Intromittent Organ in the Genus Monotocheirodon (Characiformes: Characidae)

We herein document intromittent organs in four Peruvian populations of inseminating fishes of the family Characidae, genus Monotocheirodon, which may comprise new species. All females histologically analyzed from these populations, as well as a female of Monotocheirodon pearsoni, had spermatozoa within the ovary, confirming insemination. Male M. pearsoni produced aquasperm with spherical nuclei (diameter 1.6 μm), with no intromittent organ. On the other hand, males from the Peruvian populations of Monotocheirodon produced spermatozoa with slightly ovoid to elongate nuclei (lengths 1.8–4.1 μm) and possessed distinct intromittent organs. Some populations with more elongate sperm nuclei had longer intromittent organs. Each intromittent organ consisted of an elongate extension of the body wall encompassing an extension of the common sperm duct and urinary duct, which joined with one another near the base of the organ and opened to the outside at the pointed tip of the organ. Skeletal muscle cells are present throughout the organ. At the base, the skeletal muscle is oriented around the organ (circular) possibly forming a sphincter. The skeletal muscle along the organ is mainly oriented longitudinally, although some circular bundles are present. These males are apparently able to manipulate the organs by means of this muscle. The wall of the sperm duct within the organ is folded and lined with a low cuboidal epithelium. The posterior portions of the testes of males with the longest intromittent organs lack spermatogenic tissue and serve as open storage regions for mature spermatozoa.

Figure 1

Longitudinal sections through ovaries from mature female Monotocheirodon species. (A) Monotocheirodon pearsoni, CAS 59792, 32.1 mm SL. (B) Monotocheirodon sp., MUSM 11082, 36.7 mm SL. (C) Monotocheirodon sp., MUSM 6756, 38.0 mm SL. (D) Monotocheirodon sp., MUSM 11250, 33.5 mm SL. Arrows, spermatozoa; po, previtellogenic oocyte; vo, vitellogenic oocyte. Bar = 25 µm.

Figure 2

Detached intromittent organs with testes attached for mature males from two Peruvian populations of Monotocheirodon sp. (A) MUSM 11250, 30.5 mm SL. (B) ANSP 143791, 33.9 mm SL. Arrows, intromittent organs; t, testis; bw, body wall. Bar = 1 mm.

Figure 3

Testis and intromittent organ of Monotocheirodon sp., ANSP 143791, 33.9 mm SL. (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.

Figure 4

Sections through the intromittent organ of Monotocheirodon sp., ANSP 143791, 33.9 mm SL. (A) Mid-sagittal section through base of intromittent organ showing a possible sphincter of circular skeletal muscle (arrows); e, exterior; s, sperm duct. Bar = 100 µm. (B) Longitudinal section through proximal third of organ showing a mixture of longitudinal (l) and circular (c) skeletal muscle. Bar = 25 µm. (C) Longitudinal section through middle third of organ with mainly longitudinal skeletal muscle (arrows); l, lumen of sperm duct. Bar = 50 µm. (D) Longitudinal section through distal third of organ (one side only) showing the outer epithelium (oe) that covers the organ, and the luminal epithelium (le) that lines the sperm duct; l, lumen; bv, blood vessel; e, exterior. Bar = 50 µm.

Contributor Notes

(JRB) Department of Biological Sciences, George Washington University, Washington, D.C. 20052; and (SHW) Division of Fishes, Department of Vertebrate Zoology, MRC 159, National Museum of Natural History, P.O. Box 37012, Smithsonian Institution, Washington, D.C. 20013-7012. E-mail: (JRB) jrburns@gwu.edu; and (SHW) Weitzman.Stan@NMNH.SI.EDU. Send reprint requests to JRB.

Received: 01 Aug 2005

Accepted: 16 Apr 2006