The Origins’ of Neocaridina


Population history of N. sp. in eastern Taiwan

Our study found that N. sp. was distributed only in three adjacent rivers: SK, SM and WL (Figs. 2 and 5d). Although the NST in this species was smaller than those in other species, only this species displayed a higher GST than NST (Table 3). These results suggested that most related haplotypes were found in different populations. However, the depth of the sea around eastern Taiwan was deeper than the depth in western Taiwan (Taiwan Strait), and even during the glaciations, these oceans around eastern Taiwan were not exposed. Previous studies have demonstrated that the amphidromous fish R. giurinus [43] and shrimp Caridina pseudodenticulata [42] larvae survived in seawater and could not cross this deep sea. How did N. sp. colonize or migrate among these three rivers? The geological study of Taiwan Island [44] proposed that these three adjacent rivers belonged to one river, the paleo-Siuguluan River (paleo-SK), and separated after the middle Pleistocene.

Phylogeography of N. saccam and N. ketagalan

Neocaridina saccam were divided into two lineages, NS1 and NS2 (Fig. 5c), and exhibited a southern and northern distribution, which were to the south and to the north of the Formosa Bank. The S-DIVA analysis showed that the ancestral populations of N. saccam were distributed to the south and north of Formosa Bank (Fig. 5c). Moreover, only two populations, JS, the northernmost population, and ER, the southernmost population, had private haplotypes (Table 1; Fig. 1). These results seemed to reveal that the colonization route was divided into two routes by the Formosa Bank. The Formosa Bank is located in the southern part of the Taiwan Strait. Previous studies [9171945] have suggested that the Formosa Bank divided the glacial land bridge in the Taiwan Strait; however, the role of the Formosa Bank on the population dispersion within the island was rarely described. Ju et al. [26] proposed that during the maximum glacial periods, the ridge lifted from Formosa Bank to the present coastline of Taiwan Island. Therefore, during the maximum glaciation, the dispersal between the two sides of the bank through the exposed continental shelves of the island were prevented. After N. saccam colonized the island, the northward dispersal route was interrupted by the Miaoli Plateau, and the southward dispersal route was fragmented by the Kaoping foreland basins (Fig. 1).

Neocaridina ketagalan can be divided into three lineages (NK1-NK3, Fig. 5b). Lineage NK3 was restricted to the northern region of the Taoyuan Plateau, lineage NK1 was restricted to the north region of the Miaoli Plateau, and lineage NK2 was restricted to the north region of the Formosa Bank (Figs. 1 and 5b). The S-DIVA analysis displayed that its ancestral populations were distributed north of the Taoyuan Plateau and then southward (A1 region; Fig. 5b). Finally, the population structure was shaped by the Taoyuan Plateau, Miaoli Plateau and Formosa Bank. The Taoyuan Plateau is located in northwestern Taiwan (Fig. 1). Some freshwater fishes, e.g., O. evolansSqualidus argentatusSinibrama macrops and Hemibarbus labeo, were only distributed in the Tamsui River north of the Taoyuan Plateau (excluding). Chang et al. [19] and Hsu et al. [23] also found that the Taoyuan Plateau divided the populations of M. brevirostris and Semisulcospira libertina into different lineages. Thus, the lineage NK3 was restricted to the northern region of the Taoyuan Plateau. Moreover, many studies suggest that the Miaoli Plateau prevented the dispersal of the freshwater fishes [919]. Thus, when the Miaoli Plateau emerged, the populations were isolated and diverged (lineage NK1). Last, as described above, during the maximum glaciation, the Formosa Bank interrupted the migrations of N. saccam and N. ketagalan.

Population history of N. davidi in Taiwan

Among the four Neocaridina species in Taiwan, the distribution range of N. davidi was wider than those of the others (Fig. 2). According to a previous study [918], this widely distributed species colonized Taiwan Island before the species with restricted ranges. However, the results of the TMRCA analysis showed that N. davidi colonized the islands after the other species (Table 3). In addition, our study also found that this species was widely distributed throughout the world. Neocaridina davidi is known to be an invasive species due to its importance in the aquarium trade (Englund and Cai [46] for Hawaii; Jabłońska et al. [47] for Poland; Klotz et al. [48] for Germany). Thus, some populations might have resulted from introduction into the wild from aquarium stocks.

Neocaridina davidi in Taiwan can be divided into three lineages (ND-ND3, Fig. 5a). Lineage ND2 was only distributed north of the Miaoli Plateaus, lineage ND3 was mostly distributed in northeastern Taiwan, and lineage ND1 was widely distributed. The results of the S-DIVA revealed that the ancestral populations were distributed north of the Formosa Bank and in northeastern Taiwan. Moreover, our study found that some populations did not have private haplotypes, and these populations were not distributed in ancestral areas, excluding population FG (Table 1; Figs. 1 and 5a). In addition, the shared haplotypes of the other three species were only distributed in the neighbour populations, and the shared haplotypes of N. davidi, excluding D9 and D10, were also distributed in the neighbour populations (Table 1). Thus, we suggest that the distribution of the widespread haplotype and the discontinuous distribution might have resulted from introductions to the wild from aquarium stocks, and the human-caused transformations among the wild population were rarer than the introductions of individuals to the wild from aquarium stocks.

In conclusion, our study considered that the ancestral populations of N. davidi were distributed north of the Formosa Bank and in northeastern Taiwan (Fig. 5a) and then isolated and divergent by the Central Range and Miaoli Plateau. Finally, the ND1-ND3 lineages were restricted to northeastern Taiwan (E region), north of the Miaoli Plateau (A region), south of the Miaoli Plateau and north of the Formosa Bank (B region). We suggested that the populations in regions C and D might have resulted from the introduction from aquarium stocks to the wild by humans (Fig. 5a).

Conclusions

This study found that there were four Neocaridina species in Taiwan and that they originated through four colonization events. There were five phylogeographic breaks in Taiwan: the Central Range, Taoyuan Plateau, Miaoli Plateau, Formosa Bank and Kaoping foreland basins. This study found that the population sizes of these four species all displayed declines (Fig. 4). In the census programme of the Neocaridina species in Taiwan, we found that the Neocaridina species is rare or has disappeared in many rivers. Thus, the results of the present study provide information to conservation management agencies about the patterns of genetic diversity and the structure of the Neocaridina species in Taiwan. However, this study did not determine the systematics of the genus Neocaridina in East Asia. The results of the present study provide information on the phylogeography of East Asia and the history of the genus Neocaridina. In future studies, we need more sampling and more genetic characters.

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