Open Access

Oleg E. Kosterin

• Lake Tonle Sap in NW Cambodia is the largest freshwater lake in Southeast Asia and one of the most productive freshwater ecosystems in the world, so its banks are a home for ca 1,5 million people. It serves as a natural reservoir of the excess water of the Mekong River and cyclically changes its area from 2,500 km2 in May to 16,000 km2 in October. Its banks are naturally occupied by temporarily inundated forest and scrub, at present mostly replaced by rice fields. The present day semiaquatic vegetation of the lake is to a large extent formed by invasive plant species. The hitherto existing data on Odonata of the lake are very scarce. The author briefly examined the bank and floodplain at the NW part of the lake in February/March 2017, June and November 2018 and December 2019. Five main localities studied are described and illustrated in detail. In total 41 odonate species of four families (22 in Libellulidae) were found. Most of them are common and widespread lentic species but Macrogomphus phalantus is a species hitherto known only by few specimens from swamped forests of Borneo and Sumatra; its Tonle Sap population was earlier described by the author as the subspecies M. phalantus jayavarman Kosterin, 2019. The earlier published report by Seehausen et al. (2016) of Sinictinogomphus clavatus (not found by the author) was a considerable extension of the known species’ range to the south. Six species were found in all main examined localities and ten only in one of them. At any season at the lake immediate bank (that is water front at the lowest level), Brachythemis contaminata predominates overwhelmingly, Orthetrum sabina and Crocothemis servilia are numerous, two damselfly species, Pseudagrion microcephalum and P. rubriceps, invariably occur at floating vegetation (mostly water hyacinth), and Trithemis pallidinervis, Urothemis signata, Rhyothemis phyllis, R. variegata and Tholymis tillarga are common at bushes. Agriocnemis nana, Ceriagrion praetermissum, Ischnura senegalensis, Macrogomphus phalantus, and Aethriamanta aethra were occasionally met at the lake bank. Other 26 species were found, with different occurrence and quantity, on the lake floodplain. Variation of the male occiput coloration of Amphiallagma parvum is commented.