A Long Overlooked New Species of Fanged Frog, Genus Limnonectes (Amphibia: Anura: Dicroglossidae), from Luzon Island, Northern Philippines
We describe a new species of fanged frog (genus Limnonectes) from the foothills of two, inland, south-to-north oriented, parallel mountain ranges of Luzon Island of the northern Philippines. Although the new species broadly co-occurs with its closest relative at numerous localities within the Luzon Pleistocene aggregate island complex faunal region, it can be readily diagnosed from Limnonectes macrocephalus based on its unpigmented (white) ventral surfaces of terminal digital discs, its unexpanded or minimally expanded terminal digital discs, and an allometric growth pattern indicating evidence of sexual dimorphism at a smaller overall body size. The new species, which can also be identified by its divergent 16S ribosomal RNA mitochondrial gene sequence, possesses a curious distribution unlike the range of any Philippine endemic amphibian characterized to date: it is known from nine interior (inland) localities distributed between the two, parallel, south-to-north mountain chains (the Cordillera and Sierra Madre) which characterize mainland Luzon. We interpret the presence of two broadly sympatric, genetically divergent, strongly supported haplotype clades—which correspond to morphologically diagnosable phenotypes, using traditional discrete characters and allometric growth patterns—as prima facia evidence of two, independently evolving evolutionary lineages (species) of giant fanged frogs on Luzon. The description of another new species of large-bodied fanged frog on Luzon from multiple localities in close proximity to the capital city (Manila) emphasizes the degree to which even well-studied larger Philippine landmasses possess unrecognized and overlooked biodiversity.
Map of sampling locations on Luzon Island, Philippines (right) and ML phylogenetic tree (left). Red circles on the map represent genetic sampling localities for Limnonectes cassiopeia, new species, blue diamonds represent genetic sampling for L. macrocephalus, and gray triangles represent genetic samples of L. woodworthi. Maximum likelihood tree topology estimated in IQ-TREE. Only the Limnonectes clade E (sensuEvans et al., 2003) subtree is shown here (see inset; outgroups not shown: L. finchi, L. parvus, L. micrixalus, L. palavanensis, L. leytensis, L. acanthi, and L. beloncioi). Black circles at nodes indicate strong support for all ML and BI support metrics (UFBS ≥ 95, SH-LR ≥ 80, PP ≥ 0.95). Branch length scale bar units = substitutions/site. The full ML and BI trees are available in supplementary data (see Data Accessibility).
Exploratory plots depicting uncorrected measurements plotted against SVL in specimens of Limnonectes macrocephalus (blue shades) and Limnonectes cassiopeia, new species (red shades). In all plots, adult males are represented by triangles, adult females by diamonds, and juveniles by circles. Eye–tympanum distance vs. snout–vent length is plotted separately for L. macrocephalus (A) and Limnonectes cassiopeia, new species (B). Solid black lines in panels A and B show the fitted regression lines for eye–tympanum distance vs. snout–vent length for males and combined females plus juveniles, while the vertical gray dashed line shows the snout–vent length intersect of two regression equations in each taxon. We confirmed that the apparent difference in slopes between sex classes was statistically significant with separate analyses of covariance (ANCOVA) for Limnonectes cassiopeia, new species (P = 0.002), and L. macrocephalus (P < 0.001). In Limnonectes cassiopeia, new species, the intersection point of the two regressions occurs at a substantially smaller snout–vent length (B: 41.6 mm) than in L. macrocephalus (A: 60.8 mm), which suggests that sexual dimorphism begins to emerge at a smaller size in Limnonectes cassiopeia, new species, than in L. macrocephalus. Panel C shows toe IV disk width vs. snout–vent length in L. macrocephalus and Limnonectes cassiopeia, new species, plotted together, suggesting that Limnonectes cassiopeia, new species, has relatively narrower toe disks than L. macrocephalus. All plots depict uncorrected measurements for eye–tympanum distance, toe IV disk width, and snout–vent length.
Boxplot summary comparisons for the 17 continuous morphometric variables collected in this study. Each plot depicts morphological variation between Limnonectes cassiopeia, new species (red) and L. macrocephalus (blue) and between females (left) and males (right). Horizontal lines within boxes represent the median values, upper and lower bounds of boxs represent interquartile ranges, and whiskers represent the full range of measurements.
Ordination of principal components one (x-axes) and two (y-axes) from principal component analysis (PCA) of morphometric measurements of Limnonectes cassiopeia, new species (red), and L. macrocephalus (blue). PCA analyses of males (top) and females (bottom) were conducted separately.
Dorsal (left) and ventral (right) comparison of adult male Limnonectes cassiopeia, new species, holotype (PNM 9871; A, B) and adult male L. macrocephalus (KU 345550; C, D). Scale bar = 20 mm.
Comparisons of plantar view of the left foot of Limnonectes cassiopeia, new species (top row), and L. macrocephalus (bottom row) showing differences in toe disk pigmentation between the two species. Limnonectes cassiopeia, new species, depicted are: PNM 7610 (A), KU 327466 (B), and PNM 7612 (C). Limnonectes macrocephalus depicted are KU 306058 (D), KU 326359 (E), and KU 326358 (F). Scale bars = 10 mm.
Comparison of palmar view of the left hand (left) and plantar view of the left foot (right) of adult male Limnonectes cassiopeia, new species, holotype (PNM 9871; A) and adult male L. macrocephalus (KU 345550; B). Scale bar = 10 mm.
Holotype specimen of Limnonectes macrocephalus (FMNH 40519; adult male) showing dorsal (A) and ventral (C) views of the body, the plantar view of the left foot (B), and the palmar view of the left hand (D). Scale bars = 20 mm (A, C) and 10 mm (B, D).
Dorsal variation among adult male (top row) and adult female (bottom row) paratypes of Limnonectes cassiopeia, new species. Specimens depicted are PNM 7488 (A), KU 327466 (B), KU 326045 (C), KU 329132 (D), PNM 7608 (E), PNM 7612 (F), PNM 7487 (G), KU 326043 (H), KU 327467 (I), and PNM 7610 (J). Scale bar = 20 mm.
Contributor Notes
Associate Editor: B. L Stuart.