Abstract
A species-level phylogenetic reconstruction of the Neotropical bluntnose knifefish genus Brachyhypopomus (Gymnotiformes, Hypopomidae) is presented, based on 60 morphological characters, approximately 1100 base pairs of the mitochondrial cytb gene, and approximately 1000 base pairs of the nuclear rag2 gene. The phylogeny includes 28 species of Brachyhypopomus and nine outgroup species from nine other gymnotiform genera, including seven in the superfamily Rhamphichthyoidea (Hypopomidae and Rhamphichthyidae). Parsimony and Bayesian total evidence phylogenetic analyses confirm the monophyly of the genus, and identify nine robust species groups. Homoplastic osteological characters associated with diminutive body size and occurrence in small stream habitats, including loss of squamation and simplifications of the skeleton, appear to mislead a phylogenetic analysis based on morphological characters alone–resulting in the incorrect placing of Microsternarchus + Racenisia in a position deeply nested within Brachyhypopomus. Consideration of geographical distribution in light of the total evidence phylogeny indicates an origin for Brachyhypopomus in Greater Amazonia (the superbasin comprising the Amazon, Orinoco and major Guiana drainages), with subsequent dispersal and vicariance in peripheral basins, including the La Plata, the São Francisco, and trans-Andean basins of northwest South America and Central America. The ancestral habitat of Brachyhypopomus likely resembled the normoxic, low-conductivity terra firme stream system occupied by many extant species, and the genus has subsequently occupied a wide range of terra firme and floodplain habitats including low- and high-conductivity systems, and normoxic and hypoxic systems. Adaptations for impedance matching to high conductivity, and/or for air breathing in hypoxic systems have attended these habitat transitions. Several species of Brachyhypopomus are eurytopic with respect to habitat occupancy and these generally exhibit wider geographical ranges than stenotopic species.
Highlights
The weakly electric ‘bluntnose knifefish’ genus Brachyhypopomus (Gymnotiformes, Hypopomidae) is distributed in lowland freshwaters from southern Costa Rica to southern Uruguay [1,2,3]
Using MrBayes v3.2.2 [37, 38], we conducted a Bayesian Inference (BI) phylogenetic analysis of (1) each gene separately, (2) both genes combined, and (3) both genes combined with morphological data
Bayesian analysis of the cytb and rag2 genes separately (Figs 2 and 3), both genes combined (Fig 4), and both genes combined with morphological data (Figs 5 and 7) all provide strong support for a monophyletic Brachyhypopomus
Summary
The weakly electric ‘bluntnose knifefish’ genus Brachyhypopomus (Gymnotiformes, Hypopomidae) is distributed in lowland freshwaters from southern Costa Rica to southern Uruguay [1,2,3]. Brachyhypopomus species are cryptically pigmented, nocturnally active predators of small aquatic invertebrates, and are small to moderate in size (not known to exceed 461 mm total length [3], and typically < 250 mm [3]). Brachyhypopomus generate species-typical, pulsed weakly electric organ discharges (EODs) of ca. 0.5–5 ms duration [5, 6] These EODs comprise from one to four phases of alternating polarity and are generated at rates of
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