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This study describes the microscopic anatomy of the reproductive systems of the …


Biology Articles » Anatomy & Physiology » Anatomy, Animal » Microscopic Anatomy of the Reproductive System in Two Sympatric Species of Fissurella Bruguiére, 1789 (Mollusca: Vetigastropoda) » Discussion

Discussion
- Microscopic Anatomy of the Reproductive System in Two Sympatric Species of Fissurella Bruguiére, 1789 (Mollusca: Vetigastropoda)

In both sexes in F. latimarginata and F. cumingi the germ cell line in the gonad presents a centrifugal organization like in the vetigastropod Tegula tridentata (Potiez & Michaud, 1838) (Brown, 1992). The gametes of F. latimarginata and F cumingi travel from the gonad to the right kidney through the renopericardial duct. The similar organization of the reproductive system between the two species does not show differences useful for making unequivocal anatomical distinctions between them. The histological detail of the genital duct (Figs. 3G, H) also fails to reveal interspecific differences in relation to find an isolating reproductive mechanism of morphophysiological type. Consequently, the reproductive system has therefore little value as taxonomic tool at the species level. Anatomical characteristics of the reproductive systems of these species could be useful in making differentiations at higher taxonomic levels or between less closely related species; however, differences in sperm morphology have shown to be useful to distinguish between both species (Collado & Brown, 2003).

Relations between the reproductive and excretory systems of the Fissurellidae were controversial until now. In the Ihering (1877) model, the gonad of Fissurella opens to the right kidney from which there is no renopericardial connection. Boutan (1885) was unable to find a renopericardial connection in Fissurella reticulata (da Costa) [= Diodora apertura (Montagu)], nor did Erlanger (1892) in five species of Fissurella. Boutan however described a "glande annexe" in the internal wall of the genital duct mF. reticulata females. According to Boutan and Erlanger the gonad opened to the right kidney. A renopericardial duct was recognized by Haller (1886, 1906) in a Fissurella species. In the Perrier (1896) model, the Fissurella gonad opened to the right kidney, in which there was a renopericardial connection. Ziegenhorn & Thiem determined that the reproductive system in Fissurella crassa Lamarck, 1822 consisted of a gonad that opened to the right kidney through a gonoduct which contained a gland in its internal wall, and which was also observed in females of Fissurella oriens Sowerby, 1835 (under Fissurella alba Philippi, 1845) and Fissurella limbata Sowerby, 1835. For Ziegenhorn & Thiem the gland was the same as that of Boutan and was also found in Fissurella (= Diodora) nubécula (Linnaeus, 1758) males. Ziegenhorn & Thiem found a renopericardial duct in all the species which they studied. In the Pelseneer (1898) model, the Fissurella gonad opened to the renopericardial duct and the gametes were evacuated through the right renal aperture. More recent studies, and reviews made by Gabe (1951), Fretter & Graham (1962), Delhaye (1976) and Haszprunar (1988) agreed with the Pelseneer observations. The Voltzow (1994) model, based on the literature, shows two renopericardial connections, one for each kidney, with the gonad opening to the right kidney.

The results of the present study show that in F. ¡atimarginata and F. cumingi the gametes are transported to the distal region of the right kidney through the renopericardial duct, which functions as a genital duct, as well as draining the pericardium. In this manner, the renopericardial duct represents a "nephrogonoduct" if we follow the terminology correctly assumed by Pelseneer for this double functional duct. No connection was observed between the gonad and the small left kidney in both F. ¡atimarginata and F. cumingi, which also discharges its contents to the mantle cavity. Andrews (1985), however, described two renopericardial connections in Emarginula reticulata Sowerby, 1813, one for each kidney, the left one revealed ultrastructurally. Fretter (1984) mentioned that the kidney, pericardial cavity, gonad, and gonoduct were coelomic in origin, and that the connections between them remained in some adult prosobranchs, just as in the case of the two species considered in the present study.

The anatomy of the reproductive system in F ¡atimarginata and F. cumingi is similar to that described by Ziegenhorn & Thiem for Fissurella species. A gland of unknown function associated with the genital duct like that described by these authors is also found in F ¡atimarginata and F cumingi, both in male and females. The gland present in other fissurelids seems to be related with the pattern of development of the species; adding extra material to the gametes for clustering the oocytes during spawning in females (Boutan), or packaging the sperms in spermatophores in males (Medem, 1945). In the present study, we were unable to know if the gland adds extra material to the gametes. Another interesting possibility that could not be discarded is that secretions from the gland had a communication role, releasing a chemical like a sex pheromone to mediate an appropriate gregarious response and gamete evacuation from conspecifics. The simultaneous hermaphrodite nudibranch mollusk Aplysia with internal crossed fertilization, produces water-borne attractin pheromones in the oviduct and are released during egg laying (Cummins et al., 2005). Attractins are a family of small proteins in invertebrates that stimulates the formation and maintenance of mating and egg-laying aggregations (Cummins et al.). It is possible that the gland, as a specialization of the gonoduct in Fissurella, could produce substances like attractins acting as chemical signals in sex communication to induce aggregation and release of gametes improving external fertilization. Although no gland has been associated with the gregarious behavior, sexual aggregation associated with gamete release has been described in vetigastropod species (Fretter & Graham, 1962).

Fissurella is one of the youngest genera within the Family Fissurellidae (McLean, 1984). Local species of Fissurella in the Chilean coast appeared in the Pliocene, and increased diversity in the Pleistocene (Herm, 1969). Although these species may have evolved structures adapted for radiating to new habitats in this relatively short geological period, this would not be the case for the reproductive system, which probably requires a longer time period for greater specialization and differentiation.

ACKNOWLEDGEMENTS

We thank Drs. Bernardita Campos and Doris Oliva for their valuable comments related to preparation of this MS.


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