Yuyang Zhang

A sea cucumber outbreak on a degraded coral reef in Sanya, China

The sudden mass occurrence of a new species of sea cucumber (Holothuroidea), Ocnus sanya (You et al. 2013), was found on Luhuitou Reef off the coast of Sanya, China. Ocnus sanya ranges in size from 0.5 to 4 cm and is sessile on solid substratum like rocks or coral rubble. Tube feet make it mobile enough to escape from unprotected sites so that few individuals are found living on the sand or mud around the reef. Its tentacles catch organic material in seawater, which it uses as a food source (Fig. 1a). The sea cucumber was first seen on the reef in 2008, but in low density. By April 2014, the abundance of O. sanya increased to more than 1,000 individuals/m on the seabed and occupied half of the hard substratum area of the reef itself (Fig. 1b). Most of the O. sanya are distributed at a depth of 1–6 m on the reef with the highest density at 4 m depth. Sedimentation rates on the reef were previously described as high by Li et al. (2013). The population increase of O. sanya may result from the enriched organic particles in the sediment and seawater, which increase the food source for the sea cucumbers. The outbreak of O. sanya may become a new threat to the living corals on the reef. While there is no bleaching, mucus, or dead tissue on corals touched by O. sanya, the sea cucumbers appear to compete for coral colony space (Fig. 1c). Coral larvae settlement will likely decrease where O. sanya occupies the substratum that would otherwise be available for coral recruits. Whether O. sanya kill corals directly for food or compete with corals for space is unknown. The impact of O. sanya on this coral reef ecosystem requires future study.

Temperature-Driven Local Acclimatization of Symbiodnium Hosted by the Coral Galaxea fascicularis at Hainan Island, China

The success of coral reef ecosystems largely depends on mutualistic symbiosis between scleractinian corals and the dinoflagellate photosymbiont Symbiodinium spp. However, further investigation is needed to elucidate the flexibility of coral-algae associations in response to environmental changes. In this study, we applied a molecular method (high-throughput internal transcribed spacer 2 region of ribosomal RNA gene amplicon sequencing) to explore diversity and flexibility of Symbiodinium associated with Galaxea fascicularis, an ecologically important scleractinian coral species collected at five locations around Hainan Island, South China Sea. The results revealed a high diversity of Symbiodinium subclades with C2r and D17 being dominant in G. fascicularis. Clade D Symbiodinium occurred most frequently in habitats where the annual average sea surface temperatures are the highest, suggesting that temperature is an important factor in determining Symbiodinium D abundance in G. fascicularis. The distribution of coral-Symbiodinium associations are possibly mediated by trade-off mechanisms which change the relative abundance of Symbiodinium clades/subclades under different environmental conditions. These findings provide further evidence that reef-building corals such as G. fascicularis can shuffle their symbionts to cope with environmental changes, and have implications for our understanding of the ecology of flexible coral-algal symbiosis.