The expedition of the Joint Russian-Vietnamese Tropical Center with the participation of the staff of the A.N. Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences (IEE RAS) and the Zoological Institute of the Russian Academy of Sciences (ZIN RAS) to the north and central part of Vietnam, the main objective of which was to study the composition, distribution and condition of corals and related organisms, has been successfully completed.
The expedition took place in three areas: in the north of Vietnam, in the Gulf of Tonkin near the islands of Van Don and Me, and in the central part of Vietnam near the island of Son Tra and the Son Tra Peninsula in the waters of Da Nang. The research was carried out underwater using lightweight diving equipment at depths from 1 to 30 m in difficult conditions of low transparency and strong currents. A total of 23 points were surveyed and about 170 dives were performed. During the dives, data was collected on the species composition and distribution of corals, other large invertebrates and specialized cryptofauna (symbionts).
The fauna of northern Vietnam turned out to be especially interesting. It differs significantly from central and southern Vietnam in its lower diversity and abundance of scleractinian corals. In total, about 30 species were noted, while more than 200 species were described in Nha Trang Bay alone. In addition, they do not form reefs here, and the coverage at most of the studied points does not exceed 5%. This is due to the specific habitat conditions in the Gulf of Tonkin - large temperature fluctuations during the year, low transparency, shallow depths, eutrophication and desalination. Particularly extreme environmental conditions were noted in the area of Van Don Island, where there was strong silting and water transparency rarely exceeded 20 - 50 cm.
Unexpectedly, it turned out that octocorals react to "bad" conditions differently: their diversity and abundance increase from south to north. Moreover, the fauna of alcyonarians and their symbionts is especially diverse here.
An interesting feature of the Gulf of Tonkin is the shift of mesophotic zone species to shallow depths. We found dense clusters of octocorals (gorgonian corals) at a depth of only 5-8 meters, while in central Vietnam they are found in the depth range of 15 to 50 meters. A similar situation is with black corals: in the north they form clusters at depths of 12-17 meters, and in the south, in Nha Trang Bay, they are found at depths of less than 25 meters and do not form clusters. The shift of hard-bottom fauna to shallower depths is probably explained by the low transparency and shallowness of the bay.
Despite the low diversity of the fauna of the Gulf of Tonkin, some species rare in central Vietnam reach and flourish in the north. These are some species of corals (Plesiastrea versipora and Pseudosiderastrea tayamai), gorgonians (Dichotella), sea anemones (Isactinia citrina), starfish Anthenea aspera and holothurians (Cercodemas anceps), which is probably due to the high productivity of this water area. The results obtained significantly change the understanding of the diversity and structure of settlements of marine invertebrates in the extreme conditions of the shallow water zone of the Gulf of Tonkin.