Diversity of Three Small Type’s Giant Clams and Their Associated Endosymbiotic Symbiodiniaceae at Hainan and Xisha Islands, South China Sea

Giant clams are found in a mutualistic association with Symbiodiniaceae dinoflagellates, however, the diversity of the giant clams, as well as the diversity and distribution of Symbiodiniaceae in different Tridacnine species remain relatively poorly studied in the South China Sea. In this study, a total of 100 giant clams belonging to small type’s giant clams, Tridacna maxima, T. crocea, and T. noae, were collected from Hainan and Xisha Islands. Based on mtDNA cytochrome c oxidase subunit 1 gene (COI) and 16S rRNA fragments, T. maxima and T. crocea showed a closer phylogenetic relationship than T. noae. All the three species of giant clams hosted Symbiodiniaceae including genera Symbiodinium (formerly Clade A) and Cladocopium (formerly Clade C). Geographically, symbionts in Cladocopium are restricted to Xisha Islands, probably because Cladocopium prefers to inhabit in waters with higher mean temperatures. The endosymbiont specificity among the three giant clam species was also detected. T. noae and T. crocea are found to harbor Symbiodinium preferentially, compared with Cladocopium. These results could provide important information to understand various endosymbionts occurring in giant clams in the South China Sea.

Effects of sea-level variation and sedimentary noise variation on the development of biogenic reefs since the Pliocene among the Xisha Islands, South China Sea

Carbonate strata are a vital and favorable reservoir for global oil and gas exploration, and carbonate sedimentary systems record ancient oceanic and paleoclimatic conditions, including paleoenvironmental variations throughout geologic periods. Carbonate platforms are widely distributed among the Xisha Islands in the South China Sea and contain large amounts of oil and gas resources. Biogenic reefs are the dominant parts of the carbonate platforms in the Xisha Islands; however, research on the factors that control and affect biogenic reef development is lacking. In this study, a core from well XK-1, which is located on Yongxing Island in the Xisha Islands, a sedimentary noise model, and time-series analyses were used to determine the effects of sea-level fluctuations from 5.3 Ma to present. The results show that coral reefs in the Xisha Islands are sensitive to eustatic fluctuations and that a decrease in sea level essentially corresponds to an increase in sedimentation rate. Indexes of the East Asian monsoon and other environmental indexes show that the Pleistocene and Holocene were suitable for coral growth; however, the trends shown by these indexes and the sea-level variation indicate that the future growth of coral reefs will be at a disadvantage. Research on the controlling factors of biogenic reefs is of significance for understanding reef growth, performing global reef comparisons, and encouraging the future protection of coral reefs.

Characteristics of Macroalgal Consumption by Eight Herbivorous Coral Reef Fishes From the Xisha Islands, China, as Determined by Microscopy, 18S rRNA High-throughput Sequencing and Stable Isotope Analyses

Herbivorous fishes play an important role in controlling the overabundance of macroalgae on coral reefs. Understanding the feeding selectivity and consumption of macroalgae by herbivorous fishes can be challenging in studies of their ecological role in the preservation and recovery of coral reefs. Coral reef decline, macroalgal overgrowth and overfishing are clearly visible in the Xisha Islands, China. However, there have been no studies of the feeding behaviors of herbivorous fishes in this area. We used microscopy, 18S rRNA high-throughput sequencing and stable isotope analyses to comprehensively examine the dietary spectrum of eight herbivorous reef fish species common in the Xisha Islands, including one parrotfish, two chub, two unicorn fish and three rabbitfish. Multi-technique analyses of intestinal contents revealed that Kyphosus vaigiensis, Naso unicornis and Siganus argenteus showed a high consumption potential of macroalgae, suggesting that they are the key browsers which should receive priority protection in in the Xisha Islands. Kyphosus cinerascens, K. vaigiensis, N. unicornis and S. punctatissimus fed on the entire macroalgal thallus, indicating their greater ecological importance compared with species which only consume the algal fronds. However, Calotomus carolinus can consume the red alga Pneophyllum conicum, which is widely distributed on Indo-Pacific coral reefs and can overgrow and kill live corals. Clearly, a diverse herbivorous fish fauna is very important in the Xisha coral reefs. These results not only demonstrated the various functions of different herbivorous fish species in macroalgal removal, but also provided insights into the management of herbivorous fishes on the coral reefs of the South China Sea.

Magnetic Properties and Initiation of Biogenic Reefs in Xisha Islands, South China Sea, at the Oligo–Miocene Boundary

Biogenic reefs and carbonate platforms are valuable natural resources, playing an important role in modulating the global climate and in carbon cycles through biological processes. Biogenic reefs in the Xisha (Paracel) Islands began in the late Oligocene and covaried with the deep-sea basin of the South China Sea and with the aeolian deposit in the Chinese Loess Plateau. Core XK-1 was drilled into the Xisha Islands to their granitic base and well dated by magnetostratigraphy, offering an opportunity to reveal the details of how the Xisha reefs initiated. In this report, the lower section of the biogenic reefs (23.0–24.5 Ma) was sampled for studying magnetic properties. The main results are as follows: (1) magnetic minerals in the XK-1 biogenic reefs are dominated by low-coercivity and relatively coarse-grained magnetite; (2) the variabilities of magnetic parameters can be clustered into two sections around 23.6 Ma, and the differences between the two units are evident both in the amplitudes and the means; and (3) changes in the concentration-dependent magnetic parameters can be well correlated with the records of global deep-sea oxygen and carbon isotopes, and the sea level during the Oligo–Miocene boundary. Based on these results, a close link was inferred between biogenic reef evolution in the Xisha Islands and global climate change. This link likely highlights the covariation or the dominant role of the Asian monsoon in biogenic reefs and involves different responses to global temperature, CO2, and sea-level changes on various timescales. Therefore, we proposed that the origin of biogenic reefs in the Xisha Islands was likely paced by orbital obliquity from a long-term perspective.

Cephonodes sanshaensis sp. nov. (Lepidoptera, Sphingidae) from Xisha Islands, Hainan Province, China

A new species, Cephonodes sanshaensis Deng & Huang sp. nov. is described from Xisha Islands in Sansha City of China. The photos of the adults and their genitalia are provided. The new species is similar to C. hylas (Linnaeus, 1771) and C. picus (Cramer, 1777), but it can be distinguished easily by characters in the male genitalia: the right lobe of the uncus hook-shaped with distinctly acute apex, left valva long and narrow with truncate apex, right valva broad knife-shaped. Molecular analysis based on COI barcode sequences is used to infer the phylogenetic position of the new species within the genus Cephonodes. An updated key and checklist to the species of the genus Cephonodes are also provided.

The fishermen inhabiting the Xuande and Yongle Islands in Xisha Islands

Hainan Island fishermen have been engaged in fishing in Nanhai Zhudao (the Nanhai Islands) since ancient times. From sailing to the emergence of motorboats, the South China Sea fishermen have experienced different seafaring eras. Fishermen who work and live on the islands and reefs also have experienced living on islands seasonally to long-term. This article uses the inhabiting fishermen groups of the Xuande and Yongle Islands in Xisha Qundao (the Xisha Islands) as examples to demonstrate the unique livelihoods of the fishermen and reveal their role in the use of marine ecology to achieve sustainable development.