Editorial Type:
Article Category: Research Article
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Online Publication Date: 31 Jul 2020

Two New Species of Gymnotus (Gymnotiformes: Gymnotidae) from Brazil and Historical Biogeography of the Subgenus Lamontianus

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Page Range: 468 – 484
DOI: 10.1643/CI-19-205
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Gymnotus is the most species-rich and geographically widespread genus of gymnotiform electric fishes and has been widely explored to understand mechanisms of diversification in Neotropical freshwater fishes at a continental scale. Within Gymnotus, the subgenus Lamontianus is a phenotypically distinctive clade with an elongate, cylindrical body shape currently known from four valid species (G. anguillaris, G. cataniapo, G. pedanopterus, and G. tiquie) restricted to rivers draining the Guiana Shield. Here we use aspects of body-surface coloration, meristic, morphological, and osteological data, including cranial, laterosensory pore, and postcranial characters, to diagnose two new species of Lamontianus that inhabit the Aripuanã and Arapiuns rivers that drain the Brazilian Shield. We also use geometric morphometric analyses of head shape to separate the new species from one another and other species of Lamontianus. We report biogeographic distributions for all species of Lamontianus and estimate ancestral geographic ranges and range evolution using the parametric biogeographic program BioGeoBEARS. We use the phylogeny of Lamontianus to test alternative hypotheses regarding lineage divergence times, before or after the formation of the modern East-draining Amazon at c. 10 Ma. Our analysis suggests that diversification in Lamontianus occurred primarily by geographic range fragmentation (vicariance) from an ancestral species distributed across the Western Guiana Shield. These results are similar to those of other Gymnotus and gymnotiform clades, where allopatric speciation and secondary contact due to geographic range expansion are commonly observed. This study brings to 46 the number of valid species of the genus Gymnotus, and to six the number of valid species of the subgenus Lamontianus.

Copyright: © 2020 by the American Society of Ichthyologists and Herpetologists
Fig. 1.
Fig. 1.

Collection localities of species in the Gymnotus subgenus Lamontianus in northern South America. Species inhabit basins draining the Guiana and Brazilian shields in clear and blackwater rivers, and are not collected in whitewater rivers of the Western Amazon or Orinoco lowlands. Stars indicate type localities.

Fig. 2.
Fig. 2.

Shape plots of 2D-GM analysis of head in lateral view of adult and subadult Gymnotus aripuana, new species (n = 12, 122–295 mm TL). Analysis in MorphoJ. (A) Among individual variation of landmark positions (small dots) and consensus points (numbered circles) after Procrustes superimposition. (B) Result of PCA displaying PC1 using a ball-and-stick model. This image illustrates the transposition of individual landmarks on the X- and Y-axes, using the mean distribution of the landmark positions as a starting point (ball), and drawing a best-fit line through the remaining landmark positions (Klingenberg, 2011).

Fig. 3.
Fig. 3.

Phylogeny of the Gymnotus subgenus Lamontianus from Craig et al. (2019). Branch lengths inversely proportional to species richness.

Fig. 4.
Fig. 4.

Gymnotus arapiuns, holotype, ZUEC 16727, 155 mm TL, from the Rio Arapiuns, Tocantins drainage. Scale bars = 10 mm.

Fig. 5.
Fig. 5.

CT scan of a paratype specimen of Gymnotus arapiuns (ZUEC 16392, 158 mm TL). (A) Whole body. Note relatively small head in relation to total body length, the elongate body cavity with 55 pre-caudal vertebrae, and relatively short anal-fin pterygiophores about the same length as associated hemal spines. White-dashed line indicates ventral margin of anal-fin pterygiophores at ventral body margin. (B) Close-up of head and cervical regions. Note dorso-ventrally compressed wedge-shaped anterior portion of neurocranium, well-ossified lamellar structure of dermatocranium, and relatively short and deep mandible with numerous conical dentary teeth. (C) Close-up of caudal region. Arrow indicates last pre-caudal vertebrae at posterior margin of body cavity. Note damaged, non-regenerated caudal appendage.

Fig. 6.
Fig. 6.

Gymnotus arapiuns color in life (ZUEC 16393, 120 mm TL). Photo credit: Flávio C. T. Lima.

Fig. 7.
Fig. 7.

Holotype locality for Gymnotus arapiuns, Igarapé Jararaca, near community of Cachoeirinha do Mentai, aff. Rio Mentai, Rio Arapiuns, Rio Tapajos, 02°43′48.5″S, 055°35′34.6″W. Photo credit: Will Crampton.

Fig. 8.
Fig. 8.

Averaged EOD waveforms (black) for (A) immature, (B) maturing or mature females, and (C) maturing or mature males of Gymnotus arapiuns. Gray waveforms represent individually recorded fish. Arrows report position of the return of the dominant negative P2 phase to the baseline. Note P2 extension in males.

Fig. 9.
Fig. 9.

Log EOD amplitude as a function of log total length in immature, female, and male individuals of Gymnotus arapiuns. For ease of interpretation, the axis labels report the untransformed values (i.e., the original measurements). Solid lines represent ordinary least squares regressions; dashed lines represent 95% confidence intervals. Slopes are homogenized for the males and females due to the non-significant interaction term in the general linear model: log EOD amplitude ∼ log total length * sex.

Fig. 10.
Fig. 10.

Gymnotus aripuana, holotype, INPA 6390, 284 mm TL, from the Rio Aripuanã, Madeira drainage. Scale bars = 10 mm.

Fig. 11.
Fig. 11.

PCA of caliper-based morphometrics of (A) the Gymnotus subgenus Lamontianus (n = 62). Species include: G. anguillaris (n = 6), G. arapiuns (n = 22), G. aripuana (n = 12), G. cataniapo (n = 3), G. pedanopterus (n = 13), and G. tiquie (n = 6). Ellipses represent 95% confidence intervals. Vector loadings of individual measurements are indicated by the brown arrows. Note variance in this plot of PC1 and PC2 explains 63.9% of the total variance. Note also the two new species do not overlap in this PC space, that G. arapiuns does not overlap with other species, and that G. aripuana overlaps only with G. pedanopterus. (B) Comparison between only G. arapiuns (n = 22) and G. aripuana (n = 12) using all specimens including juveniles. Note PC1 is influenced positively by body depth (BD) and body width (BW), and negatively by pectoral-fin length (P1), postorbital distance (PO), mouth width (MW), and anal-fin length (AF). PC2 is positively influenced by head length (HL) and negatively influenced by head width (HW), pre-orbital distance (PR), and head depth (HD).

Fig. 12.
Fig. 12.

Principal components analysis of 15 landmarks for 2D-GM analysis of (A) n = 44 adult specimens representing seven species of the Gymnotus subgenus Lamontianus. Species include: G. anguillaris (n = 9), G. arapiuns (n = 13), G. aripuana (n = 11), G. cataniapo (n = 2), G. pedanopterus (n = 4), and G. tiquie (n = 5). Note diagnostic differences separating G. pedanopterus and G. arapiuns from all other species. Landmarks loading most heavily on PC1 correspond to axial elongation of the head, such that G. pedanopterus has the most dolichocephalic and G. arapiuns the most brachycephalic morphologies. Landmarks loading most heavily on PC2 are located in the posterior region of the head and in the post-cranial region, such that G. pedanopterus and G. arapiuns have the most dorsoventrally compressed head and G. tiquie the deepest head. Gymnotus pedanopterus and G. tiquie exhibit the greatest variation in head-shape morphospace and G. arapiuns the least, as assessed by the confidence ellipses. (B) Comparison between only G. arapiuns (n = 20) and G. aripuana (n = 11). Plot of PC1 and PC2 illustrating non-overlapping distribution of data for the two species. Ellipses represent 95% confidence intervals for means.

Fig. 13.
Fig. 13.

Best supported model output from BioGeoBEARS analysis with the three-area states: Blue = Western Guiana (WG), Green = Eastern Guiana (EG), and Red = Brazilian Shield (BS). Ancestral range estimation is WG, with dispersal events occurring east to EG and south to BS.

Fig. 14.
Fig. 14.

Alternative biogeographic models for the Gymnotus subgenus Lamontianus. Centers of origin (shaded region), dispersal (arrows), and vicariance events (phylograms). Phylogenetic tree overlay is for the Gymnotus subgenus Lamontianus denoting Western Guiana (WG), Eastern Guiana (EG), and the Brazilian Shield (BS) at the nodes. Hypotheses shown are: (A) H1: Center of origin in Eastern Highlands; (B) H2: Center of origin in Western Guiana Shield; (C) H3: Vicariance only; (D) H4: River runs through it.

Contributor Notes

1 Department of Biology, University of Louisiana at Lafayette, P.O. Box 43602, Lafayette, Louisiana 70504; Email: lyk4468@louisiana.edu. Send reprint requests to this address.
2 Department of Biology, University of Central Florida, Biological Sciences Bldg., 4110 Libra Drive, Orlando, Florida 32816-2368; Email: william.crampton@ucf.edu.
3 Department of Biology, University of Louisiana at Lafayette, P.O. Box 43602, Lafayette, Louisiana 70504; Email: jalbert@louisiana.edu.

Associate Editor: R. E. Reis.

Received: 13 Mar 2019
Accepted: 27 Dec 2019
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