Phenotypic indicators for cultured pearl size improvement in the black‐lipped pearl oyster (Pinctada margaritifera): towards selection for the recipient growth performance

The black‐lipped pearl oyster, Pinctada margaritifera, is the most important farmed species in French Polynesia and the basis of the most valuable export industry. Mass production of black pearls relies on a surgical operation requiring tissue from a donor pearl oyster to be grafted, together with a nucleus made of shell, into the gonad of a recipient oyster. Improving pearl size through family selection remains one of the main challenges for future aquaculture development. This study analyses the relative contribution of donor and recipient oysters to pearl size. To this end, hatchery‐produced donor oysters of two batches, large and small (based on shell height), were used to supply grafts for recipients, which were then monitored individually for their growth performance by recording shell height, width, and thickness, and total live weight (flesh + shells) every 6 months (four biometric measurement times) over 20 months of culture. Pearls issued from the two batches of donors showed no significant differences in nacre weight or thickness. In contrast, recipient oyster shell height and total weight were increasingly positively correlated with these pearl size parameters over the culture period, becoming significant at 8 months post‐grafting. Potential therefore exists to use shell height and oyster weight as phenotypic indicators for selective breeding of recipient oysters with high growth performance to increase pearl size in P. margaritifera.     

Donor effect on cultured pearl nacre development and shell matrix gene expression in Pinctada margaritifera reared in different field sites

Understanding the respective roles played by donor and recipient pearl oysters in pearl quality determination in relation to the environment is a challenge for the pearl industry. In most Pinctada species, pearl size is mainly related to recipient oyster growth performance but also relies to some extent on the biomineralisation activity of the pearl sac, a tissue that originates from the donor oyster mantle. We examined donor effect on pearl size in response to culture in the lagoons on Arutua and Apataki atolls. Overall, nacre weight and thickness were greater in Arutua than in Apataki, but sensitivity to the environment differed between donors. Some donors were associated with significantly heavier and thicker nacre in Arutua (I group), while others had similar results at the two sites (NI group). On average, up to 20% of the pearl size could be attributed to the donor but, in group I, donor effect was responsible for up to 36% of nacre weight determination. Additionally, a real‐time PCR expression study of eight matrix protein genes related to biomineralisation in the pearl sac showed that MSI60, pearlin and pif177 were significantly and positively correlated with nacre weight and thickness, with the latter two genes explaining the larger pearl size observed in Arutua. Donor oysters in P. margaritifera therefore play a key role in pearl size improvement, related to the role of the shell matrix protein genes. Understanding such contributions could help in the design of genetic selection plans for specific and adapted donor oyster lines.     

Comparison of harvested rate and nacre deposition parameters between cultured pearls issued from initial graft and second nucleus insertion in P. margaritifera

Cultured pearls produced with Pinctada margaritifera, using the surgreffe method (implantation of a second nucleus following pearl harvest) were studied for the first time to: (1) examine family effect on nacre thickness, nacre weight and nacre deposition speed and (2) compare variation in these three traits with that obtained from the cultured pearls previously harvested after the corresponding initial grafts. A surgreffe experiment using 783 recipient oysters was realized in Rangiroa atoll (French Polynesia). After 24 months of culture, 389 cultured pearls were harvested. Significant donor family effect was found for the harvested pearl rate from surgreffe (P = 0.046). Highly significant donor family effect was recorded for nacre thickness (P = 0.004). Very highly significant donor family effects were recorded for nacre weight and nacre deposition speed (P < 0.0001). Comparison between surgreffe and initial graft showed: (1) no significant effect for the average cultured pearl rate harvested (P = 0.052) and average cultured pearl nacre deposition speed (P = 0.622) and (2) very highly significant differences (P < 0.0001) for the average cultured pearl nacre thickness and nacre weight. This study highlighted three major implications for pearl industry management: (1) donor family effect was maintained from initial graft to surgreffe, for nacre thickness, weight and deposition speed, (2) the persistence of the pearl sac metabolic activity over three years of culture and (3) the relation between harvested pearl rate and the size of the nucleus inserted in the pearl sac.     

Age‐dependence of cultured pearl grade and colour in the black‐lipped pearl oyster Pinctada margaritifera

Pinctada margaritifera is an economically important marine bivalve species for cultured pearl production in French Polynesian aquaculture. In order to evaluate the influence of donor oyster age on pearl quality traits, experiments were conducted over 6 years using both grafts and surgreffe operations. At harvest, six pearl quality traits were recorded and compared: surface defects, lustre, grade, darkness level and visual colour. Analysing the quality traits of pearls harvested in the initial graft process and those of pearls obtained from surgreffe experiments allowed a comparison of the influence of pearl sac cells originating from the initial mantle graft, which aged together with their recipient oysters. The results demonstrated a significant decrease between these successive grafts in lustre, grade (A‐B‐C), darkness level, and green colour – traits that are of major importance in the pearl market. The duplicated graft experiment allowed the comparison of donor oyster families at 2 and 5 years old, where a mantle graft was inserted into recipient oysters aged 2.5 years. The results showed the same tendencies to a lesser extent, with (i) an improved pearl grade, predominantly through a most important rate of 0 surface defect category, and (ii) a green/grey ratio in favour of the younger donor. A comparison between the graft‐surgreffe and the duplicated graft experiments also highlighted: (i) the indirect role played by the younger recipient oysters, which must be optimized for optimal pearl quality realization, and (ii) the complex interplay between the donor and recipient oysters.     

Factors influencing the quality of half‐pearls (mabé) produced by the winged pearl oyster,Pteria penguin (Röding, 1758)

The winged pearl oyster, Pteria penguin, is cultured primarily to produce half‐pearls (mabé). The mabé quality is influenced by culture techniques, but there is limited information in this field. P. penguin with mean (±SE) dorso‐ventral height of 250 ± 6.5 mm were used to investigate the influence of culture period and nucleus position on mabé quality. Oysters were relaxed using 1‐propylene phenoxetol, and five nuclei were glued at different positions to the inner surfaces of the oyster shells; three on the more concave left valve and two on the right valve. Nucleated oysters were then cultured for 10 months under commercial pearl farming conditions at Savusavu in Fiji. Nacre deposited at the base and top of the nuclei was measured monthly, from the 6th to the 10th months of culture and the different qualities of mabé produced at different positions were scrutinized. Nacre thicknesses at the base and top of the resulting mabé were significantly different at different months (P < 0.05) and the rate of nacre deposition was highest during the warmer months. The different positions of nuclei on the valve greatly affected the quality of mabé formed. After a 10‐month culture period, around 1 mm of nacre covered the nuclei although the best quality mabé were obtained after 9 months.     

Cost–benefit analysis of two culture methods that influence pearl production from the black‐lip pearl oyster,Pinctada margaritifera

The black‐lip pearl oyster, Pinctada margaritifera, used for round pearl production in Polynesia, is generally cultured using “ear‐hanging” where they are attached to a rope to form “chaplets.” In other countries, pearl oysters are cultured using panel (pocket) nets that are more expensive than chaplets but afford more protection to cultured oysters. Prior research has shown panel nets produce pearls of higher quality and value, potentially generating higher profits. This study used cost–benefit analysis to compare pearl production using chaplet‐based and panel net‐based culture methods. Whole farm data, including gross revenues and annual production costs, fixed and variable, were analyzed. Average production cost per pearl using panel net‐based culture was USD 22.47 and for chaplet‐based culture was USD 21.55. However, use of panel nets saved around 3,430 hr (USD 6,860) of labor a year, offsetting the greater capital investment. A chaplet‐based pearl farm generated USD 65,738 in annual profits compared to USD 88,774 for a panel net‐based farm. Positive cash flow was achieved 1 year earlier (Year 7) for the panel net‐based farm. This is the first economic analysis of different pearl culture methods for P. margaritifera and evidence of profitability will support further development of the black‐lip pearl industry in the Indo‐Pacific region.     

Macro‐geographical differences influenced by family‐based expression on cultured pearl grade,shape and colour in the black‐lip ‘pearl oyster’ Pinctada margaritifera: a preliminary bi‐local case study in French Polynesia

In French Polynesia, the aquaculture of Pinctada margaritifera is carried out in numerous grow‐out sites, located over three archipelagos (Gambier, Society and Tuamotu). To evaluate the impact of macro‐geographical effects of these growing sites on pearl quality traits, five hatcheries produced families were used as homogeneous donor oysters in an experimental graft. The molluscs were then reared in two commercial locations: Tahaa Island (Society) and Rangiroa atoll (Tuamotu). At harvest, eight pearl quality traits were recorded and compared: surface defects, lustre, grade, circles, shape categories, darkness level, body and secondary colour and visual colour categories. Overall inter‐site comparison revealed that: (1) all traits were affected by grow‐out location except for lustre and round shape, and (2) a higher mean rate of valuable pearls was produced in Rangiroa. Indeed, for pearl grade, Rangiroa showed twice as many A‐B and less reject samples than Tahaa. This was related to the number of surface defects (grade component): in Rangiroa, twice as many pearls had no defects and less pearls had up to 10 defects. Concerning pearl shape, more circled and baroque pearls were found in Tahaa (+10%). For colour variation, 10% more pearls have an attractive green overtone in Rangiroa than in Tahaa, where more grey body colour were harvested. Lustre does not seem to be affected by these two culture site (except at a family scale). This is the first time P. margaritifera donor family have been shown to vary in the quality of pearls they produce depending on their grow‐out location.     

Contribution of donor and host oysters to the cultured pearl colour in Pinctada martensii

Cultured free round pearls are produced by implanting a spherical nucleus with a small piece of nacre‐secreting mantle graft from a donor oyster into the gonad of a recipient oyster (host). To examine the possible contribution of host and donor oysters to the colouration of the harvested pearls, the CSE‐1 Imaging and Color‐Measuring System were used to quantitatively measure the L*a*b* values of donor and host shells and the produced pearls in Pinctada martensii. Results showed that the colour of pearls had significant positive correlation (r = 0.1–0.22, P = 0.00) with that of donors, but had no correlation with that of host oysters, thus convincingly confirmed the contribution of nacre colour of donor to the realization of pearl quality of colour. To further clarify the relationship between the donor and the pearl colour, the donors from pearls of good and poor colour quality were further compared and the results demonstrated the significant difference in L* values (P < 0.05) and insignificant difference in a* and b* values, suggesting the necessity of selecting donors with bright and lustrous nacre in pearl production.     

Imidazole‐dicarboxylic Acid Nickel Complex as a Novel Grafting Agent for Pearl Oyster,Pinctada martensii

In the pearl cultivation industry, the implantation of the nucleus is a critical step in the production process. The surgical insertion of the nucleus and the mantle results in a wound and stress that affect not only the survival chances of the pearl oysters but also the quality of the pearls produced. This study proposes a synthesized imidazole‐dicarboxylic acid nickel complex that may hold potential in assisting the wound healing and nacre formation processes. The acute toxicity of the complex to pearl oysters, Pinctada martensii, was examined in a dose–response study over a 90‐h exposure. The safe concentrations of the nickel complex determined using probit analysis were 0.916 mg/L and 0.471 mg/L for 72‐h and 96‐h treatments, respectively. The reliability of the regression was verified using the chi‐square test. The alkaline phosphatase (ALP) activity in different regions of the treated and control oysters, along with relevant blood biochemical indices, were measured over a 90‐d period to predict the potential effects of the nickel complex on wound healing and nacre formation. A significant increase in ALP activity in the mantle, together with elevated blood levels of Mg (II) and globulin, and reduced blood levels of Zn (II) and blood urea nitrogen were observed. These results indicate that the prepared nickel complex may hold promise in helping wound healing post mantle implantation, promote the secretion of nacre, and enhance the immunity of the oysters, thus ultimately improving the yield and quality of the pearls.     

Pearl grafting: Tracking the biological origin of nuclei by straightforward immunological methods

French Polynesia is renowned for the production of Tahitian black pearl. These gems are obtained by grafting a nucleus into the gonad of a receiving oyster together with a graft, i.e. a small section of mantle tissue of a donor oyster. This procedure initiates the formation of a pearl sack around the nucleus, and subsequently, the deposition of concentric layers of nacre. The nucleus plays a key‐role in pearl formation and its characteristics influence markedly the quality of the final product. As it is manufactured from mollusc shells, it contains a small percentage of organics. In the present paper, we used a set of biochemical techniques to characterize and compare the organic matrices from two types of nuclei that are currently used in French Polynesia: that from the freshwater mussel Amblema sp., and that from the pearl oyster Pinctada sp. To this end, we extracted the matrices from nuclei and performed FT‐IR, monodimensional electrophoresis, and enzyme‐linked immuno‐sorbent assay (ELISA). Our data show that the matrix associated with Amblema nuclei has a very different biochemical signature from that of Pinctada nuclei, a fact that may explain the improved tolerance of grafted oysters to nuclei of Pinctada origin. In the absence of complex physical methods of investigation, simple immunological techniques and FT‐IR performed on the extracted organic matrix are extremely reliable and effective for discriminating nuclei from these two sources. We assert that such techniques can be used as a diagnostic test to track unambiguously the biological origin of nuclei to avoid fraud.     

How cultured pearls acquire their colour

Round nucleated pearls are produced through a surgical operation, where a round nucleus and a mantle tissue ‘saibo’ from donor oyster are inserted into the gonad of the host oyster. The epithelial cells in the mantle tissue proliferate around the nucleus, and thus, the pearl sac is formed. Pearl sac secrets nacre and forms a pearl. The quality and economic value of pearls are assessed by pearl features such as colour, brightness, lustre and shape. Among all these features, colour has been reported as an important economic indicator and has been widely studied by researchers. Generally, pearl colour is affected by the donor oyster which is determined genetically and biological pigments (melanin and carotenoid). Organic matrices, metal ions and other factors have also been reported to influence the colour of a cultured pearl. Recently, multi‐omics methods have been used to study the colour formation of pearl, and some key genes and signal pathways related to the colour formation of pearls have been identified. Nevertheless, the specific mechanism of pearl formation needs further research. The review combines both fresh and sea water pearls focusing on Hyriopsis cumingii and pearl oysters to provide a general overview and understanding for pearl colour formation.     

Freshwater nucleated pearl quality is influenced by host mussel growth traits in Hyriopsis cumingii

Hyriopsis cumingii is one of the most important freshwater pearl mussels in China. Recently, this species can produce freshwater nucleated pearls of high quality. Here, we investigated whether nucleated pearl quality is influenced by the growth traits of the host mussel or other factors like cultivation period. We implanted host mussels with a spherical nucleus consisting of a small piece of mantle tissue from donor mussels. After 24 and 36 months of culture, host mussel growth traits including body weight and shell length, height, width and weight were recorded. These factors were then correlated with the quality traits of the pearls they produced, such as nacre thickness, size, weight, lustre and colour. Results indicated pearls obtained at 36 months after seeding were significantly larger in terms of nacre thickness, size and weight compared to those harvested at 24 months. In particular, nacre thickness (r = 0.33–0.48, P = 0.00), pearl size (r = 0.39–0.43, P = 0.00) and pearl weight (r = 0.35–0.47, P = 0.00) were showed to be significantly correlated with host mussels shell length, body weight and shell weight at 24 or 36 months. Larger and heavier host mussels tended to produce bigger pearls. In contrast, host mussels did not affect pearl colour. Cumulatively, our results suggest that longer culturing times and a larger host mussel may help produce better quality nucleated pearl. This information can help guide selective breeding programs designed to improve pearl quality produced by H. cumingii.     

Non‐spherical pearl layers in the Polynesian ‘black‐lipped’ Pinctada margaritifera: The non‐nacreous deposits compared to microstructure of the shell growing edge

Cultivated pearls frequently exhibit morphological irregularities making obvious that mineral deposition was irregularly distributed onto nucleus surface. Taking advantage of experimental cultivations with short durations (from 10 days to few months), these irregular deposits predating occurrence of the nacre were investigated in Polynesian pearls by biochemical characterizations and a series of physical methods. Diversity in the resulting data suggests that various in‐depth alterations of the biomineralization mechanism may have occurred during the grafting process, leading to diversity in the biochemical pathways to nacreous deposition. This allows a precise discussion of current views about pearl formation. The “reversed shell theory” is formally disproved through point to point comparison with development of the shell growing edge. Similarity of pearl formation with “regeneration” or “shell repair” is also discussed, emphasizing the differences between these concepts.     

Influence of nacre deposition rate on cultured pearl grade and colour in the black-lipped pearl oyster Pinctada margaritifera using farmed donor families

Cultured pearl nacre deposition rate in Pinctada margaritifera oysters was studied to examine its influence on Tahitian classification grade and colour and to further our understanding of using selected donor oysters produced in a hatchery system for the improvement of these traits. A large-scale grafting experiment using 150 farmed donor oysters was realised in Rangiroa atoll (French Polynesia). After 18 months of culture, 874 pearls were harvested and 7 pearls quality traits were recorded: (1) nacre deposition rate assessed by nacre thickness and weight, (2) classification grade, lustre and surface defects, and (3) colour assessed by pearl colour darkness level and visual colour categories. Nacre deposition rate significantly influence pearl grade, surface defects, darkness level and colour categories, but not the lustre. Grade A pearls had the thickest and heaviest nacre on average, compared with grade D pearls and rejects. In addition, pearls without any surface defect had thicker and heavier nacre. Pearls with low darkness were the thinnest and the lightest. Pearls of aubergine–peacock secondary colours were the thickest and the heaviest. These results highlight the importance of rapid nacre deposition rate for increasing the production of grade A pearls, with less surface defects and dominance of a black tone colour. Fast nacre deposition may, however, have a negative impact on other pearl quality traits, such as pearl lustre. This study demonstrates the importance of examining relationships between pearl quality traits in a way that provides solid information that can be directly used for breeding and genetic selection programmes.     

Is pearl colour produced from Pinctada margaritifera predictable through shell phenotypes and rearing environments selections?

The black‐lipped pearl oyster, Pinctada margaritifera, is the most important farmed mollusc species in French Polynesia. Donor oyster selection among wild P. margaritifera individuals, chosen according to their inner shell colour, makes it possible to obtain the broadest range of cultured pearl colours of any species. This study demonstrates the relative influence of using black [B] or red [R] outer shell phenotypes, combined with green [G] or yellow [Y] inner shell phenotypes, on pearl darkness level, colour categories and lustre. A large scale grafting experiment was designed and carried out over five grow‐out locations, covering three archipelagos: Tuamotus, Society and Gambier. Results revealed that the [B + G] phenotypes may be used as donors to produce dark green pearl, which suit the demands of the Asian market; whereas, phenotypes incorporating [R] and/or [Y] phenotypes may be used to obtain multicolour pearls of medium/light darkness, which suit the demands of the European market. From an environmental point of view, the 1) [B] phenotype showed no significant variation for light and other pearl colour production, and 2) [Y] phenotype produced both the same rate of pearl darkness level and green colour pearls whatever the grow‐out location. A classification tree model was built to predict, according to shell phenotype and culture location, the colour and darkness level of harvested pearls. Lustre was shown to be more influenced by the environment than by phenotype. These results should be taken into account in pearl farm production management and in selective breeding programmes.     

Fouling and predation; how do they affect growth and survival of the blacklip pearl oyster, Pinctada margaritifera, during nursery culture?

This paper reports on 5 experiments conducted to assess the effect of cleaning regime and predation on growth and survival of blacklip pearl oyster (Pinctada margaritifera) juveniles in north Queensland, Australia. P. margaritifera juveniles with a mean (±SE) dorso-ventral shell height (DVH) of 4.5 ± 0.1 mm were placed into plastic mesh trays and cleaned either every 4 or 8 weeks or left uncleaned for 16 weeks. Cleaning regime had a significant effect on growth and survival (P < 0.005). Lowest DVH (16.2 ± 1.0) was shown by oysters in uncleaned trays during 16 weeks compared to oysters in cleaned trays; however, there was no significant difference in DVH between oysters held in trays cleaned every 4 (19.4 ± 1.2) or 8 weeks (21.2 ± 0.8). In contrast lowest survival was shown by oysters held in trays that were cleaned every 4 weeks (30 ± 5%), but no differences were noted between oysters cleaned every 8 weeks (63 ± 4%) and oysters that were left uncleaned for 16 weeks (75 ± 8%). Predators of P. margaritifera in northern Australia included crabs, stomatopods, flatworms, gastropods and fish. The stomatopod, Gonodactylus falcatus, was the most destructive predator with individuals consuming in excess of 20 juvenile pearl oysters per week. The leather jacket, Paramonocanthus japonicus, did not kill pearl oysters, but trimmed the margin of oysters shells significantly reducing DVH when compared to control groups cultured without fish. Removing predators monthly had a significant effect on growth of pearl oysters compared to oysters in non-inspected trays; however monthly inspection of culture trays did not significantly improve oyster survival. This revised version was published online in August 2006 with corrections to the Cover Date.     

Expression profiles of nine biomineralization genes and their relationship with pearl nacre thickness in the pearl oyster,Pinctada fucata martensii Dunker

Pinctada fucata martensii is an ideal animal for study of biomineralization. Although dozens of genes have been identified, the molecular mechanism of biomineralization remains still unclear. The purpose here was to discover the expression profiles of nine biomineralization genes in related tissues: mantle edge (ME), mantle centre (MC) and pearl sac (PS), and to explore the relationships between expression level and nacre thickness. The expression levels of seven genes (ACCBP, aspein, CaM, EFCBP, KRMP, nacrein and prismalin‐14) showed no significant difference between ME and MC, but were significantly higher than those in PS. The expression level of N19 was the same in ME and PS, significantly higher than that in MC. However, msi60 in ME, MC and PS showed no significant differences in expression level. In addition, the correlations of expression levels between ME, MC and PS were highly significant, indicating that there are some similar function and mechanism among the three tissues. Also the nine genes had significant correlation of expression levels between one another. Furthermore, no other genes from oysters which produced pearls of different grades showed significant difference in the expression level, except CaM in MC, which had significantly higher expression level in the medium pearl grade than in the plain nucleus grade. Moreover, the expression level of msi60 in MC of host oyster was significantly correlated with produced pearl layer thickness, suggesting that msi60 might play an important role in pearl formation and host oyster may also participate in the development of pearl.     

Factors influencing half‐pearl (mabe) production and quality in the red abalone Haliotis rufescens

The factors influencing half‐pearl (mabe) production in the red abalone Haliotis rufescens were investigated, as a strategy to optimize the technology for the development of a pearling industry in Chile. The effects of abalone size (small, large), implant position (dorsal, central, ventral and their combinations), and number of nuclei (one, two, three) were analysed on the quantity, thickness of the nacre layer and quality (shape, colour, lustre, defects) of the mabe produced. The size of abalone did not exert a clear influence on the quantity and quality of mabe, but the position on the shell (firstly) and number of nuclei (secondly) did. More mabe, with thicker nacre, brighter lustre and fewer defects occurred in small or large abalone implanted with only one nucleus on dorsal position. In contrast, small and large abalone implanted in the ventral position formed less mabe, with thinner nacre, poor lustre and more defects. This likely occurs because mantle tissue folds at the ventral position and does not always cover the nucleus, leaving it partially or totally exposed. Most of the pearls were categories AA in small abalone and A in large abalone; only three AAA gems were produced in large abalone with one nucleus on dorsal position or with three nuclei (DCV). Based on these results, we recommend implanting abalone at 60–100 mm shell length, with only one nucleus on the dorsal position.     

Effects of glycopeptides on development,growth and non‐specific immunity of pearl oyster Pinctada fucata (Gould)

The effects of glycopeptides, prepared from pearl oyster Pinctada fucata, on embryonic development, larval and juvenile growth and adult non‐specific immunity of P. fucata were investigated in this study. Glycopeptides had a pronounced stimulatory effect on embryonic development and larval and juvenile growth of P. fucata. enhancing with increased glycopeptide concentrations. All of haemocytes, phagocytosis, aggregation, serum microbiostatic activity and bacteriocidal activity all showed significant increases after 60‐day feeding, relative to unfed controls. The major conclusion is that glycopeptides had a pronounced stimulatory effect on the non‐specific immunity of pearl oysters.     

Hot water immersion lowers survival,shell growth rate and lysosomal membrane stability of oysters Crassostrea virginica (Gmelin)

This study compares the effect of two anti‐fouling treatments, hot water immersion (15 s at 60°C) and air drying (72 h) on the physiological status of the Eastern oyster Crassostrea virginica. The negative impact of hot water immersion was greater than that of air drying, but varied depending on the initial size of the oysters (40 vs. 60 mm shell height) and the time of the year (June vs. August). Groups treated with hot water exhibited a higher proportion of haemocytes with destabilized lysosomal membranes (HDLM; 47.5 ± 3.1%) than those exposed to air drying (37.5 ± 2.9%). This suggests that the oyster immunocompetency may be lowered by hot water immersion. Overall, the large oysters had lower HDLM values (32.9 ± 3.5%) than the small individuals in June (45.7 ± 2.8%) but similar values in August (46.6 ± 3.5%). Small oysters subjected to hot water immersion in June exhibited a 50% reduction in shell growth and a 50% mortality rate after one month. Our results indicate that air drying is more suitable than hot water immersion as an anti‐fouling treatment for <45 mm oysters.