Low Genetic and Morphological Differentiation Within Brachyalestes lateralis in Southern Africa (Characiformes: Alestidae), and a Redescription of Brachyalestes humilis from the Kwanza River System
Several southern African freshwater fishes have distribution patterns that defy conventional distribution boundaries because they are either characterized by disjunct ranges or have unexpectedly broad distribution ranges. Accurate delimitation of species boundaries and mapping of distribution ranges is a critical requirement to gain insights on the mechanisms that gave rise to these peculiar distribution patterns. This study applied an integrative taxonomic approach incorporating molecular analysis and detailed morphological examination to investigate potential hidden diversity within a widely distributed freshwater fish, Brachyalestes lateralis, in southern Africa. Samples collected from representative populations of this species showed shallow genetic divergence (0.0–1.0%) in the mitochondrial cytochrome c oxidase I (COI) marker and no geographic structuring. Morphological data did not reveal consistent differences between populations of this species. Findings from this study indicate that the synonymy of Brachyalestes natalensis (described from KwaZulu-Natal Province of South Africa), Brachyalestes thamalakanensis, and Brachyalestes langi (both described from the Okavango River system) with B. lateralis (described from Lake Dilolo in the Zambezi River Basin) should be sustained. The study further clarifies the identity of the Kwanza River system specimens that had been tentatively assigned to B. lateralis due to similar morphology and color pattern. This study ascertains that B. lateralis does not occur in the Kwanza River system. The most recently collected specimens from this system were found to be conspecific with Brachyalestes humilis, a species that has rarely featured in the ichthyological literature from the region. A redescription of B. humilis is provided to aid future identification and distinction with congeneric species.
Distribution of specimens and genetic samples of Brachyalestes lateralis and B. humilis used in this study.
(A) Illustration of point-to-point measurements and counts performed in this study. Morphometrics included: total length (TL), standard length (SL), head length (HL), snout length (SNL), orbit diameter (OD), postorbital length (POL), pre-dorsal length (PreDL), post-dorsal length (PostDL), pre-pelvic length (PrePL), pre-anal length (PreAL), dorsal-fin base (DFB), dorsal-fin height (DFH), anal-fin base (AFB), body depth (BD), caudal-peduncle length (CPL), caudal-peduncle depth (CPD), pectoral fin to pelvic fin (PPF), pelvic fin to anal fin (PFA), dorsal fin to adipose fin (DA), and adipose fin to caudal fin (AC). (B) Meristic counts included: lateral line scales (LL), lateral line to dorsal fin (LD), lateral line to anal fin (LA), scales around the caudal peduncle (CS), dorsal-fin base scales (DBC); pre-dorsal scales (DS), scales between dorsal and adipose fins (AD), scales between adipose and caudal fin (CA), and symphysis to pelvic fin (SP).
Bayesian inference tree of the COI gene for samples of Brachyalestes from southern Africa. Node colors indicate Bayesian posterior probabilities: black ≥ 0.95; gray < 0.95. The black bars represent candidate species identified by the species delimitation methods that include: (A) Bayesian implementation of the Poisson tree processes (bPTP), (B) general mixed Yule coalescent (GMYC), (C) automatic barcode gap discovery (ABGD), and (D) assemble species by automatic partitioning (ASAP). See Data Accessibility for tree file.
Maximum likelihood inference tree of the COI gene for samples of Brachyalestes from southern Africa. Node colors indicate bootstrap values: black ≥ 70; gray < 70. The black bars represent candidate species identified by the species delimitation methods that include: (A) Bayesian implementation of the Poisson tree processes (bPTP), (B) general mixed Yule coalescent (GMYC), (C) automatic barcode gap discovery (ABGD), and (D) assemble species by automatic partitioning (ASAP). See Data Accessibility for tree file.
(A) Principal component analysis of the meristic characters of specimens of Brachyalestes lateralis and B. humilis from southern Africa. (B) Principal component analysis of the measured characters of specimens of Brachyalestes lateralis and B. humilis from southern Africa. Brachyalestes humilis (inverted gray triangles) from the Kwanza River, syntypes of B. lateralis (open circles), B. lateralis from the Congo River (asterisks), B. lateralis from the Kunene River (black squares), B. lateralis from the Okavango River (diamonds), B. lateralis from the Zambezi River (black circles), B. lateralis from the Mkuze River (black triangles), and syntype of B. natalensis (open triangle).
Scatterplot of meristic and morphometric characters that showed clear separation between the Brachyalestes lateralis and B. humilis from southern Africa specimens examined in the study. (A) Number of dorsal-fin rays. (B) Number of scales between the dorsal and adipose fins. (C) Interorbital width. (D) Caudal-peduncle depth. Brachyalestes humilis (inverted gray triangles) from the Kwanza River, syntypes of B. lateralis (open circles), B. lateralis from the Congo River (asterisks), B. lateralis from the Kunene River (black squares), B. lateralis from the Okavango River (diamonds), B. lateralis from the Zambezi River (black circles), and B. lateralis from the Mkuze River (black triangles).
(A) Brachyalestes humilis (SAIAB 84635, 82 mm SL) had three premaxillary (1–3) teeth in the outer row. (B) Brachyalestes lateralis (SAIAB 120105, 74 mm SL) had four premaxillary (1–4) teeth in the outer row. Premaxilla (pm), maxilla (m), and dentary (de). Scale bars = 5 mm.
(A) Brachyalestes humilis, holotype, NHMUK 1905.5.29.66; (B) Brachyalestes humilis, holotype, x-ray radiograph. Photographs copyright The Trustees of the Natural History Museum, London (used with permission). Scale bars = 1 cm.
Recently collected specimens of Brachyalestes humilis from the Kwanza River: (A) male B. humilis, SAIAB 84635; (B) female B. humilis, SAIAB 84635; (C–D) field photographs of live specimens, SAIAB 84635. Scale bars = 1 cm.
Contributor Notes
Associate Editor: R. E. Reis.