A molecular phylogeny of the freshwater‐fish genusRasbora(Teleostei: Cyprinidae) in Sri Lanka reveals a remarkable diversification—And a cryptic species

Abstract

The diversity of the freshwater‐fish genus Rasbora (Cyprinidae) on Sri Lanka (five species) is high compared with the four species reported from the peninsula of India, from which the island's cyprinid fauna is derived. The paucity of characters by which species of Rasbora can be phenotypically distinguished renders field identification difficult, adversely affecting the estimation of populations and distributions, with consequences for conservation and management, increasing also the risk of taxonomic inflation. From a sampling of 90 sites across Sri Lanka and based on phylogenetic and haplotype analyses of sequences of cox1 and cytb mitochondrial, and rag1 and irbp nuclear markers, we review the species diversity and phylogeography of Rasbora on the island. Molecular analyses recover, in addition to the five species previously reported, a new (cryptic) species: Rasbora adisi sp. nov. Uncorrected pairwise cox1 genetic distances between species range from 2.0 to 12.3 percent. The Sri Lankan diversification derives from a common ancestor which arrived from India during a sea‐level low‐stand in the mid‐Miocene (15.1 Ma [95% HPD: 11.5–19.8 Ma]), when the present‐day island was subaerially connected to the Indian subcontinent by a broad isthmus. This gave rise to a clade comprising five species—R. adisi sp. nov.,Rasbora armitagei, Rasbora microcephalus, Rasbora naggsi and Rasbora wilpita—with a crown age of 9.9 Ma (95% HPD: 7.1–13.3 Ma) and to a clade comprising Indian and Sri Lankan populations of Rasbora dandia, which themselves are reciprocally monophyletic. Morphological analysis of 334 specimens discriminates between most species which, however, are most reliably diagnosed by chromatic characters. The four endemic species exhibit a pattern of inter‐basin dispersal via headwater capture, followed by vicariance, explaining the high diversity of the genus on the island.

Molecular phylogeny and phylogeography of the freshwater-fish genus Pethia (Teleostei: Cyprinidae) in Sri Lanka

“…Nevertheless, perhaps owing to the wet zone's greater topographic complexity [51], and despite its extent being only about a quarter that of the dry zone, nucleotide and haplotype diversity in the wet-zone endemics P. nigrofasciata, P. cumingii and P. reval are greater than in P. melanomaculata. This phenomenon has been observed previously in species pairs in which one is confined to the wet zone while the other is distributed across the dry zone, such as Devario micronema vs D. malabaricus, Laubuka varuna vs. L. lankensis, Systomus pleurotaenia vs S. sarana, and Rasbora wilpita vs R. dandia [11,14,15,39]. As Potter et al [52] show, genetic diversity in low-dispersal vertebrate species tends to be higher in mesic, topographically complex biomes, compared to that of species inhabiting dry and topographically less complex biomes.…”

“…4) recovered P. nigrofasciata as two well-supported, sympatric subclades, one spanning the distribution of the species in Sri Lanka, from the Attanagalu to the Walawe basins, and the other confined to the region between the Kalu and Gin basins, inclusive. Such a pattern has not been observed in the other phylogeographic studies of Sri Lankan cyprinids published so far [5,11,14,15,39]. Given that our genetic dataset is limited, it is difficult to offer an explanation for this observation.…”

“…Hence, much of the island’s remarkable biodiversity derives from insular diversifications stemming from a small number of immigrant dispersals, as in the case of its 59 endemic treefrog species and 50 endemic crab species, shown to be monophyletic by Meegaskumbura et al [ 12 ] and Beenaerts et al [ 13 ], respectively. The same has been shown to be true also for several freshwater-fish diversifications, such as in the cypriniform genera Devario , Rasbora and Systomus [ 11 , 14 , 15 ].…”

Diversity, phylogeny and biogeography of Systomus (Teleostei, Cyprinidae) in Sri Lanka

“…While haplotypes are not shared between these four haplogroups, compared with S. pleurotaenia , their haplotype and nucleotide diversities are low. Such a phylogeographic pattern has been observed also in other Sri Lankan cyprinids, such as Laubuka, Rasbora and Devario (Sudasinghe, Pethiyagoda, & Meegaskumbura, 2020a; 2020b; Sudasinghe, Pethiyagoda, Ranasinghe, et al, 2020). Given that S. sarana are abundant in both lotic and lentic waters in the lowland floodplain that surrounds the island's central mountains, the barriers to dispersal across the basins remain to be determined.…”

Patterns of Cryptic Diversity and Phylogeography in Four Freshwater Copepod Crustaceans in European Lakes

“…Studies on the genetic diversity of widespread freshwater taxa, largely applying DNA barcoding methods, are increasingly revealing complex patterns of cryptic variation. Apart from investigations of freshwater fishes [2] and other macroscopic taxa [3], studies on the phylogeny of smaller freshwater crustaceans, such as cladocerans [4][5][6], isopods [7], amphipods [8][9][10], and copepods [11] have demonstrated the presence of hidden genetic diversity and occurrence of cryptic species across presumed conspecific populations.…”