This study presents an analysis of the spectral characteristics of remote sensing reflectance (R_rs) in northwestern South China Sea based on the in situ optical and water quality data for August 2018. R_rs was initially divided into four classes, classes A to D, using the max-classification algorithm, and the spectral properties of whole R_rs were characterized using the empirical orthogonal function (EOF) analysis. Subsequently, the dominant factors in each EOF mode were determined.The results indicated that more than 95% of the variances of R_rs are partly driven by the back-scattering characteristics of the suspended matter. The initial two EOF modes were well correlated with the total suspended matter and back–scattering coefficient. Furthermore, the first EOF modes of the four classes of R_rs (A–D R_rs–EOF₁) significantly contributed to the total variances of each R_rs class. In addition, the correlation coefficients between the amplitude factors of class A–D R_rs–EOF₁ and the variances of the relevant water quality and optical parameters were better than those of the unclassified ones. The spectral shape of class A R_rs–EOF₁ was governed by the absorption characteristic of chlorophyll a and colored dissolved organic matter (CDOM). The spectral shape of class B R_rs–EOF₁ was governed by the absorption characteristic of CDOM since it exhibited a high correlation with the absorption coefficient of CDOM (a_g (λ)), whereas the spectral shape of class C R_rs–EOF₁ was governed by the back-scattering characteristics but not affected by the suspended matter. The spectral shape of class D R_rs–EOF₁ exhibited a relatively good correlation with all the water quality parameters, which played a significant role in deciding its spectral shape.

Aquaporins (AQPs) are a family of integral membrane proteins that have been shown to be important for osmoregulation in many vertebrates. To identify potential stress resistance-related aqp genes in salinity adaptation of the roughskin sculpin Trachidermus fasciatus, we investigated the time-course expression dynamics of seven aquaporin genes (aqp1, 4, 7, 8, 10, 11 and 12) in three osmoregulatory tissues (kidney, gill and intestine) and one metabolic tissue (liver). The fish were subjected to two different acute osmotic treatments (seawater-to-freshwater transfer respectively achieved in 1 h and 24 h, namely, E-acute and acute group). The expression profiling of the seven aqp genes were performed using quantitative real-time PCR (qRT-PCR). At the time of all sampling time points (0 h, 12 h, 24 h and 48 h), no expression of aqp4 was found in the gill, liver and intestine; no expression of aqp7 was found in the gill and liver. Significant differences of aqp expression were determined in the four target tissues, and the mRNA levels were largely variable among gene members and tissues. Similar patterns of the time-course expression were detected in most of the aqp genes in T. fasciatus between the two acute groups, except that only one gene (aqp12) in the kidney and three genes (aqp7, aqp8 and aqp10) in the intestine revealed different expression patterns. These results suggest that the expression response of aqp genes was similar under osmotic changes with different rates.

Calreticulin (CRT) is a highly conserved Ca²⁺-binding protein and chaperone in the endoplasmic reticulum, which mainly participates in adjusting calcium level and directing proper conformation of proteins. Here, we cloned the crt gene of the scleractinian coral Galaxea astreata, named Gacrt, and analysed its ability to drive bacterial agglutination. The full-length Gacrt cDNA consisted of 1 792 nucleotides and contained a 77 bp 5’ untranslated region (UTR), a 380 bp 3’ UTR and a 1 335 bp open reading frame (ORF) that encoded a 444 amino acid protein. The deduced peptide possessed a signal peptide domain, an endoplasmic reticulum retrieval signal sequence (KDEL), two potential calreticulin family signature motifs and a set of triplicate repeats. We also found that the recombinant GaCRT protein could promote agglutination of both the Gram-positive bacterium Micrococcus luteus and the Gram-negative bacterium Escherichia coli. These results show that the GaCRT protein can enhance bacterial agglutination, hinting that GaCRT is an immune-relevant molecule involved in host defense against bacterium.

In order to realize the spatiotemporal variations of benthic macrofaunal communities at the “Amphioxus Sand” habitat, six surveys including four seasons and three consecutive summers (i.e., 2014, 2015 and 2016) were conducted in two core sites, i.e., Huangcuo (HC) and Nanxian-Shibaxian (NX), in the Xiamen Amphioxus Nature Reserve in China. A total of 155 species of macrofauna were recorded, therein, polychaetes were dominant in terms of species number and density. Significant spatiotemporal variations of macrofaunal communities were observed. The density of polychaetes and the biomass of molluscs in the HC site were higher than those in the NX site. Macrofauna were more diverse and abundant in the cold seasons (winter and spring) than that in the warm seasons (summer and autumn). The annual variations of macrofaunal communities may be attributed to the changes in sediment texture among the three years of the survey. The variations in macrofaunal communities were mainly related to the proportion of polychaetes within the community. In addition, the density of amphioxus (include Branchiostoma japonicum and B. belcheri) was negatively correlated to that of polychaetes, bivalves, and crustaceans. Amphioxus was less likely to be found in the sediments with higher silt and clay content. Five biotic indices including Margaref’s richness index (d), Peilou’s evenness index (J′), Shannon-Wiener diversity index (H′), AMBI and M-AMBI were calculated in the present study. AMBI seems suitable in assessing benthic health at the “Amphioxus Sand” habitat, and a potential risk of ecological health in Xiamen Amphioxus Nature Reserve should be aware.

Marine ecosystems provide a wide variety of diverse habitats that frequently promote migration and ecological adaptation. The extent to which the geographic distribution of marine organism has reshaped by human activities remains underappreciated. The limitations intrinsic to morphology-based identification systems have engendered an urgent need for reliable genetic methods that enable the unequivocal recognition of fish species, particularly those that are prone to overexploitation and/or market substitution. In the present study, however, an attempt has been taken to identify two locally adapted fish species, Siganus sutor (Valenciennes, 1835) and Seriolina nigrofasciata (Rüppell, 1829) of order Perciformes, which happens to be the first record in Odisha coast, Bay of Bengal. The diagnostic characteristics of Siganus sutor are: dorsal fin XIII-10, anal fin VII-9, pectoral fin 15, pelvic fin II-3, while that of Seriolina nigrofasciata dorsal fin VI-I-35, anal fin I-17, pectoral fin 16, pelvic fin 5. All COI barcodes generated in this study were matched with reference sequences of expected species, according to morphological identification. Bayesian and likelihood phylogenetic trees were drawn based on DNA barcodes and all the specimens clustered in agreement with their taxonomic classification at the species level. The phylogeographic studies based on haplotype network and migration rates suggest that both the species were not panmitic and the high-frequency population distribution indicates successful migration. The result of this study provides an important validation of the use of DNA barcode sequences for monitoring species diversity and changes within a complex marine ecosystem.

As one of the important microzooplankton in marine pelagic habitats, tintinnids are classified as neritic genera, oceanic genera and cosmopolitan genera. Until now, we know little about the interaction between neritic and oceanic genera in continental shelves. Low species richness area was found in the mixing area between neritic and oceanic genera in the East China Sea. In this study, we investigated the spatial distribution of tintinnid community throughout the entire Bohai Sea in May (spring) of 2014 to find out: (1) whether the tintinnid assembalges differed in the three major bays bordering this shallow inland sea; (2) whether there was low species richness area in the Bohai Sea. Three genera and 11 species attributed to neritic and cosmopolitan taxa were reported from this region. Tintinnid abundance averaged (81±216) ind./L (0–1 234 ind./L). Tintinnid community differed within the three major bays, with each characterized by different tintinnid taxa. Referring to the average abundance of three bays, tintinnids were most abundant in the Laizhou Bay ((328±445) ind./L), and least abundant in the Liaodong Bay ((34±57) ind./L). A low species richness area occurred in the central Bohai Sea (20–40 m isobaths), in some stations of which no tintinnid was recorded. Our study demonstrates that tintinnid community varied greatly over small spatial scales, and low species richness area was found in the mixing area between neritic and oceanic genera within this inland sea.

Uncovering the role of environmental factors and finding critical factors which harbor significant fractions in governing microbial communities remain key questions in coastal marine systems. To detect the interactions between environmental factors and distributions of virio- and bacterioplankton in trophic coastal areas, we used flow cytometry to investigate the abundance of virio- and bacterioplankton covering 31 stations in the Bohai Sea of China. Our results suggested that the average abundance of total virus (TV) in winter (~2.29×10⁸ particles/mL) was slightly lower than in summer (~3.83×10⁸ particles/mL). The mean total bacterial abundance (TB) was much lower in winter (~2.54×10⁷ particles/mL) than in summer (~5.43×10⁷ particles/mL). Correlation analysis via redundancy analysis (RDA) and network analysis among virioplankton, bacterioplankton and environmental factors revealed that the abundances of viral and bacterial subpopulations depend on environmental factors. In winter, only temperature significantly influenced the abundances of virio- and bacterioplankton. In summer, in addition to temperature, both salinity and nutrient (SiO₂) had a remarkable impact on the distribution of virio- and bacterioplankton. Our results showed a clear seasonal and trophic pattern throughout the whole water system, which revealed that temperature and eutrophication may play crucial roles in microbial distribution pattern.

Habitat suitability index (HSI) models have been widely used to analyze the relationship between species abundance and environmental factors, and ultimately inform management of marine species. The response of species abundance to each environmental variable is different and habitat requirements may change over life history stages and seasons. Therefore, it is necessary to determine the optimal combination of environmental variables in HSI modelling. In this study, generalized additive models (GAMs) were used to determine which environmental variables to be included in the HSI models. Significant variables were retained and weighted in the HSI model according to their relative contribution (%) to the total deviation explained by the boosted regression tree (BRT). The HSI models were applied to evaluate the habitat suitability of mantis shrimp Oratosquilla oratoria in the Haizhou Bay and adjacent areas in 2011 and 2013–2017. Ontogenetic and seasonal variations in HSI models of mantis shrimp were also examined. Among the four models (non-optimized model, BRT informed HSI model, GAM informed HSI model, and both BRT and GAM informed HSI model), both BRT and GAM informed HSI model showed the best performance. Four environmental variables (bottom temperature, depth, distance offshore and sediment type) were selected in the HSI models for four groups (spring-juvenile, spring-adult, fall-juvenile and fall-adult) of mantis shrimp. The distribution of habitat suitability showed similar patterns between juveniles and adults, but obvious seasonal variations were observed. This study suggests that the process of optimizing environmental variables in HSI models improves the performance of HSI models, and this optimization strategy could be extended to other marine organisms to enhance the understanding of the habitat suitability of target species.

Phytoplankton growth rates and mortality rates were experimentally examined at 21 stations during the 2017 spring intermonsoon (April to early May) in the northern and central South China Sea (SCS) using the dilution technique, with emphasis on a comparison between the northern and central SCS areas which had different environmental factors. There had been higher temperature but lower nutrients and chlorophyll a concentrations in the central SCS than those in the northern SCS. The mean rates of phytoplankton growth (μ₀) and microzooplankton grazing (m) were (0.88±0.33) d^–1 and (0.55±0.22) d^–1 in the central SCS, and both higher than those in the northern SCS with the values of μ₀ ((0.81±0.16) d^–1) and m ((0.30±0.09) d^–1), respectively. Phytoplankton growth and microzooplankton grazing rates were significantly coupled in both areas. The microzooplankton grazing impact (m/μ₀) on phytoplankton was also higher in the central SCS (0.63±0.12) than that in the northern SCS (0.37±0.06). The microzooplankton abundance was significantly correlated with temperature in the surface. Temperature might more effectively promote the microzooplankton grazing rate than phytoplankton growth rate, which might contribute to higher m and m/μ₀ in the central SCS. Compared with temperature, nutrients mainly affected the growth rate of phytoplankton. In the nutrient enrichment treatment, the phytoplankton growth rate (μ_n) was higher than μ₀ in the central SCS, suggesting phytoplankton growth in the central SCS was nutrient limited. The ratio of μ₀/μ_n was significantly correlated with nutrients concentrations in the both areas, indicating the limitation of nutrients was related to the concentrations of background nutrients in the study stations.

The West Pacific Ocean is considered as the provenance center of global marine life and has the highest species diversity of numerous marine taxa. The phytoplankton, as the primary producer at the base of the food chain, effects on climate change, fish resources as well as the entire ecosystem. However, there are few large-scale surveys covering several currents with different hydrographic characteristics. This study aimed to explore the relationships between the spatio-temporal variation in phytoplankton community structure and different water masses. A total of 630 water samples and 90 net samples of phytoplankton were collected at 45 stations in the Northwest Pacific Ocean (21.0°–42.0°N, 118.0°–156.0°E) during spring and summer 2017. A total of 281 phytoplankton taxa (>5 μm) belonging to 61 genera were identified in the study area. The distribution pattern of the phytoplankton community differed significantly both spatially and temporally. The average abundances of phytoplankton in spring and summer were 797.07×10² cells/L and 84.94×10² cells/L, respectively. Whether in spring or summer, the maximum abundance always appeared in the northern transition region affected by the Oyashio Current, where nutrients were abundant and diatoms dominated the phytoplankton community; whereas the phytoplankton abundance was very low in the oligotrophic Kuroshio region, and the proportion of dinoflagellates in total abundance increased significantly. The horizontal distribution of phytoplankton abundance increased from low to high latitudes, which was consistent with the trend of nutrient distributions, but contrary to that of water temperature and salinity. In the northern area affected by the Oyashio Current, the phytoplankton abundance was mainly concentrated in the upper 30 m of water column, while the maximum abundance often occurred at depths of 50–75 m in the south-central area affected by the Kuroshio Current. Pearson correlation and redundancy analysis (RDA) showed that phytoplankton abundance was significant negatively correlated with temperature and salinity, but positively correlated with nutrient concentration. The phytoplankton community structure was mainly determined by nutrient availability, especially the N:P ratio.