A remarkably diverse and well-organized virus community in a filter-feeding oyster.

BACKGROUND: Viruses play critical roles in the marine environment because of their interactions with an extremely broad range of potential hosts. Many studies of viruses in seawater have been published, but viruses that inhabit marine animals have been largely neglected. Oysters are keystone species in coastal ecosystems, yet as filter-feeding bivalves with very large roosting numbers and species co-habitation, it is not clear what role they play in marine virus transmission and coastal microbiome regulation. RESULTS: Here, we report a Dataset of Oyster Virome (DOV) that contains 728,784 nonredundant viral operational taxonomic unit contigs (≥ 800 bp) and 3473 high-quality viral genomes, enabling the first comprehensive overview of both DNA and RNA viral communities in the oyster Crassostrea hongkongensis. We discovered tremendous diversity among novel viruses that inhabit this oyster using multiple approaches, including reads recruitment, viral operational taxonomic units, and high-quality virus genomes. Our results show that these viruses are very different from viruses in the oceans or other habitats. In particular, the high diversity of novel circoviruses that we found in the oysters indicates that oysters may be potential hotspots for circoviruses. Notably, the viruses that were enriched in oysters are not random but are well-organized communities that can respond to changes in the health state of the host and the external environment at both compositional and functional levels. CONCLUSIONS: In this study, we generated a first "knowledge landscape" of the oyster virome, which has increased the number of known oyster-related viruses by tens of thousands. Our results suggest that oysters provide a unique habitat that is different from that of seawater, and highlight the importance of filter-feeding bivalves for marine virus exploration as well as their essential but still invisible roles in regulating marine ecosystems. Video Abstract.

Hypoxia-induced oxidative stress and transcriptome changes in the mud crab (Scylla paramamosain).

Mud crab (Scylla paramamosain) is an economically important cultured species in China. Hypoxia is a major environmental stressor during mud crab culture. In the present study, we investigated the oxidative stress and transcriptome changes in the gills of mud crab after intermediate hypoxia stress with dissolved oxygen (DO) 3.0 ± 0.2 mg/L (named as "DO3") and acute hypoxia stress with DO 1.0 ± 0.2 mg/L (named as "DO1") for 0, 3, 6, 12 and 24 h. The superoxide dismutase (SOD) activity of DO1 increased significantly at 3, 6 and 24 h after hypoxia stress, while SOD activity of DO3 increased significantly at 6 and 24 h. The total antioxidant capacity (T-AOC) increased significantly at 6, 12 and 24 h after hypoxia stress. The malondialdehyde (MDA) concentration of DO1 increased significantly at 6, 12 and 24 h after hypoxia stress, while MDA concentration of DO3 only increased significantly at 6 h. The lactate dehydrogenase (LDH) activity of DO1 increased significantly at 3, 6, 12 and 24 h after hypoxia stress, while LDH activity of DO3 increased significantly at 12 and 24 h. Transcriptomic analysis was conducted at 24 h of gill tissues after hypoxia stress. A total of 1052 differentially expressed genes (DEGs) were obtained, including 394 DEGs between DO1 and DO3, 481 DEGs between DO1 and control group, 177 DEGs between DO3 and control group. DEGs were enriched in the pathways related to metabolism, immune functions, ion transport, and signal transduction. Transcriptional analysis showed that glycolysis and tricarboxylic acid cycle genes were the key factors in regulating the adaptation of mud crab to hypoxia stress.

First report of black-heart disease in Kumamoto oyster Crassostrea sikamea spat caused by Polydora lingshuiensis in China.

A black-heart disease caused by polydorid infestation is reported for the first time in Kumamoto oyster Crassostrea sikamea Amemiya, 1928 spat in a pond at Beihai city, Guangxi province, China, with a prevalence of 100% and a cumulative mortality rate of 50% within 2 mo. In heavily infected oyster spat, blisters extended toward the center of the inner shell surface, around the adductor muscle scar area to form a large black area occupying approximately 50% of the area of the inner shell surface. Morphological analysis identified the pathogen as Polydora lingshuiensis Ye et al., 2015, which was reconfirmed by comparison of its corresponding 18S rRNA and mitochondrial CO1 gene sequences with those in the GenBank database. The mean abundance of mud blisters was significantly higher in live spat than in dead spat, suggesting that P. lingshuiensis preferentially infests live oyster spat. Additionally, P. lingshuiensis larvae were detected in the inlet near the dam, which suggests that the source of P. lingshuiensis larvae infecting the spat may be larvae entering the ponds through the water current from the sea.

Supplementary studies on Henneguya doneci Schulman, 1962 (Myxozoa: Myxosporea) infecting the gill filaments of Carassius auratus gibelio (Bloch) in China: histologic, ultrastructural, and molecular data.

Henneguya doneci Schulman, 1962 was collected from the gill filaments of Carassius auratus gibelio (Bloch) in Hubei Province, China. The plasmodia located on the surface of the gill arches deformed the neighboring gill filaments. The size of the plasmodia ranged from 0.6 to 4.5 mm in diameter in different months. The myxospores in the plasmodia measured 10.1 (9.2-11.5) µm long × 8.0 (7.5-8.5) µm wide × 7.5 (7-8) µm thick, with two equal capsules at 4.7 (4-5.5) µm long × 3.3 (2.5-4) µm wide, and two caudal processes 32.7 (24-38.5) µm long, respectively. Polar filaments were coiled 5-6 turns. Ultrastructural observation of the plasmodia showing the capsulogenesis of H. doneci is described briefly. The external tubule initially invaginated into the polar capsule. The rudimentary polar filaments were observed to undergo a series of considerable modification, finally developing into mature polar filaments. Molecular analysis demonstrated that although the myxosporean species were collected from different tissues of hosts in various geographic locations, they clustered with the Cyprinidae-infecting myxosporean species in the phylogenetic tree.

Prevention of Ichthyophthirius multifiliis infestation in goldfish (Carassius auratus) by potassium ferrate(VI) treatment.

Ichthyophthirius multifiliis is an important freshwater teleost pathogen that often leads to significant economic losses to the aquaculture industry. The purpose of this study was to assess the acute toxicity of potassium ferrate(VI) to I. multifiliis theront and the concentration needed to prevent I. multifiliis infestation in goldfish, Carassius auratus. Five hundred theronts were exposed to concentrations of potassium ferrate(VI) in each well of a 96-well microtiter plate and observed for 4h to determine the acute toxicity. Results showed that the exposure of I. multifiliis theronts to potassium ferrate(VI) at concentrations of 4.80mg/L or more resulted in 100% mortality by 4h; the LC(50) value was estimated to be 1.71mg/L. Aqueous static renewal 96-h bioassays were carried out to determine the acute toxicity of potassium ferrate(VI) to goldfish. The LC(50) value for potassium ferrate(VI) in goldfish was 42.51mg/L. Goldfish were exposed to 4000 theronts/fish in aerated tap water (a dose previously shown to result in consistent infestation) and treated with a single dose of potassium ferrate(VI) after 30min contact with theronts. Infection level and prevalence were recorded everyday after exposure. The results revealed that potassium ferrate(VI) at the 4.80mg/L or more concentrations can significantly reduce not only the number of trophonts on the fin of goldfish on day 3 (P<0.05), but also the prevalence of ichthyophthiriasis (P<0.05). Potassium ferrate(VI) at a concentration of 4.80mg/L was considered to be the lowest effective dose to prevent infestation of I. multifiliis in goldfish.