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REPRODUCTIVE CYCLE OF Megalobulimus abbreviatus 459 THE ANNUAL REPRODUCTIVE CYCLE OF THE SNAIL Megalobulimus abbreviatus (BEQUAERT, 1948) (GASTROPODA, PULMONATA) HORN, A. C. M.,1 ACHAVAL, M.1 and ZANCAN, D. M.2 1Laboratório de Histofisiologia Comparada, Departamento de Ciências Morfológicas 2Laboratório de Neurobiologia Comparada, Departamento de Fisiologia, Instituto de Ciências Básicas da Saúde, Universidade Federal do Rio Grande do Sul, Rua Sarmento Leite, 500, CEP 90050-170, Porto Alegre, RS, Brazil Correspondence to: Ângelo Cássio Magalhães Horn, Laboratório de Histofisiologia Comparada, Departamento de Ciências Morfológicas, Instituto de Ciências Básicas da Saúde, Universidade Federal do Rio Grande do Sul, Rua Sarmento Leite, 500, CEP 90050-170, Porto Alegre, Brazil, e-mail: [email protected] Received June 2, 2003 – Accepted November 19, 2003 – Distributed August 31, 2005 (With 1 figure) ABSTRACT Morphological changes in the sexual organs of the pulmonates were observed throughout a year and correlated with reproductive-cycle periods. Reproductive-organ weights of the snail Megalobulimus abbreviatus were recorded seasonally and gonad sections were analyzed morphologically. The weights were used to obtain the organosomatic index. Mean oocytic diameter and oocytic maturation index were based on gonad sections. It was concluded that M. abbreviatus is an iteroparous snail whose annual reproductive cycle is characterized by mating and egg laying throughout spring and early summer, and also by reproductive system preparation, occurring over the remainder of the summer until the end of winter, for a new breeding season. Key words: Pulmonata, Megalobulimus, reproductive cycle, reproductive system, ovotestis. RESUMO Ciclo reprodutivo anual do caracol Megalobulimus abbreviatus (Bequaert, 1948) (Gastropoda, Pulmonata) Variações morfológicas nos órgãos do sistema reprodutor dos gastrópodes pulmonados são observadas ao longo do ano e podem ser correlacionadas a fases de seu ciclo reprodutivo. A partir dessa observação, a massa dos órgãos do sistema reprodutor do caracol Megalobulimus abbreviatus foi tomada em cada estação do ano e foram obtidas secções histológicas das gônadas. Os valores de massa foram utilizados para a obtenção do índice organo-somático e as secções, para calcular o diâmetro médio dos ovócitos e o índice de maturação ovocitária. Concluímos que M. abbreviatus é um caracol “iteroparous”, apresentando ciclo reprodutivo anual caracterizado por acasalamento e oviposição durante a primavera e o início do verão e pela preparação do sistema reprodutor para um nova fase reprodutiva, que se inicia em janeiro e se estende até o final do inverno. Palavras-chave: Pulmonata, Megalobulimus, ciclo reprodutivo, sistema reprodutor, gônada. INTRODUCTION to its size (Pitoni et al., 1976), M. abbreviatus and other congenerics have proved to be good experi- Megalobulimus abbreviatus is a giant South mental models for physiological (De Jorge et al., American snail that can be found in Argentina, 1965; Jaeger, 1965; Marques & Pereira, 1970; Paraguay, and sub-tropical regions of Brazil Romero & Hoffmann, 1991; Belló-Klein et al., 1993; (Bequaert, 1948; Sawaya & Petersen, 1962). Due Rossi & Da Silva, 1993; Da Silva & Zancan, 1994; Braz. J. Biol., 65(3): 459-467, 2005 460 HORN, A. C. M., ACHAVAL, M. and ZANCAN, D. M. Romero et al., 1994) and morphological (Zancan month, fixed in a Bouin’s solution, and embedded et al., 1994; Zancan & Achaval, 1995; Zancan et in paraffin wax. Using a microtome (Minot), sagittal, al., 1997; Donelli et al., 1998; Faccioni-Heuser et frontal, or transverse serial sections (10 um) were al., 1999; Santos et al., 2002) studies. cut and stained with hematoxylin-eosin (H.E.) as Like all pulmonates, M. abbreviatus is a well as by using periodic acid-Schiff (P.A.S.) hermaphroditic snail having three identifiable regions procedures (Pearse, 1968). Sections were examined in their reproductive apparatus: hermaphrodite, male, and photographed with a Nikon Optiphot-2 and female (Duncan, 1975; Tompa, 1984; Lutchel microscope and measured with a micrometer ocu- et al., 1997). Reproductive-cycle characteristics of lar lens. stylommatophora based on a morphological analysis Organ weight was used to calculate the of the reproductive system have been established organosomatic index (O.S.I.) by the following for- for several species, most of which are of temperate mula: OSI = [(organ weight/body weight) × 100]. climates (Bett, 1960; Berry, 1963a; Smith, 1966; These indices were grouped by season and analysed Hunter, 1968; Runham & Laryea, 1968; Parivar, by means of both one-way ANOVA and Duncan’s 1978). Similar reproductive-cycle studies of tropical multiple range test (p < 0.05). A new index was or sub-tropical snails such as M. abbreviatus are created, referred to by us as the oocytic maturation rare (Zannini, 1958; Galangau, 1964). index (O.M.I.) = [(number of oocytes with diameter The use of M. abbreviatus (Gastropoda, equal to or higher than 80 µm/ total number of Pulmonata) as an experimental animal model requires oocytes) × 100], yielding the proportion of mature basic knowledge of its biology. Therefore, the aim oocytes found each time. The 80 µm value was based of this research was to determine the reproductive on the diameter of mature oocytes previously found cycle of this snail based on the annual variation of in M. oblongus (Tompa, 1984). the morphological parameters of the reproductive organs and a histological analysis of the ovotestis. RESULTS MATERIAL AND METHODS The O.S.I. showed some variation in different organs during the year (Table 1). Spring saw the Adult specimens of the snail Megalobulimus highest values for all sexual accessory organs when abbreviatus (Pulmonata, Stylommatophora) (Bequaert, compared to those for the other seasons. With the 1948) were collected in Charqueadas, Rio Gran- exception of the albumen gland, these values were de do Sul State, Brazil (lat. 30º56’S; long. 51º40’W) significantly different from those found in the other every month from March 1991 to July 1992. The seasons. snails were then housed in humid cages at the When compared to the other reproductive Universidade Federal do Rio Grande do Sul, exposed organs, the ovotestis in autumn showed an elevated to ambient temperature and natural light/dark cycles, O.S.I., which differed significantly from those in and fed with lettuce ad libitum twice weekly. The the other seasons (Table 1). mean ambient temperature was 21.6ºC in spring, The ovotestis of M. abbreviatus, which 24.3ºC in summer, 16.7ºC in autumn, and 14.9ºC appeared as a large, yellowish organ partially in winter. embedded in the digestive gland, is located in the Every month, 8 to 17 animals weighing 39.6- last turn of the shell’s apex. A hermaphroditic organ, 101.5 g were anesthetized by immersion in a it consists of four lobes, each of which presented menthol-saturated solution, following which the a large number of acini or follicles intermingled with ovotestis, spermoviduct, oviduct-vagina, albumen loose connective interacinar tissue. Each acinus was gland, penis complex, Bursa copulatrix, and a pear-shaped sac (Fig. 1A) opening into an acinar accessory gland were dissected and removed. Each duct. The ducts of several acini joined together was immediately isolated, washed with a Ringer formed the hermaphroditic duct. In each acinus four solution (Jaeger, 1961), dried on filter paper, and types of cells were identified: (a) male germ cells, weighed on a precision balance (Sartorius). The mainly occupying the lumen of the acini; (b) female weight of each organ was classified by season. At germ cells, initially found on the wall of the acini; (c) the same time, three ovotestes were obtained every Sertoli cells; and (d) follicle cells. A fifth type of cell Braz. J. Biol., 65(3): 459-467, 2005 REPRODUCTIVE CYCLE OF Megalobulimus abbreviatus 461 was occasionally observed among the others on the spermatocytes were small, rounded cells organized acinar wall, which, due to its position and in clusters. The highly heterochromatic nucleus, characteristics, corresponds to the cells of the smaller than those of the primary spermatocytes, germinal epithelium as described for the ovotestis was spherical in shape containing no observable of other snails and slugs. nucleolus. Although all the spermatogenetic forms Two basic types of Sertoli cells lining the appeared in the ovotestes throughout the year, there basal lamina of the acini were identified. One was was a clear predominance of some forms in specific an active Sertoli cell in which spermatogonia, periods. spermatocytes, and spermatids were embedded. Such In August and September, acini are largely cells were observed in each acinus, occupying an constituted of spermatogonia, and primary and area from the basal to the apical domain. This cell, secondary spermatocytes (Fig. 1B). Spermatogonia attached broadly at one end to the acinar wall, had were large, round cells with a narrow, basophilic an irregular shape varying from squamous to cytoplasm around the nucleus. The nucleus of this columnar and often producing cytoplasmic cell had a granular aspect with fine chromatin extensions into the acinar lumen to which the male particles in which one nucleolus could be observed. gametic cells adhered. The large coarsely granulated These cells were arranged in clusters, each having nucleus with deeply stained chromatin was spherical a variable number of cells assembled around a Sertoli or oval, depending on cell shape. In

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