Search

Saccostrea are common oysters of Indo-Pacific rocky shores. However, the taxonomy of this genus is confused. In this study, we report the complete sequence of the mitochondrial genome of S. kegaki. The complete S. kegaki mitogenome is 16,260 bp in size. The 16S rRNA gene is split into two parts by a large fragment of genes. Neighbour-joining molecular phylogenetic analysis appears to support the current taxonomic framework of Ostreidae. We expect that the complete mitogenome of S. kegaki will provide a useful resource for studies on the molecular phylogeny and conservation genetics of Bivalvia.

Two Saccostrea oysters (S. glomerata and S. cucullata) are considered to be protandric hermaphrodites based on indirect observations. However, no other information is available on the sexual systems of Saccostrea oysters and the potential for bi-directional sex change has not previously been reported in them. Therefore, the present study was undertaken to investigate the sexual systems of S. kegaki and S. mordax in western Japan, utilizing field sampling and in situ experiments. In the field sampling, shell length (SL) and sex were determined during the reproductive season of these oysters (June-October) in 2012. Then, sex change was directly observed between 2013 and 2016 by determining the sex of individuals through gonadal biopsy, re-attaching the individuals in the field, collecting the survivors in the following year, and determining their sex by dissection. In both species, the proportion of males decreased with increasing SL, although the SL of males and females greatly overlapped. Direct observations showed that both male-to-female and female-to-male sex change occurred. Thus, our results indicated that both Saccostrea species tend to be protandric, but have the ability to change sex in both directions under natural conditions. [ABSTRACT FROM AUTHOR]

Abstract: We first characterized the morphology and immune-related activities of hemocytes in the subtropical oysters Saccostrea kegaki, Ostrea circumpicta, and Hyotissa hyotis using light microscopy and flow cytometry. Hemocytes of these three oyster species were classified into three main types: 1) granulocytes containing numerous granules in the cytoplasm, 2) hyalinocytes with no or fewer granules, and 3) blast-like cells characterized by the smallest size and very thin cytoplasm. The percentage of each hemocyte population was similar in all species; hyalinocytes were the most abundant cell in the hemolymph accounting for more than 59%, followed by granulocytes (23–31%) and blast-like cells (3–5%). The size of granulocytes of S. kegaki was smaller (P < 0.05) than those of O. circumpicta and H. hyotis. Light microscopy also allowed the description of vacuolated cells characterized by large vacuoles in the cytoplasm. Flow cytometry analysis confirmed that the granulocytes of the three oyster species were the major hemocytes engaged in cellular defense with the largest lysosome content, and the most active phagocytosis activity and oxidative activity, as was previously reported in several marine bivalves. Phagocytic activity was the lowest in S. kegaki hemocytes, and PMA-stimulated oxidative activity was the lowest in H. hyotis hemocytes. Our results provide the basic information of hemocytes population of three subtropical oysters for further investigations associated with various environmental disease stresses. [Copyright &y& Elsevier]

Abstract: During the molluscan evolution leading to the bivalves, the single dorsal shell was doubled. To elucidate the molecular developmental basis underlying this prominent morphological transition, we described the cell cleavage and expression patterns of three genes, brachyury, engrailed, and dpp in the Japanese spiny oyster Saccostrea kegaki, and examined the function of dpp in this species. The cleavage pattern of the S. kegaki embryo was nearly the same as the previously described pattern of other bivalve species, suggesting that the pattern itself is highly important for the establishment or the maintenance of the bivalve body plan. The expression pattern of a brachyury homolog in S. kegaki (SkBra) was similar to the pattern in gastopods even at the single cell level despite the deep divergence of gastropods and bivalves. Engrailed and dpp were previously found to be expressed around the shell anlagen in gastropods. Like that of gastropods, an engrailed homolog in S. kegaki (SkEn) was found to be expressed around the shell anlagen. However, the dpp homolog in S. kegaki (SkDpp) was expressed only in the cells along the dorsal midline. ZfBMP4 treatment experiments revealed the importance of dpp in establishing the characteristic shape of the bivalve shell anlagen. [Copyright &y& Elsevier]

Saccostrea are common oysters of Indo-Pacific rocky shores. However, the taxonomy of this genus is confused. In this study, we report the complete sequence of the mitochondrial genome of S. kegaki. The complete S. kegaki mitogenome is 16,260 bp in size. The 16S rRNA gene is split into two parts by a large fragment of genes. Neighbour-joining molecular phylogenetic analysis appears to support the current taxonomic framework of Ostreidae. We expect that the complete mitogenome of S. kegaki will provide a useful resource for studies on the molecular phylogeny and conservation genetics of Bivalvia. [ABSTRACT FROM AUTHOR]

Hemocyte parameters have been used as a proxy to characterize the health condition of marine bivalves, as the effects of external and internal stresses are reflected well in these parameters. Marine bivalve hemocyte functions are often depressed during the post-spawning period due to physiological stress and energy depletion. In this study, we analyzed temporal changes in hemocyte parameters of the oyster Saccostrea kegaki on Jeju Island, off the south coast of Korea, using flow cytometry. Total hemocyte count (THC), hemocyte types, hemocyte mortality, and phagocytosis capacity were analyzed. S. kegaki spawned during June and August, when the sea surface temperature increased from 18 to 23 °C. Most of the oysters were in the spent and resting phases from September to January. THC dropped dramatically from September to October, when most oysters completed spawning. Histology revealed that the residual eggs or sperm were actively resorbed through phagocytosis by hemocytes during the spent stage. Hemocyte mortality also showed its annual peak in October, possibly due to increased resorbing activities. The phagocytosis capacities of the granulocytes decreased dramatically from September to February. The level of energy reserves (glycogen) in post-spawning tissues was significantly lower than that prior to spawning. The low energy level reserve during the post-spawning period likely reduced the THC and immune capacities, as oysters may have been unable to acquire sufficient food from the ambient environment. • We analyzed seasonal changes in the hemocyte parameters of Saccostrea kegaki using flow cytometry. • Hemocyte functions in the post-spawning oyster were significantly depressed. • Hemocyte density and phagocytosis capacity declined and the hemocyte mortality increased. • It was believed that the low level of energy reserves during post-spawning period impaired the immune capacities. [ABSTRACT FROM AUTHOR]