DOI: 10.5061/dryad.sqv9s4ndh
title: Data from: Gill evolution in Neotropical electric fishes: Comparative phylogenetic evidence for hypoxia-driven adaptation

description: Tropical freshwater fishes are often exposed to hypoxia, which limits
 aerobic metabolism and drives the evolution of diverse physiological,
 behavioral, and morphological adaptations. In species from chronically
 hypoxic habitats, gill morphology is frequently modified through traits
 that increase branchial surface area. However, the adaptive basis of these
 traits and the evolutionary origins of hypoxia tolerance remain poorly
 understood, partly because few studies have examined species-rich clades
 in a phylogenetic context. The Neotropical electric knifefish
 family Hypopomidae, which inhabits environments with a broad range of
 dissolved oxygen (DO) conditions, represents an ideal model for examining
 respiratory adaptations to DO variability. Using phylogenetic comparative
 methods, we examined correlations between environmental DO and gill
 morphology in 27 species, focusing on three independently varying traits
 associated with branchial surface area: mean filament length (MFL), mean
 hemibranch length (MHL), and total filament number (TFN). Species
 specialized to hypoxic habitats exhibited significantly greater MFL (but
 not MHL or TFN) after phylogenetic corrections, supporting an adaptive
 basis for increased gill filament length. In contrast, species specialized
 to normoxic habitats had lower MFL, likely reflecting trade-offs involving
 the energetic costs of gill development or functional constraints from
 adjacent morphological structures. Many hypopomid species are
 eurytopic, with demes inhabiting both normoxic and hypoxic habitats. Like
 species specialized to hypoxic systems, hypoxia-tolerant demes exhibit
 elevated MFL, but not MHL or TFN, suggesting a shared pathway for hypoxia
 tolerance across different evolutionary scales. Ancestral state
 reconstructions indicate that ancestors with lower MFL occupied normoxic
 environments, whereas those with elevated MFL likely evolved in hypoxic
 systems. A marked transition to greater MFL, accompanied by a simultaneous
 shift in ancestral habitat, occurs at the base of
 the Brachyhypopomus beebei group. The emergence of
 hypoxia tolerance and the onset of diversification in the B.
 beebei group occurred approximately 3.9 Ma, coincident with a Pliocene
 expansion of seasonally hypoxic Amazon whitewater floodplains—likely
 driven by increased Andean sedimentation during the Amazon’s transition to
 its modern transcontinental course. Our findings highlight how
 environmental changes shaped by ancient geological events can drive
 ecomorphological innovation, potentially accelerating species
 diversification.
description: Following Chapman et al. (2008), three independently varying
 morphological traits correlated with total lamellar surface area were
 measured: (1) <em>Mean filament length</em> (MFL)—the average
 length of gill filaments across the eight left hemibranchs; (2)
 <em>Mean hemibranch length</em> (MHL)—the average distance
 between the bases of the first and last gill filaments, calculated across
 all eight left hemibranchs; (3) <em>Total filament numb</em>er
 (TFN)—the total count of primary gill filaments on the eight left
 hemibranchs multiplied by two. Filament counts were taken at the tapering
 end (every second filament) and central portion (every fifth filament),
 with the remainder interpolated (Fig. 1). All specimens
 were fixed in 10% formalin and preserved in 70% ethanol. Only adult
 specimens, defined as those exceeding the minimum size for gonadal
 expression (Crampton<em> et al.</em> 2016) were measured. Body
 mass was recorded to ±0.01g using a digital scale after blotting excess
 ethanol. The branchial arches were dissected, and the left and medial
 hemibranchs from each arch were mounted on slides and imaged with an
 SDCM-3 digital camera attached to a Meiji-RZ stereo microscope fitted with
 an ocular micrometer. Measurements were obtained using Motic 2.0 software
 (www.motic.com). <strong>References:</strong> Chapman, C.A., Albert, J.S. & Galis, F. (2008) Developmental plasticity, genetic differentiation,and hypoxia-induced trade-offs in an African cichlid fish. <em>The Open Evolution Journal,</em> <strong>2,</strong> 75-88. DOI: 10.2174/1874404400802010075. Crampton, W.G.R., de Santana, C.D., Waddell, J.C. & Lovejoy, N.R. (2016) A taxonomic revision of the Neotropical electric fish genus <em>Brachyhypopomus </em>(Ostariophysi: Gymnotiformes: Hypopomidae), with descriptions of 15 new species. <em>Neotropical Ichthyology,</em> <strong>14,</strong> 639-790. DOI: 10.1590/1982-0224-20150146.
description: # Gill evolution in Neotropical electric fishes: Comparative phylogenetic
 evidence for hypoxia-driven adaptation
 [https://doi.org/10.5061/dryad.sqv9s4ndh](https://doi.org/10.5061/dryad.sqv9s4ndh) ## Description of the data and file structure Raw gill and body measurements, locality data, and biodiversity collection data for 248 hypopomids and 10 outgroup specimens, taxon mean gill measures for 32 hypopomid species. **Files and variables:** #### File: Crampton\_etal\_Functional\_Ecology\_data.csv #### File: Crampton\_etal\_Functional\_Ecology\_taxon\_means.csv **Description:**  Crampton_etal_Functional_Ecology_data.csv: species, dissolved oxygen and specialization categories, body measurements, gill trait measurements (raw and log10-transformed), collection data, biodiversity collection data. Crampton_etal_Functional_Ecology_taxon-means: log10 taxon-mean values for hypopomid gill traits at a standardized log10 body mass of 0.526 g (average mass for all 32 hypopomid species) ##### Variables Raw gill and body measurements, locality data, and biodiversity collection data for 248 hypopomid and 10 outgroup specimens: Habitat categories: H = hypoxic, N = normoxic Specialization categories: E = eurytopic, S = stenotopic Body measurements: TL                = Total length (mm) LEA             = Length to end of anal fin (snout to anal-fin terminus) (mm) Mass             = Total body mass of specimen stored in 70% ethyl ethanol (g) Gill Trait:      MHL            = Mean hemibranch length TFN              = Total filament number MFL             = Mean filament length Institutional acronyms for biodiversity collections: ZMH            = Centrum für Naturkunde, Zoologisches Museum, Universität Hamburg, Germany FMNH          = Field Museum of Natural History, Chicago, USA MBUCV-V   = Museo de Biología de la Universidad Central de Venezuela, Caracas, Venezuela MUSM         = Museo de Historia Natural, Universidad Nacional Mayor de San Marcos, Lima, Peru MCP             = Museu de Ciências e Tecnologia, Pontifícia Universidade Católica do Rio Grande do Sul, Porto Alegre, Brazil MZUSP        = Museu de Zoologia da Universidade de São Paulo, Brazil USNM          = Smithsonian Institution, National Museum of Natural History, Washington DC, USA NMR            = Swedish Museum of Natural History, Stockholm, Sweden UMSS           = Universidad Mayor de San Simón, Cochabamba, Bolivia UF                = University of Florida, Florida Museum of Natural History, Gainesville, USA KU               = University of Kansas University Biodiversity Institute, Lawrence, USA TNHC          = University of Texas Biodiversity Collections, Austin, USA. uncat.            = uncataloged. Species authors: | Family | Genus | species | Author/s | | :--------------- | :------------------- | :-------------- | :--------------------------------------- | | Hypopomidae | *Brachyhypopomus* | *alberti* | Crampton, De Santana, Waddell, & Lovejoy | | Hypopomidae | *Brachyhypopomus* | *arrayae* | Crampton, De Santana, Waddell, & Lovejoy | | Hypopomidae | *Brachyhypopomus* | *batesi* | Crampton, De Santana, Waddell, & Lovejoy | | Hypopomidae | *Brachyhypopomus* | *beebei* | (Schultz) | | Hypopomidae | *Brachyhypopomus* | *benjamini* | Crampton, De Santana, Waddell, & Lovejoy | | Hypopomidae | *Brachyhypopomus* | *bennetti* | Sullivan, Zuanon & Fernandes | | Hypopomidae | *Brachyhypopomus* | *bombilla* | Loureiro & Silva | | Hypopomidae | *Brachyhypopomus* | *brevirostris* | (Steindachner) | | Hypopomidae | *Brachyhypopomus* | *bullocki* | Sullivan & Hopkins | | Hypopomidae | *Brachyhypopomus* | *diazae* | (Fernández-Yépez) | | Hypopomidae | *Brachyhypopomus* | *draco* | Giora, Malabarba, & Crampton | | Hypopomidae | *Brachyhypopomus* | *flavipomus* | Crampton, De Santana, Waddell, & Lovejoy | | Hypopomidae | *Brachyhypopomus* | *gauderio* | Giora & Malabarba | | Hypopomidae | *Brachyhypopomus* | *hamitoni* | Crampton, De Santana, Waddell, & Lovejoy | | Hypopomidae | *Brachyhypopomus* | *hendersoni* | Crampton, De Santana, Waddell, & Lovejoy | | Hypopomidae | *Brachyhypopomus* | *janeiroensis* | Costa & Campos-da-Paz | | Hypopomidae | *Brachyhypopomus* | *occidentalis* | (Regan) | | Hypopomidae | *Brachyhypopomus* | *palenque* | Crampton, De Santana, Waddell, & Lovejoy | | Hypopomidae | *Brachyhypopomus* | *pinnicaudatus* | (Hopkins, Comfort, Bastian & Bass) | | Hypopomidae | *Brachyhypopomus* | *provenzanoi* | Crampton, De Santana, Waddell, & Lovejoy | | Hypopomidae | *Brachyhypopomus* | *regani* | Crampton, De Santana, Waddell, & Lovejoy | | Hypopomidae | *Brachyhypopomus* | *sullivani* | Crampton, De Santana, Waddell, & Lovejoy | | Hypopomidae | *Brachyhypopomus* | *verdii* | Crampton, De Santana, Waddell, & Lovejoy | | Hypopomidae | *Brachyhypopomus* | *walteri* | Sullivan, Zuanon & Fernandes | | Hypopomidae | *Hypopomus* | *artedi* | (Kaup) | | Hypopomidae | *Microsternarchus* | *bilineatus* | Fernández-Yépez | | Hypopomidae | *Racenesia* | *fimbriipinna* | Mago-Leccia | | Rhamphichthyidae | *Gymnorhamphichthys* | *rondoni* | (Miranda Ribeiro) | | Rhamphichthyidae | *Hypopygus* | *lepturus* | Hoedeman | | Rhamphichthyidae | *Rhamphichthys* | *heleios* | Carvalho & Albert | | Rhamphichthyidae | *Steatogenys* | *duidae* | (La Monte) |