Abstract:

Aims: Marine benthic shell-bearing mollusks are one of the most important components of marine benthic fauna and one of the key groups influencing marine ecosystem and sediments. The west coast of Guangdong is located in the Northern South China Sea (NSCS) and is rich in marine benthic shell fauna. However, it is also the area with a great lack of surveys of the marine benthic shell fauna, which seriously hinders the development of geographical divisions of offshore benthic fauna. Donghai Island and Naozhou Island have been well studied in terms of marine benthic mollusks. They are also the key areas to reveal the geographical pattern of benthic animals in the NSCS. Here we aim to report on the species diversity and geographical distribution of benthic mollusks found on the coast of the two islands and adjacent waters.

Methods: Based on historical records from previous studies, historical specimens preserved in the collection of the South China Sea Institute of Oceanology, and more than 2,600 specimens collected in 2021-2023 by the School of Geography and Planning, Sun Yat-sen University, an up-to-date checklist of known marine benthic shell-bearing mollusks living around Donghai Island and Naozhou Island was compiled. The geographical distribution patterns of all species were analysed based on their northern limits in the East Asian continental shelf.

Results: A total of 207 species from 144 genera and 73 families were identified in the field collections. Thirty-eight of these were new to the area. So far, a total of 602 benthic mollusks from 330 genera and 115 families have been recorded in the waters around Donghai Island and Naozhou Island, making this area the most species-rich area on the continental coast of the NSCS in terms of marine benthic shell-bearing mollusks. Of the 602 benthic mollusk species, 26.2% were tropical warm-water species, 49.3% were tropical-subtropical warm-water species and 24.4% were eurythermal species. The number of species that were also recorded in the nearshore waters of Hainan, Beibu Gulf and Eastern Guangdong accounted for 89.0%, 80.2% and 80.2% of the total species, respectively.

Conclusion: The results show that the coast of Donghai Island, Naozhou Island and the adjacent waters are rich in marine benthic shell-bearing mollusks. The shell fauna here has typical tropical characteristics and is most closely related to that of Hainan Island. It is recommended that these two areas could be placed in the same ecoregion with regard to the bioregionalization of the coastal and shelf benthic fauna. The present work enriches the benthic shell-bearing mollusks in the sea east of Leizhou Peninsula and provides up-to-date data to support the bioregionalization of benthic fauna in the NSCS.

Key words: Gastropoda, Bivalvia, Mollusca, species diversity, Northern South China Sea