Hypoxia in aquatic invertebrates: Occurrence and phenotypic and molecular responses.

Global deoxygenation in aquatic systems is an increasing environmental problem, and substantial oxygen loss has been reported. Aquatic animals have been continuously exposed to hypoxic environments, so-called "dead zones," in which severe die-offs among organisms are driven by low-oxygen events. Multiple studies of hypoxia exposure have focused on in vivo endpoints, metabolism, oxidative stress, and immune responses in aquatic invertebrates such as molluscs, crustaceans, echinoderms, and cnidarians. They have shown that acute and chronic exposure to hypoxia induces significant decreases in locomotion, respiration, feeding, growth, and reproduction rates. Also, several studies have examined the molecular responses of aquatic invertebrates, such as anaerobic metabolism, reactive oxygen species induction, increased antioxidant enzymes, immune response mechanisms, regulation of hypoxia-inducible factor 1-alpha (HIF-1α) genes, and differently expressed hemoglobin/hemocyanin. The genetic basis of those molecular responses involves HIF-1α pathway genes, which are highly expressed in hypoxic conditions. However, the identification of HIF-1α-related genes and understanding of their applications in some aquatic invertebrates remain inadequate. Also, some species of crustaceans, rotifers, sponges, and ctenophores that lack HIF-1α are thought to have alternative defense mechanisms to cope with hypoxia, but those factors are still unclear. This review covers the formation of hypoxia in aquatic environments and the various adverse effects of hypoxia on aquatic invertebrates. The limitations of current hypoxia research and genetic information about the HIF-1α pathway are also discussed. Finally, this paper explains the underlying processes of the hypoxia response and presents an integrated program for research about the molecular mechanisms of hypoxic stresses in aquatic invertebrates.

Early Larval Development and Annual Gametogenesis of the Brooding Oyster Ostrea circumpicta (Pilsbry, 1904) in the Shallow Subtidal Benthic Ecosystem in Jeju Island, Off the South Coast of Korea.

The subtropical brooding oyster Ostrea (= Striostrea) circumpicta (Pilsbry, 1904) occurs at high density in the shallow, subtidal, rocky bottom in Jeju Island, off the south coast of Korea, where the sea surface temperature and salinity varies annually from 13 to 25°C and 30 to 33 ppt, respectively. In this study, the annual gametogenesis and early larval development of O. circumpicta was examined, using histology and scanning electron microscopy (SEM). Histology indicated that the females and males initiated gonial mitosis in September, shortly after sexual resting in August. In December, ripe eggs first appeared in the follicles, and most of the females exhibited fully mature oocytes in May, as the water temperature reached 17 to 18°C. Spawning females were dominant in June and July, when the trochophore and strait- hinged veliger larvae were also identified in the branchial chambers, their size ranging from 111 to 130 µm and 135-205 µm in diameter, respectively. The veliger larvae in the brooding chamber exhibited a well- developed velum and digestive tract, suggesting that the larvae are engaged in feeding in the branchial chamber. Unlike other marine bivalves in temperate coastal ecosystems, O. circumpicta has a long period of gonad maturation and a short resting phase. It has been believed that such a long period of reproductive maturation is associated with a low level of food in the environment and the comparatively large size of the oocytes, which may require a relatively longer time to accumulate necessary nutrients to produce large eggs in a food-poor environment.

High parasite burden increases the surfacing and mortality of the manila clam (Ruditapes philippinarum) in intertidal sandy mudflats on the west coast of Korea during hot summer.

BACKGROUND: Over the past few decades, mass mortality events of Manila clams have been reported from several tidal flats on the west coast of Korea during hot summers. During such mortality events, once clams simultaneously surface, they fail to re-burrow, perishing within a week. The present study aimed to identify the possible causes of the mass mortality of this clam species by investigating the Perkinsus olseni parasite burden and immune parameters of surfaced clams (SC) and normal buried clams (NBCs) when sea water or sediment temperature in the study area varied from 25 °C to 34 °C from late July through mid-August 2015. RESULTS: We collected 2 groups of clams distributed within a 10-m2 area when a summer clam mortality event occurred around Seonyu-do Island on the west coast of Korea in 2015. The clams were collected 2 days after they surfaced on the sediment and still looked healthy without any gaping. The clams were transported to the laboratory, and we compared P. olseni infection intensity and cell-mediated hemocyte parameters between the NBCs and SCs. SCs showed significantly higher levels of P. olseni burden, lower condition index, and lower levels of cell-mediated immune functions than those of NBCs. CONCLUSIONS: Our study suggests that high P. olseni infection weakens Manila clams' resistance against thermal stress, causing them to surface. We surmise that the summer mass mortality of Manila clams on the west coast of Korea is caused by the combined effects of high P. olseni infection levels and abnormally high water temperature stress.

Purification and Characterization of Vitellin from the Egg of the Suminoe Oyster Crassostrea ariakensis and Cross-Reactivity of Anti-vitellin Antibody with Other Marine Invertebrate Egg Proteins.

A polyclonal antibody specific to an egg protein of Suminoe oyster Crassostrea ariakensis was previously developed in our laboratory to assess the reproductive life cycle of the oyster. The present study was undertaken to investigate vitellin of C. ariakensis (CAVt). Vitellin is an essential component of egg proteins in marine invertebrates as it provides energy and nutrients to the embryo and larvae. CAVt was purified from eggs of the oyster using ammonium sulfate precipitation followed by affinity chromatography with Concanavalin A-agarose. Native polyacrylamide gel electrophoresis (PAGE) and sodium dodecyl sulfate PAGE showed that CAVt is a high molecular weight [532 kiloDaltons (kDa)] protein, with multiple subunits. Similar to other vitellin proteins, it is a phospholipoglycoprotein composed of phospholipids (12.06%), carbohydrates (mannose, 10.08% or glucose, 9.84%), and alkali-labile phosphates (4.16%). Affinity chromatography, enzyme-linked immunosorbent aasay (ELISA) and western blot analysis revealed that CAVt is only present in the ovary, and two subunits of CAVt (72 and 35 kDa) are believed to be incorporated from the hemolymph into the oocyte. The antibody specific to CAVt (anti-CAVt), raised in rabbit, strongly cross reacted with the egg proteins of oyster species and scallops, suggesting that the antigenic epitopes are highly conserved among species. Our results suggest that the anti-CAVt antibody can be used to develop a tool similar to ELISA or western blotting for investigation of the effect of microorganisms on reproduction as well as the effect of chemicals on the endocrine system in C. ariakensis.

Popular edible seaweed, Gelidium amansii prevents against diet-induced obesity.

The popular edible seaweed, Gelidium amansii is broadly used as food worldwide. To determine whether G. amansii extract (GAE) has protective effects on obesity, mice fed a high-fat diet (HFD) treated with GAE (1 and 3 %) were studied. After 12 weeks of GAE treatment, body weight was greatly decreased in mice fed a high-fat diet. This effect could be due to decreased adipogenesis, as evidenced by the fact that GAE suppressed adipogenic gene expression in adipocytes. In addition, blood glucose and serum insulin levels were reduced by GAE treatment in mice fed a high-fat diet, suggesting improvement in glucose metabolism. GAE supplementation also led to a significant decrease in total cholesterol and triglyceride levels. These data are further confirmed by H&E staining. Our findings indicate that Gelidium amansii prevents against the development of diet-induced obesity, and further implicate that GAE supplementation could be the therapeutical option for treatment of metabolic disorder such as obesity.

Molecular and histological identification of Marteilioides infection in Suminoe Oyster Crassostrea ariakensis, Manila Clam Ruditapes philippinarum and Pacific Oyster Crassostrea gigas on the south coast of Korea.

The oyster ovarian parasite Marteilioides chungmuensis has been reported from Korea and Japan, damaging the oyster industries. Recently, Marteilioides-like organisms have been identified in other commercially important marine bivalves. In this study, we surveyed Marteilioides infection in the Manila clam Ruditapes philippinarum, Suminoe oyster Crassostrea ariakensis, and Pacific oyster Crassostrea gigas, using histology and Marteilioides-specific small subunit (SSU) rDNA PCR. The SSU rDNA sequence of M. chungmuensis (1716 bp) isolated from C. gigas in Tongyoung bay was 99.9% similar to that of M. chungmuensis reported in Japan. Inclusions of multi-nucleated bodies in the oocytes, typical of Marteilioides infection, were identified for the first time in Suminoe oysters. The SSU rDNA sequence of a Marteilioides-like organism isolated from Suminoe oysters was 99.9% similar to that of M. chungmuensis. Marteilioides sp. was also observed from 7 Manila clams of 1840 individuals examined, and the DNA sequences of which were 98.2% similar to the known sequence of M. chungmuensis. Unlike Marteilioides infection of Pacific oysters, no remarkable pathological symptoms, such as large multiple lumps on the mantle, were observed in infected Suminoe oysters or Manila clams. Distribution of the infected Manila clams, Suminoe oysters and Pacific oysters was limited to small bays on the south coast, suggesting that the southern coast is the enzootic area of Marteilioides infection.

Cloning and localization of MCdef, a defensin from Manila clams (Ruditapes philippinarum).

A defensin-like peptide was previously detected in hemocytes of Manila clams (Ruditapes philippinarum). In the current study, we cloned and characterized this defensin, designated MCdef. Cloning produced a full-length gene sequence of 201 bp predicted to encode a 66-amino-acid precursor protein maturing to a 44-amino-acid residue. Amino acid sequence analysis showed that MCdef is similar to defensins from marine mollusks and ticks. Phylogenetic analysis suggested that MCdef is closely related to defensins from Mytilus galloprovincialis (Mediterranean mussel) and Crassostrea gigas (Pacific cupped oyster). The three-dimensional structure of MCdef was modeled using the solution structure of C. gigas defensin as a template. With the exception of three variable loop areas, the modeled structure of MCdef was identical to that of C. gigas defensin. MCdef antiserum was raised against a synthetic MCdef peptide and verified by Western blotting using recombinant MCdef. RT-PCR analysis demonstrated high levels of MCdef mRNA in hemocytes and adductor, foot, gill, mantle, palp, and siphon tissues of Vibrio tapetis-infected Manila clams, whereas in V. tapetis-uninfected Manila clams, the level of MCdef mRNA was low in adductor, palp, and siphon tissues and even lower in the other tested tissues. Immunohistochemical analysis revealed high MCdef expression was detected in the gill, the mantle, and the digestive tubules of the diverticulum of V. tapetis-infected Manila clams. Minimum inhibitory concentration (MIC) of the purified rMCdef was determined. MCdef showed highest activity against Streptococcus iniae and Staphylococcus aureus.