the External Genital Structures and Mating Behaviour ...

Abstract

How to Copulate without an Intromittent Organ: the External Genital Structures and Mating Behaviour of Xenotoca eiseni (Goodeidae)

▼ The modified anal fin of male goodeids undoubtedly is a copulation device, but surely does not act as an intromittent organ. Videotapes showed that for copulation males of Xenotoca eiseni approach the female laterally with the anal fin bent towards her. Then both tilt, touching each other with the belly. The male bends the tail towards the female; his anal fin forms a pocket around the female urogenital opening. In this moment transfer of spermatozeugmata must take place. Because the female and male genital openings are directed posteriorly and that of the female is very small, neither parts of the male’s anal fin can be inserted nor can the genital openings be pressed together during copulation, unless animals would mate head to tail with one mate supine. Strong muscle fibres running between the intestine and the male’s ductus spermaticus surround the duct in a semicircle. These fibres are suggested to facilitate ejaculation. Ultrastructurally, these fibres do not differ from the remaining body muscles. Casual observations show that males of X. eiseni attempt copulations with non-cooperative females suggesting the existence of sneakers which gain copulations by rape.

Hartmut Greven and Marcel Brenner

Institut für Zoomorphologie und Zellbiologie der HeinrichHein-Univessität Düsseldorf, Universitätsstr. 1, D-40225 Düsseldorf, Germany.

Introduction Viviparous goodeids practice internal fertilization. The male anal fin is modified in some respect – the reduction of the fin rays 2 to 7 leads to a small anterior and a larger posterior lobe of the fin– and has been called a “gonopodium”, “spermatopodium” or “andropodium” or even “intromittent organ” in the literature (e.g., Mohsen, 1961, 1965; Turner, 1962; Hieronimus, 1995; for review see Meisner, 2005). However, as previously observed in the relatively large Goodea atripinnis, the smaller lobe of the anal fin wraps the female genital pore during copulation forming a pocket. Muscles around the ductus spermaticus (= vas deferens) and the urinary canal called “pseudopenis” or “urogenital organ” seems to produce a spurt of spermiozeugmata into the pocket (Nelson, 1975). These observations suggest that the anal fin of male goodeids is not inserted in the female genital pore and, thus, does not act as an intromittent organ. During the course of our studies on the sexual dimorphic traits and the reproductive biology of the goodeid, Xenotoca eiseni, we used videography and morphological techniques to record physical contacts between mates during copulation. Our observations supplement the courtship of this species as described by Fitzsimons (1976) and Bisazza (1997). Material and Methods We observed and videotaped several copulations and copula-

Viviparous Fishes II Mari Carmen Uribe and Harry Grier, book editors. New Life Publications, Homestead, Florida, 2010. pp. ##-##

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Figure 1a-b. Lateral view of the male (a) and ventral view of the female (b) outer genitals. Genital opening (arrows); anus (larger black arrow). Note the small anterior lobe of the male anal fin (right side in a). Bar = 300 μm

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Fig. 2a Figure 2a-b. 1 µm cross section (LM) of the ductus spermaticus (ds). Note the semicircular arrangement of muscles (arrows) ventral to the duct (a). Bar 100 µm. Low power TEM-micrograph of muscle fibres of the ventral constrictor (b). Bar = 5 µm

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tion attempts of X. eiseni kept in pairs or in mixed groups using a conventional camcorder with high-speed shutter (25 frames/ sec; exposure time 1/2000 sec) and a high speed videocamera (motion pro 500; 250 or 500 frames/sec). Video sequences were processed by a PC (see Brenner and Greven, 1999). For scanning electron microscopy (SEM) the genital regions of a male and a female were fixed (2.5% GA in 0.1 mol/l cacodylate buffer, pH 7.2), critical point-dried, sputtered with gold, and viewed in a Zeiss Leo 1430 VP scanning electron microscope. For light (LM) and transmission electron microscopy (TEM) the urogenital organ of a male was fixed in 2.5% GA in 0.1 mol/l cacodylate buffer, pH 7.2, postfixed in

2% osmium tetroxide, and embedded in Spurr’s resin (Spurr, 1969). Semithin sections (approximately 1.0 μm thick) were stained with toluidine-blueborax. Ultrathin sections were stained with aqueous uranyl acetate and lead citrate and viewed in a Zeiss EM 9S. Results In both sexes the genital opening lies immediately at the base of the anal fin. That of the male is a narrow transverse slit (Fig. 1a), and that of the female is a small rounded pore close to the anus without any noticeable modifications (Fig. 1b). Semithin sections show that the ductus spermaticus and the urinary tract are embedded in a U-shaped mass of muscles, situ-

ated between the intestine and the duct. Muscle fibers are arranged transversely to the longitudinal axis of the fish (Fig. 2a). These muscles have relatively short sarcomeres measuring approx. 2.6 μm in length as shown by ultrathin sections (Fig. 2b). Ultrastructurally, these fibres do not differ from the remaining body muscles (not shown). During courtship, the male approaches the female laterally swinging his anal fin laterally towards the female (see Fig. 3a, b, d). Cooperative females remain stationary and the mates incline their anal fins together. The male bends his tail and pushes it under the tail of the female raising his head upwards. Finally he clasps her genital pore (Fig. 3ac) forming a pocket around the small female urogenital opening.

Mates remain in this position for some seconds on the bottom of the tank or swim in circles. The sexual act ends with the sudden separation of the mates. Duration of the physical contact of mates was approximately 0.61.7 sec. Non-cooperative females quiver with the anterior body, attack the male or flee, but will be often pursued by a male that again swings his anal fin laterally trying to clasp her genital pore (Fig. 4). In most cases, wrapping of the continuously swimming or fleeing female failed, but once we observed that a male succeeded. In this case mates separated violently after an extremely short physical contact. Sectioning of the females did not reveal sperm within the genital tract and showed that she was not pregnant.

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The mating of Xenotoca eiseni described herein resembles that of Goodea atripinnis (Nelson, 1975). Nelson’s (l.c.) study, our observations on X. eiseni and some other goodeids (unpublished) allow for some generalisations to be made regarding the mating process in these fishes, i.e. males approach the female. A male bends his anal fin towards the female and finally clasps her genital pore forming a pocket with his anal fin. Further details were not seen due to the relative small size of X. eiseni and the resolving capacity of the camera. However, Nelson (1975) observed in the larger G. atripinnis that the anterior lobe forms a pocket, while the notch in the male anal fin touches the anterior margin of the female anal fin. In this moment spermatozeugmata should be released into the pocket. The strong muscle fibres running between the intestine

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Discussion

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and the male’s ductus spermaticus are suggested to facilitate ejaculation (Mohsen, 1961, 1965; Nelson, 1965). How sperm reach the urogenital sinus either as spermatozeugmata or already as individual spermatozoa –if spermatozeugmata would be dissolved already in the pocket– is still unknown. Apart from the behavioral observations, there are also morphological reasons to doubt insertion of the male anal fin. Because the male’s genital opening is directed posteriorly, and that of the female is very small, neither parts of the male’s anal fin can be inserted nor can the genital openings be pressed to-

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gether during copulation, unless animals would mate head to tail with one mate supine. The present data confirm that the male’s anal fin is not used as intromittent organ, as for instance various poeciliids do (for further readings see Greven, 2005). Therefore, it is somewhat misleading to call such “copulation devices” intromittent organs “despite the fact that for most groups…the modified structures do not function as true intromittent organ….” (Meisner, 2005; p. 168). In goodeids the male anal fin obviously aids to prevent the loss of spermatozeugmata guiding them to the genital pore. Clasp-

260 ms Figure 3a-d. Selected frames of a copulation (a, b, c), and a copulation attempt (d) recorded with the high speed shutter of a camcorder (a, b) and with a high speed video camera (c-d). Note the position of the male’s anal fin (arrows). Numbers indicate time distance

Hartmut Greven and Marcel Brenner How to Copulate without an Intromittent Organ: the External Genital Structures and Mating Behavior of Xenotoca eiseni (Goodeidae)

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ing the female’s genital region with a more or less modified fin is widespread in oviparous fishes (e.g., Polypterus spp.: Bartsch and Britz, 1996; Mormyridae: Kirschbaum, 1987; Characiformes: for further readings see Meisner, 2005) and is also retained by some viviparous taxa, e.g., some halfbeaks (see Greven 2010, this volume). Therefore, this is character may be considered as plesiomorphic in goodeids. According to Bisazza (1997) this mode of mating and insemination has probably favoured size-assortative mating in X. eiseni. If this assumption is correct, size-assortative mating should hold also for other goodeids, but this has not been tested as yet. Our observations suggest that also goodeids, which normally court and display, execute rape copulations alternatively and that sneakers may exist gaining copulations by coercion, e.g., when the female is not cooperative or is monopolized by a dominant male in species that defend territories, or the male is subdominant, or the general situation for excessive displaying is bad (for various live bearers, see Farr, 1989; Bisazza, 1993; Greven, 2005). In goodeids this has to be examined in detail. In addition, obviously nonpregnant females are not necessarily receptive and/or cooperative (for poeciliids see Greven, 2005). W hether the above mentioned size depending mating (Bisazza, 1997) plays a significant role in cooperation and willingness remains to be investigated.

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Figure 4. The attempt to rape a copulation; the male shows a strong bending. Numbers indicate time distance

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References Bartsch,P., Britz R. 1996. Zucht und Entwicklung von Polypterus ornatipinnis. DATZ 49:15-20. Bisazza A. 1993. Male competition, female mate choice and sexual size dimorphism in poeciliid fishes. Mar Behav Physiol 23:257-286. Bisazza A. 1997. Sexual selection constrained by internal fertilization in the livebearing fish Xenotoca eiseni. Anim Behav 54:1347-1355. Brenner M., Greven H.1999. PC-unterstützte Bearbeitung von Videosequenzen. Acta Biol Benrodis 10:79-90. Farr JA. 1989. Sexual selection and secondary sexual differentiation in poeciliids: Determinants of male mating success and the evolution of female choice. In: Meffe GK, Snelson FF; editors. Ecology and Evolution of livebearing fishes (Poeciliidae). Prentice Hall, Englewood Cliffs, New Jersey, p 91-123. Fitzsimons JM. 1976. A revision of two genera of goodeid fishes (Cyprinodontiformes; Osteichthyes) from the Mexican plateau. Copeia 1972:728-756. Fitzsimons JM. 1976. Ethological isolating mechanisms in goodeid fishes of the genus Xenotoca (Cyprinodontiformes, Osteicthyes). Bull South Cal Acad Sci 75:84-99. Greven H. 2005. Structural and behavioral traits associated with sperm transfer in Poeciliinae. In: Uribe, MC, Grier HJ, editors. Viviparous fishes. New Life Publications, Homestead. Florida, p 145-163. Greven H. 2006. Lebendgebärende Halbschnabelhechte. Morphologische Besonderheiten, Nahrungsaufnahme und Fortpflanzung. In: Greven H, Riehl R, editors. Biologie der Aquarienfische. Tetra-Verlag, Berlin Velten, p 271-296.

Greven H. this issue. What do we know on the reproduction of hemirhamphids. Hieronimus H. 1995. Die Hochlandkärpflinge. Die Neue Brehm Bücherei. Bd. 610. Westarp, Spektrum, Magdeburg and Heidelberg. Kirschbaum F. 1987. Reproduction and development of the weakly electric fish Pollimyrus isidori (Mormyridae, Teleostei) in captivity. Env. Biol. Fishes 20:11-31. Meisner AD. 2005. Male modifications associated with insemination in teleosts. In: Uribe, MC, Grier HJ, editors. Viviparous fishes. New Life Publications, Homestead. Florida, p 165-190. Mohsen T. 1961. Sur la dimorphisme sexuel et la presence d´un organe copulateur tres evolue chez le cyprinodonte Goodeidae Skiffia lermae Meek. Am Fac Sci Univ Dakar 6:163-180. Mohsen T. 1965. A new key character in males of the family Goodeidae (Cyprinodontiformes). Nature 4976:1127. Nelson GG. 1975. Anatomy of the male urogenital organs of Goodea atripinnis and Characodon lateralis (Atheriniformes: Cyprinodontoidei), and G. atripinnis courtship. Copeia 1975:475-482. Spurr AR. 1969. A low viscosity epoxy resin embedding medium for electron microscopy. J. Ultrastruct Res 26:31-43. Turner C. 1962. Development and comparative morphology of the gonopodium of goodeid fishes. Iowa Acad Sci 69:571-586.

Hartmut Greven and Marcel Brenner How to Copulate without an Intromittent Organ: the External Genital Structures and Mating Behavior of Xenotoca eiseni (Goodeidae)

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