Novaculina spp.

Win, Than, Bolotov, Ivan N., Vikhrev, Ilya V., Lunn, Zau, Chan, Nyein, Konopleva, Ekaterina S., Gofarov, Mikhail Y., Tomilova, Alena A. & Kondakov, Alexander V., 2021, Phylogeography and distribution of the freshwater razor clams Novaculina myanmarensis and N. gangetica in Myanmar, with notes on two doubtful nominal taxa described as Novaculina members (Bivalvia: Pharidae), Ecologica Montenegrina 40, pp. 59-67 : 60-62

Ranges of Novaculina spp. and biogeographic issues

New occurrences of Novaculina species presented in this study expand our knowledge on the distribution of several species ( Fig. 1 and Tables 1-2). Here, we show that the range of Novaculina myanmarensis extends eastward to the Ataran River basin, another tributary of the massive Salween Estuary. Furthermore, new occurrences from the Lower Ayeyarwady extend its range in this large freshwater system and support the historical report of Theobald (1877) on records of a Novaculina species in the Ayeyarwady Delta. In its turn, Novaculina gangetica was found to occur in the Dalet River, approximately 80 km SE from the previously known localities in the Kaladan – Lemro Basin, western Myanmar ( Bolotov et al. 2018a, 2018b).

Ng et al. (2020) recently discovered a population of Novaculina siamensis in the Tonle Sap River ( Cambodia). Thus, this species occurs not only in the Mekong Delta and coastal rivers of the Gulf of Thailand ( Morlet 1889; Brandt 1974; Sayenko et al. 2017; Bolotov et al. 2018a) but also in the Lower Mekong as far upstream as the Tonle Sap (approximately 300 km away from the coast).

The habitats of Novaculina myanmarensis we have recorded ( Fig. 2 ) align with those described in earlier works on this species ( Bolotov et al. 2018a). The collecting localities represent freshwater downstream sections of large and medium-sized rivers with clay bottom ( Fig. 2 A-B) and a large oxbow lake in the downstream of Ayeyarwady River ( Fig. 2C ). Novaculina gangetica is known to occur in similar environments ( Bolotov et al. 2018a, 2018b). Ng et al. (2020) also collected Novaculina siamensis from cylindrical holes in clay bottom of the Tonle Sap River.

The network analyses reveal that Novaculina gangetica shares six COI haplotypes, two of which are common between different rivers (i.e. Kaladan + Lemro and Kaladan + Dalet) ( Fig. 3A ). The haplotypes are separated by one or two nucleotide substitutions only. Such a weak phylogeographic pattern may reflect relatively recent (Late Pleistocene) colonization of coastal rivers in Myanmar from the Ganges Basin but the DNA sequences of Novaculina gangetica from India and Bangladesh are not available.

Conversely, Novaculina myanmarensis shares clear phylogeographic structure with five COI haplotypes in rivers emptying to the Salween Estuary (Donthami and Ataran) and one unique COI haplotype in the Ayeyarwady Basin ( Fig. 3B ). This pattern could indicate that the species has evolved somewhere in rivers of the Salween Estuary, with a subsequent immigration into the Ayeyarwady Basin via a (sub)recent dispersal event. A shallow divergence (single nucleotide substitution) between the COI haplotypes from Ayeyarwady and Salween basins could indicate that this dispersal event took place in the Late Pleistocene, when the sea level decreased greatly, allowing connections between downstream sections of rivers ( Voris 2000).

Figures

Figure 1. Map of Novaculina spp. occurrences based on reliable georeferenced records (Table 2). The circles indicate records from our earlier work (Bolotov et al. 2018a). The stars indicate new occurrences (Ng et al. 2020; this study)

Figure 2. Habitats of Novaculina myanmarensis in Myanmar (new occurrences). (A) Winyaw River, Ataran Basin [sample ID: RMBH biv1030]. (B) Downstream of the Hlaing River, Ayeyarwady Basin [sample ID: RMBH biv1015]. (C) Large oxbow lake, downstream of the Ayeyarwady River [sample ID: RMBH biv1014]. (Photos: Than Win [A] and Ilya V. Vikhrev [B-C])

Figure 3. Median joining networks of Novaculina spp. based on the COI gene fragment. (A) N. gangetica (N = 8). (B) N. myanmarensis (N = 15). The circle size is proportional to the number of available sequences belonging to the given haplotype (the smallest = 1). The red numbers near branches indicate the number of nucleotide substitutions between haplotypes. The color filling indicate river drainage basins.