Abstract

Three new species of the genus Monotocheirodon, family Characidae, are described. The single previously known species, M. pearsoni, was described by Eigenmann in 1922 from specimens collected in the Rio Beni, Bolivia. In a recent paper Burns and Weitzman (2006) noted that some museum specimens catalogued as M. pearsoni possessed an intromittent organ and that there was some variation in sperm type and gonad structure across the specimens.

The present research focuses on determining the number of distinct Monotocheirodon species already present in museum collections and discerning and describing the morphological differences between them.

Methods and Materials

• All specimens were obtained from museum collections
• Meristic counts were taken of scales, tooth cusps, lateral line pores, vertebrae, gill rakers, and fin rays
• 30 morphological measurements were taken using 1/10mm calipers
• Reproductive characters were analyzed using both published and unpublished histological data
• Inquiries were made into the history of the specimens to clarify their actual locality data

Species Concept

The species concept used in this project was that of Cracraft (1983) as modified by Nixon and Wheeler (1990) according to which a species is a population sample with a unique combination of character states.

Results

• Four distinct species of Monotocheirodon were identified from six localities and eleven lots

• Morphological characters provide a tentative set of relationships with only one atavism

Scientific Illustrations of Monotocheirodon
M. pearsoni Rio Beni drainage, Bolivia No intromittent organ Aquasperm Unmodified pelvic fin 5-7 cusps on some teeth Narrow body Smaller caudal fin scales No aspermatogenic region in testes Found in two localities	M. sp. A Madre de Dios drainage, Peru Short intromittent organ Aquasperm Unmodified pelvic fin 5 cusps on all teeth Deep body Large caudal fin scales No aspermatogenic region in testes Found in one locality	M. sp. C Madre de Dios drainage, Peru Long intromittent organ Elongate sperm Modified pelvic fin 5 cusps on all teeth Deep body Large caudal fin scales Aspermatogenic region present in testes Found in two localities

• Diagnostic factors for the genus: blocky head with nearly horizontal mouth, caudal scales in both sexes

• Distinguishing characters for the genus: lateral stripe, humeral spot, absence of adipose fin, dark pigmentation along the dorsal ridge of the body, large caudal scales, gill glands and a long, narrow caudal peduncle

Distinguishing characters
Caudal scales with posteriormost two enlarged and elongated. The presence of these scales on both caudal fin lobes and in both species is a diagnostic character of the genus.	This five cusp dentition pattern on premaxillary, maxillary and dentary teeth was found in M. sp. A and M. sp. C. The only atavism recognized in this study was an increase in the number of cusps on the maxillary and premaxillary teeth of M. sp. B.

Histology

Testis and intromittent organ of Monotocheirodon sp. C. (A) Sagittal section through the testis and intromittent organ. Arrow, intromittent organ; asterisk, urinary duct; sg, anterior spermatogenic region; st, posterior, aspermatogenic sperm storage region. Bar = 1.0 mm. (B) Higher magnification of oblique section through distal end of intromittent organ showing open tip (arrow). Bar = 0.1 mm. (Burns and Weitzman, 2006)

Discussion

Within this genus there is a remarkably clear gradient of reproductive adaptation. Evolution toward a highly specialized reproductive mode is evident in sperm morphology, the development of the intromittent organ and structures involved in its control (pelvic fin and associated muscles), and gonad structure.

The rarity of Monotocheirodon specimens prohibited us from employing destructive methods such as genetic analysis and clearing and staining, therefore further collection is crucial for continued study. Sampling throughout more of the Madre de Dios drainage and farther down the Beni will permit us to make inferences about the geographic distribution and evolutionary history of this genus. New specimens will provide the opportunity for genetic analysis and live specimens will allow us to study the behavioral adaptations associated with a gradient of reproductive adaptations.

Acknowledgements

This research was supported by a donation to the Research Training Program by the Honorable Max Berry. The first author would like to thank John Burns (GWU) for his input and Stan Rachootin (MHC Biology Department) for his advice, support and guidance.