Largest of All Electric-Fish Snouts: Hypermorphic Facial Growth in Male Apteronotus hasemani and the Identity of Apteronotus anas (Gymnotiformes: Apteronotidae)
South American gymnotiform electric fishes exhibit sexual dimorphism of shape within species, and divergence of shape among species. Recent collecting in floating vegetation mats near Manaus, Brazil, yielded a remarkable association of female and “normal” males of Apteronotus hasemani plus a series of sexually mature male specimens with greatly hypertrophied snouts and gapes. We argue that these fish represent a single species based on shared distinctive features of morphology and coloration, continuous variation of morphometric characters including allometric and dimorphic facial growth in males, ecological and possible reproductive association, and identity in 16S mt rDNA sequences. The degree of dimorphism shown by the large males greatly exceeds previously known limits of intraspecific variation for A. hasemani. The males with the most extreme snouts and gapes closely approach the holotype of Apteronotus anas that is also a mature male. We conclude that A. anas is based on a large male of A. hasemani; the older-named A. hasemani is the senior synonym. Os Gymnotiformes, peixes elétricos da América do Sul, apresentam variação morfológica não só interespecífica como também intraespecífica, ou seja entre os sexos de uma mesma espécie. Recentemente foi coletado, em capins flutuantes próximos a Manaus, Brasil, uma notável associação de fêmeas e machos “normais” de Apteronotus hasemani, juntamente com uma série de indivíduos machos com focinho e abertura bucal hipertróficos. Nós deduzimos que esses indivíduos representam uma única espécie dimórfica, baseado nas características morfológicas distintas compartilhadas entre eles e na contínua variação morfométrica dos caracteres ligados ao sexo, que incluem a alometria e o crescimento da face. Da mesma forma, estes peixes compartilham a coloração do corpo, uma associação ecológica e possivelmente reprodutiva, e ainda a identidade na sequência do rDNA no 16S. O grau de dimorfismo apresentado pelos machos grandes excede os limites de variação previamente conhecidos para A. hasemani e aproximam-se da descrição do holótipo de Apteronotus anas, que também é um macho maduro com focinho longo e abertura bucal grande. Consequentemente, concluímos que A. anas foi descrita baseada em um exemplar macho, grande e nitidamente dimórfico de A. hasemani; e que A. anas deve ser considerada sinônima de A. hasemani.Abstract
Resumen
(A) Series of Apteronotus hasemani (INPA 13996) showing sex dimorphism and variation of snout in males; lower two fish female; upper four fish male. (B) Upper specimen, holotype of Apteronotus anas (CAS 56510 ex IU 15433), and two lower specimens, large males A. hasemani (INPA 13996, INPA 7680)
Scatterplots of facial length versus nonfacial measurements for Apteronotus hasemani (measurements in millimeters). (A) Head length versus interorbital distance. Dotted lines show limits of the ratio of head length/interorbital distance that is traditionally used to distinguish relatively short-snouted (< 4.75) from relatively long-snouted species (> 5). Squares = female (bold face square = CAS 15425); circles = males (upper open circle = CAS 15434, lower open circle = CAS 15425, see arrows), and holotype of Apteronotus anas (black diamond). (B) Distance from eye to opercle versus snout length. squares = female (bold face square = CAS 15425); triangles = small males; circles = large males (upper open circle = CAS 15434, lower open circle = CAS 15425, see arrows), and holotype of A. anas (black diamond). (C) Log distance from eye to opercle versus log snout length, with least squares regression lines fitted separately to 43 females, 6 small males, and 31 large males; dashed line = slope of 1 (isometry); see Table 1 for regression statistics. Squares = female (bold face square = CAS 15425); triangles = small males; circles = large males (upper open circle = CAS 15434, lower open circle = CAS 15425, see arrows), and holotype of A. anas (black diamond). The four specimens from museums were not included in the regression analysis
Log snout length versus sexual maturity stage assessed by size of the gonads (measurements in mm); Apteronotus hasemani; squares = female, circles = male, black diamond = holotype of Apteronotus anas)
Graphical results of principal components analyses of size and facial shape in Apteronotus hasemani (measurements in millimeters); squares = female, circles = male. (A) Ordination of specimens by their scores on first two principal components (PC1, PC2) based on all 12 measurements and sexes combined. (B) Ordination of specimens by their scores on first two principal components (PC1, PC2) based on five postocular measurements and sexes combined, PC I referred to as “face-free size.” (C) “Face-free” size (PC-I) for sexes combined versus log snout length
Scatterplots of facial shape versus nonfacial measurements for male Apteronotus hasemani (circles) compared to holotype of Apteronotus anas (diamond; measurements in millimeters), with least-squares regression lines and t-test results for comparison of residuals. (A) Log eye to opercle versus log snout length. (B) Log eye to opercle versus log eye to posterior naris. (C) Log interorbital distance versus log snout length. (D) Log interorbital distance versus log eye to posterior naris