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Research into the contourite deposits in the Upper Meishan Formation of the southern Qiongdongnan Basin in South China Sea is weak; their characteristics, distribution and original geological conditions are not clear. Using geological and geophysical methods including seismic and drilling data, based on seismic reflection characteristics, geometrical configuration description, and wave impedance inversion, two types of contourite deposits are recognized. Contourite deposits have blurred boundaries between each deposit and disordered internal seismic reflections; They are mound‐shaped only in transverse section, and banded in the longitudinal direction. Type I contourite deposits are conical, with medium‐high amplitude, low‐continuity, low‐frequency mound‐shaped seismic facies, and subparallel‐chaotic reflections internally. These deposits are conical with sharp tops, the canal between mounds is V‐shaped and deep. The western wing is gentle and the eastern wing is steep, with the slope toe mostly between 10° and 20°, and width height ratio about 1–2. Type II contourite deposits are flat, exhibiting medium‐amplitude, medium‐continuity, low‐frequency mound‐shaped seismic facies, with subparallel weak reflections internally. Their mounds are flat with gently arced tops, with shallow canals between. The slope toe is between 5° and 10°, with a width height ratio of about 2–5. The wave impedance value of these contourite deposits is 4.6 kg/m3×m/s to 6.8 kg/m3×m/s, about 5.8 kg/m3×m/s on average, which is presumed to represent marly‐calcareous clastic sediments. The contourite deposits mainly develop beneath the slope break at the margin of the fault‐controlled platform in the Southern Uplift zone of the basin. In plane view, they are distributed approaching a west‐to‐east direction, and in section, lie in low‐lying areas near the faults at fault‐controlled terraces of the Southern Uplift zone, with a paleo‐current direction nearly west‐to‐east. The paleotectonic setting of the gentle monoclinic platform was favorable for the development of such contourite deposits. The intensification of the Mid‐Miocene deepest bottom current gave rise to the contourite‐forming currents around the Southern Uplift zone in the northern South China Sea, which flow from Hainan Island to the Xisha Trough in a nearly west‐to‐east direction leading to the contourite deposits developing in the late Mid‐Miocene transgressive environment, with multiple slow sea‐level fall cycles. [ABSTRACT FROM AUTHOR]

Group B Streptococcus (GBS) is the major causative pathogen of streptococcosis in tilapia, leading to a massive economic loss in the aquaculture industry worldwide. To date, the epidemiological characterization of tilapia GBS in China remains limited. Thus, the aim of this study was to determine the phenotype and genotype of GBS strains in tilapia in China from 2006 to 2020. A total of 678 GBS strains were isolated from farmed tilapia in the past decade. The CAMP reaction of GBS strains was investigated, and the cfb gene of the CAMP negative strains was sequenced and analysed. The genotypic distribution of these strains was determined by molecular serotyping, multi-locus sequence typing (MLST), virulence genotyping, and prophage genotyping. All GBS strains were divided into two serotypes (Ia and Ib), two sequence types (STs; ST7 and ST261), five virulence genotypes (V1–V5), and three prophage genotypes (P1-P3). A total of six genotypes (I–VI) were obtained based on serotyping, MLST, virulence genotyping, and prophage genotyping. Among them, strains in genotypes I, II, III, and IV were primarily prevalent from 2006 to 2015, and those in genotypes V and VI were the dominant epidemic strains from 2012 to 2020. Moreover, genotype VI strains showed a substantial population explosion from 2011 to 2020 and were mainly distributed in the geographically proximate provinces of Guangdong, Guangxi, Hainan, and Fujian. This is first report of tilapia Ia-GBS CAMP negative strains, and the strains were probably from the same hatchery. Genetic diversity in tilapia GBS strains has been high in China over the past decade, especially from 2010 to 2015. The genotypes of the tilapia GBS strains underwent obvious variation, and the prevalent strains in recent years have harboured more virulence-related genes. This study provides a better understanding of the epidemiology, pathogenesis, and evolutionary mechanism of tilapia GBS strains. • Six genotypes of tilapia GBS strains were found in China over the past decade. • The serotype Ia tilapia GBS strain is the predominant strain in China. • The genetic diversity of tilapia GBS strains is most abundant in Hainan province. • Genotype VI tilapia GBS strains showed a significant population explosion. • This is the first report of Ia-CAMP negative GBS strains in tilapia. [ABSTRACT FROM AUTHOR]