The Role of Old Relicts in Structuring the Boreal/Tropical Transitional Zone: The Case of East Asian Planktonic Cladocera (Crustacea)

Abstract

Previous studies provided evidence of the mixing of boreal–tropical cladocerans (Crustacea) in the Far East of Eurasia, as well as the presence of numerous pre-Pleistocene relict endemics. In this study, we hypothesize that the colonization history is reflected in the proportions of endemic/boreal/tropical taxa among different habitat types within this region. We analyzed 442 qualitative samples collected from seven sub-regions of the Far East of Russia and South Korea along a latitudinal transect of approximately 2200 km, where we identified 101 Cladocera species. Our results showed a significantly higher proportion of endemic taxa in the plankton compared to the littoral and benthic zones. The proportions of endemic/boreal/tropical taxa in the permanent and temporary waters were similar. We observed a distinct shift in dominance across our transect: boreal taxa were prevalent in the northern sub-regions (1–4), while tropical taxa were prevalent in the southern sub-regions (5–7), with a pronounced boreal–tropical transition zone (sub-regions 4–5). Our findings suggest that this pattern arose due to the unhindered dispersion of the two faunas in a latitudinal direction until they met at the boundary during their arrival in the Late Pleistocene. In contrast, the proportions of the boreal and tropical taxa did not exceed 16% in any of the studied sub-regions, and the proportion of boreal taxa decreased from 16% to 10%, while that of the tropical taxa increased from 0% to 10% from north to south. We propose that the lower proportions and the absence of mixing of fauna in the plankton can be explained by the strong presence (sometimes dominance) of relics that occupy these water bodies before the arrival of colonizers from the north and south. Our findings highlight the significant role of biotic interactions in the formation of biogeographic boundaries, which was previously underestimated.

Ovalona cambouei (Guerne et Richard, 1893) (Cladocera: Anomopoda: Aloninae): the first record for Russia

“…Most probably, after our records, just Ciscaucasia must be regarded as its northernmost distribution region (up to 43°N). Therefore, we have demonstrated that Ciscaucasia is a region of penetration of the tropical fauna to European Russia similarly to the earlier studied Primorie in the Far East of Russia (Kotov, 2016;Chertoprud et al, 2023;Krolenko et al, 2023).…”

“…This thermophylic and eurybiotic species lives in the coastal zone of lakes, slow flowing rivers, temporary water bodies and rice paddies (Korovchinskyi et al, 2021). We can confirm that O. cambouei belongs to the "southern tropical" faunistic complex, according to Kotov (2016), see also Krolenko et al (2023) andChertoprud et al (2023) for a further development of these ideas. Previously, the species was found in Africa, including Madagascar, the Iberian Peninsula, subtropical and tropical regions of Asia; vicinities of the city of Aksakan (Northwest Kazakhstan, 48°N) were regarded as its most northern distribution point (Sinev, 2015;Korovchinskyi et al, 2021).…”