Aplocheilus blockii Arnold 1911
Authors: Sreesha, Gadadharan, Dahanukar, Neelesh, Sudasinghe, Hiranya, Sasikala, G., Ranasinghe, Tharindu, Wijesooriya, Kumudu, Anoop, V. K., Thomas, Liju, Meegaskumbura, Madhava, Raghavan, Rajeev
Type: Taxonomic treatment
Published: 2026-03-31
Keywords: Biodiversity, Taxonomy, Animalia, Chordata, Cyprinodontiformes, Aplocheilidae, Aplocheilus, Aplocheilus blockii

<p><b>Morphological diagnosis and distribution of <i>A. blockii</i> and <i>A. parvus</i></b></p><p>Morphometric and meristic characters of <i>A. blockii</i> and <i>A. parvus</i> are provided in Table 3. The measurements and meristics overlapped between <i>A. blockii</i> and <i>A. parvus</i>. However, examination of series of specimens of <i>A. blockii</i> and <i>A. parvus</i> showed them to be reliably separable based on the pattern of iridescent scales in their lateral scale series. While <i>A. blockii</i> has continuous rows of iridescent scales in the lateral series, giving an impression of iridescent horizontal lines, iridescence in the lateral series of <i>A. parvus</i> occurred only on alternate scales, giving an impression of iridescent vertical lines (Fig. 1). <i>Aplocheilus blockii</i> is distributed in peninsular India, while <i>A. parvus</i> occurs in peninsular India and Sri Lanka (Fig. 2A). On the western side of peninsular India, <i>A. blockii</i> has a distribution from the north of Vembanad Lake in Central Kerala, to rivers in Goa (including in major drainage systems such as Muvattupuzha, Periyar, Chalakudy, Bharathapuzha, Chaliyar, Kuttyadi, Sowparnika, Sita, Sharavati and Zuari), while the distribution of <i>A. parvus</i> is restricted to the wetlands and rivers to the south of Vembanad (including the Kallada River, Ashtamudi Lake, Karamana River and Veli Lake). On the south-eastern side of the peninsula, <i>A. parvus</i> is distributed throughout the lowlands in the state of Tamil Nadu, while <i>A. blockii</i> occurs along the northern borders of the state, extending westwards to Karnataka, and northwards to Andhra Pradesh and Telangana. In Sri Lanka, <i>A. parvus</i> occurs throughout the lowlands (Fig. 2A).</p><p><b>Phylogenetic analysis</b></p><p>The best nucleotide substitution model for the dataset was Hasegawa-Kishino-Yano (Hasegawa <i>et al.</i> 1985) with gamma distribution (HKY+F+G4, lnL = -2914.101, BIC = 6688.729, df = 132) for the combined partition of the three codon positions (Table 4). Maximum likelihood phylogenetic analysis revealed the presence of two reciprocally monophyletic groups corresponding to the two nominal species <i>Aplocheilus blockii</i> and <i>A. parvus</i>, supported by a high bootstrap value, and an uncorrected inter-specific genetic distance of 2.4–3.7% (Fig. 2B, 2C). The maximum intra-specific genetic divergence was 1.5% for <i>A. blockii</i> and 0.7% for <i>A. parvus</i>. As a result, the genetic gap that separated maximum intra-specific genetic divergence and minimum inter-specific genetic divergence was between 1.5 and 2.4% (Fig. 2C).</p><p><b>Population genetics of <i>Aplocheilus blockii</i> and <i>A. parvus</i></b></p><p>A total of 37 haplotypes were observed in <i>A. blockii,</i> together with 32 in <i>A. parvus</i> (Table 5), with significant differences in the total haplotype diversity in <i>A. blockii</i> compared to <i>A. parvus</i> (t = 7.28, P <0.0001).Within <i>A. blockii</i> there was no significant difference between the haplotype diversity of western and eastern Indian populations (t = 0.13, P = 0.8978). Similarly, within <i>A. parvus</i> there was no significant difference between the haplotype diversity of western and eastern Indian populations (t = 1.03, P = 0.3220); however, there was a significant difference in the haplotype diversity between the Indian and Sri Lankan populations of <i>A. parvus</i> (t = 6.35, P <0.0001) with a greater number of haplotypes in India.</p><p>Mismatch distribution for <i>A. blockii</i> was bimodal (Fig.3A), suggesting stable population and no recent expansion; however, the Bayesian skyline plot suggested recent decrease in the median expected population size (Fig. 3B). In the case of <i>A. parvus</i>, we observed a unimodal distribution suggestive of recent population expansion (Fig. 3C). A separate analysis of Indian and Sri Lankan populations (Fig. 4) suggested that the mismatch distribution was multimodal for Indian populations, but unimodal for Sri Lankan populations, indicating that the recent population expansion in this species occurred in Sri Lanka. Similar to <i>A. blockii</i>, the Bayesian skyline plot of <i>A. parvus</i> also suggested recent decrease in the median expected population size (Fig. 3D).</p><p>* n, number of individuals; Hd, Haplotype diversity; π, nucleotide diversity; sd, standard deviation; P, p-value.</p><p>The haplotype network (Fig. 5) showed relatively greater genetic diversity in <i>A. blockii</i> than <i>A. parvus</i>. Though there was a single shared haplotype between the western and eastern Indian populations of <i>A. blockii</i>, such shared haplotypes could not be observed between eastern and western Indian populations of <i>A. parvus</i>. Further, there were also no shared haplotypes between Indian and Sri Lankan populations of <i>A. parvus</i>.</p><p><b>Population genetic structuring and phylogeography</b></p><p>AMOVA (Table 6) revealed significant genetic differences between the two species (FCT = 0.77, P <0.0001), confirming their genetic distinctness. Further, the significant genetic difference among populations within the species (FSC = 031, P <0.0001) suggests geographical population structuring. However, there was no genetic structuring evident within populations (FST = 0.84, P = 0.11) indicating that individuals from different localities encompassing a wider geographical population were genetically mixed.</p><p>To better understand the genetic structuring suggested by AMOVA, we examined pairwise genetic differences (Table 7). <i>Aplocheilus blockii</i> and <i>A. parvus</i> were genetically distinct (FST = 0.8055, P <0.0001). For <i>A. blockii</i>, the genetic differentiation between western and eastern populations in peninsular India was not significant (FST = 0.0850, P = 0.0541); in the case of <i>A. parvus,</i> however, there was a clear genetic differentiation between the eastern and western populations (FST = 0.3719, P <0.0001), between southwestern Indian and Sri Lankan (FST = 0.6643, P <0.0001), and between southeast Indian and Sri Lankan (FST = 0.4942, P <0.0001) populations.</p>