showed lower shell and soft tissues mass growth, and a drop in the levels of certain FAs of the spat after their transfer to the natural environment, such as 14:0, 16:1n-7, 18:1n-9, 18:4n-3 and 20:5n-3 FA. This effect probably reflected a compensation strategy for the scarce availability of food and higher temperatures observed in the natural environment.

Effects of cadmium and zinc on gene expression of novel molecular biomarkers in the mangrove oyster Crassostrea gasar

Metal contamination impacts various aquatic species, and mollusk bivalves are appropriate sentinel organisms in coastal pollution assessment. Metal exposure can disrupt homeostasis, alter gene expression, and harm cellular processes. However, organisms have evolved mechanisms to regulate metal ions and counteract their toxicity. This study examined the effect of acute cadmium (Cd) and zinc (Zn) on metal-related gene expression in gills of Crassostrea gasar following 24 and 48 h of laboratory exposure. We focused on Zn transport, metallothionein (MT), glutathione (GSH) biosynthesis, and calcium (Ca) transporter genes to understand the underlying Cd and Zn-accumulating mechanisms that prevent metal toxicity. Our findings revealed increased Cd and Zn levels in oyster gills, with significantly higher accumulation after 48 h. C. gasar accumulated high Cd concentrations even in scarce conditions and increased Zn levels, suggesting a strategy to cope with toxicity. While no significant gene expression differences were observed after 24 h, the increased metal accumulation after 48 h led to upregulation of CHAC1, GCLC, ZnT2, and MT-like genes in oysters exposed to Cd, and increased ZnT2-like expression following exposure to higher Cd/Zn mixtures. We found evidence of oysters may mobilize metal-related genes to mitigate Cd-induced toxicity by both chelating metals and/or reducing their intracellular concentrations. The observed genes upregulation also indicates their sensitivity to changes in metal bioavailability. Overall, this study offers insights into oyster mechanisms for coping with metal toxicity and suggests ZnT2, MT, CHAC1, and GCLC-like as molecular biomarkers for monitoring aquatic metal pollution using C. gasar as sentinel species.

Effects of Salinity on the Reproductive Cycle of the Mangrove Oyster Crassostrea tulipa in Hatchery Conditions

Understanding the effects of environmental parameters on the reproductive cycle of shellfish is an essential tool for successful hatchery conditioning management and, consequently, for seed supply to the industry. For the tropical oyster, Crassostrea tulipa, little is known about the factors affecting the maturation process and reproduction, but there are indications that salinity may be of importance. Two experiments were carried out to evaluate the effects of salinity on the reproductive cycle of broodstock of C. tulipa. In the first experiment, oysters were reared in five salinities (40, 35, 30, 25, and 20) for 40 days and gonad histological samples were collected regularly. In a follow up experiment, oysters were reared in variable salinity conditions changing from 40 to 30 or from 20 to 30 over a period of 40 days. Salinity was found to affect the reproductive cycle of C. tulipa. Oysters kept in constant salinity of 30 showed a higher condition index compared to animals in other salinities, with most of the animals in the prespawning stage. Salinity variation was also shown to affect C. tulipa maturation. The prespawning stage was the most observed reproductive stage in low salinity, and high salinity induced a resting stage in the female animals. The results are considered the first step towards developing a protocol for the conditioning of C. tulipa in hatchery conditions.