Pearl oysters are found in the marine ecosystem. Due to their frequent exposure to microbial pathogens and environmental perturbations, they have developed a multifaceted innate immune system with differentially expressed immune‐related genes in response to a range of stress via vast coordinated immune reactions, such as immune recognition, signal transduction, synthesis of antimicrobial peptides, encapsulation and phagocytosis of the circulating haemocytes. The species are farmed to produce pearls. With the increasing adoption of aquaculture techniques in pearl production, there is the need to understand their culture environment and their immune response mechanisms. Multi‐omic studies have documented responses to nuclei insertion operation, diseases and various environmental conditions in Pinctada fucata martensii (Dunker, 1880), Pinctada maxima (Jameson, 1901), Pinctada margaritifera and Pteria penguin (Roding, 1798). Among the various stressors, nuclei insertion operation has been identified as a prevailing factor modifying gene expression. Other factors include life stage, air exposure, temperature, salinity, metals and organic contaminants. This manuscript details the response mechanism of these pearl oysters to the various stressors.

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Environmental microbiota plays a vital role in the intestinal microbiota of aquatic organisms. However, data concerning the association between the intestinal microbiota of pearl oyster Pinctada fucata martensii and the surrounding seawater are limited. The existing bacterial communities in pearl oyster intestine and surrounding water from two sites (D and H, within Liusha Bay in Guangdong, China) were investigated using 16S rRNA-based sequencing to explore the relationship among the two. D located in the inner bay, and H located in the open sea area outside bay. Results revealed the richness and diversity of pearl oyster intestinal microbiota to be less than those of the surrounding water, with 38 phyla and 272 genera observed as a result of the classifiable sequence. The microbiota compositions in the intestine and the surrounding water were diversified at the phylum and genus levels, with the sequencing data being statistically significant. However, the functional prediction of microbiota emphasized the overall similarity in the functional profile of the surrounding seawater and intestinal microbiomes. This profile was associated with metabolism of cofactors and vitamin, carbohydrates metabolism, amino acids metabolism, metabolism of terpenoids, and polyketides, metabolism of other amino acids, lipids metabolism, and energy metabolism. Seven common operational taxonomic units (OTUs), which belonged to phyla Tenericutes, Cyanobacteria, and Planctomycetes, were noted in the intestines of pearl oysters from two different sites. These OTUs may be affiliates to the core microbiome of pearl oyster. Significantly different bacterial taxa in the intestines of pearl oysters from two different sites were found at the phylum and genus levels. This finding suggested that the bacterial communities in pearl oyster intestines may exhibit some plasticity to adapt to changes in the surrounding water-cultured environment. This study generally offers constructive discoveries associated with pearl oyster intestinal microbiota and provides guidance for sustainable aquaculture.

Pearl oysters have high economic value. These species have gained much interest of research in growth, producing disease resistant and viable stocks for aquaculture facilities. Growth‐related genes, such as insulin‐related peptide receptor and the IGF system, and its components ERKs and MKK4 have been documented to influence growth in pearl oysters. Temperature, salinity, culture condition among other environmental factors were also reported to influence growth by influencing the physiological functions of pearl oysters such as clearance rate, oxygen consumption, respiration rate, excretion rate and expressed in parameters such as the scope of growth and the net growth efficiency. Genetic and artificial breeding techniques have been utilized in breeding programmes to improve growth and pearl quality in pearl oysters with mass selection and hybridization documented as efficient methods to improve growth traits. One problem observed during the growth of pearl oysters is the unsynchronized growth. Transcriptome and metabolomics studies have been undertaken to understand the underlying mechanism of this problem. The pearl oyster immune response to stress has also been reported to cause growth differences and needs to be thoroughly investigated to help produce viable stocks with synchronized growth for the aquaculture industry.

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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.