The present investigation was undertaken to understand the reproductive traits in
Macrognathus pancalus from the upper Assam region of India. It has been observed that environmental factors such as photoperiod, temperature, rainfall, food availability,
etc. have pronounced effects on the timing of gametogenesis, vitellogenesis, and maturation in fishes
(Louiz et al., 2009; Miranda et al., 2009). Variations in such factors, thus, may lead to differential breeding patterns in different geographical locations. The uniformity of studies and their interpretation regarding the overall biology of a particular species from a particular region is of utmost necessity for future management practices. The current study, thus, is an attempt to study the spawning behaviour through GSI and histology studies that may shed light on a better understanding of their reproductive traits.
GSI is an ideal and accepted tool to examine the seasonal gonadal changes and to predict the spawning season of a fish species (
Htun-Han, 1978c;
Barros and Regidor, 2002;
Tsikliras et al., 2013). Studies on GSI of
M. pancalus indicated variation in GSI values in both males and females during the study period (Fig 1). The highest value of GSI was recorded in August in both males (3.45±0.18) and females (8.85±0.35) while the lowest values were observed during January in males (0.39±0.04) and in December in females (1.04±0.15) (Fig 1). In females, a gradual increase in GSI was observed that peaked in August. Such an increase in GSI may be attributed to an increase in gonad weights during the spawning period due to the uptake of fluid by developing and ripe gonads (
Htun-Han, 1978a,
b). It was followed by a drastic drop in GSI values indicating the culmination of the spawning season. A similar trend was also recorded in males. The present investigation indicated an annual synchronous breeding pattern in
M. pancalus in the study area. Similar findings were also reported by
Abujam and Biswas (2020) and
Faridi et al., (2020). Earlier studies by
Abujam and Biswas (2020) revealed almost similar trends in GSI peak values in
M. pancalus during May and June for males and females respectively. However, their observation was based on specimens collected from different habits in contrast to the present study. Furthermore, contradictory results of the spawning period were reported earlier by
Zahid et al., (2013) in
M. pancalus. They reported two main breeding periods in
M. pancalus; one during February/March and the other one in July/ August. They interpreted that observed variations may be due to the influence of day length on spawning activity. The notion may hold true as the environmental conditions reported by
Zahid et al., (2013) are in contrast to our study and that of
Abujam and Biswas (2020). Variations in reproductive traits in different geographical regions and habitats, therefore, warrant further studies to confirm their semelparous or iteroparous nature in
M. pancalus. Recently,
Faridi et al., (2020) studied reproductive strategies on
M. aculeatus from the Ganga River and reported peak GSI values in June for both sexes. The current investigation further revealed that the males maintain uniform GSI values for a longer period with its peak in August.
Htun-Han (1978b) in this regard suggested that such tendencies in male gonads facilitate and ensures successful fertilization in fishes.
Gonad histology is often regarded as a powerful tool to analyse reproductive health and maturity in fishes (
Pieterse, 2004;
Flores et al., 2015). It is considered the most accurate method to determine maturity stages through the unambiguous interpretation of maturity status (
West, 1990). In the case of Indian teleosts, a literature survey indicates four to six maturity stages (
Sathyanesan, 1962;
Guraya et al., 1975; Dey et al., 2004). Reports of seven to eight maturity stages have also been documented (
Nagahama, 1983;
Mayer et al., 1988; West, 1990;
Treasurer, 1990;
Fishelson et al., 1996; Unal et al., 1999; Verma, 2013). The current endeavour revealed five maturity stages in both sexes of
M. pancalus based on macroscopic and microscopic observation (Table 1 and 2). Morphological examination of male and female gonads of
M. pancalus revealed five principal stages-immature virgin, maturing or recovering spent, ripening, mature or ripe and spent stage. The gonads are paired, elongated and remain suspended in mesenteries containing the fat bodies. The testes were thin, ribbon-like, and transparent during immature stages. They developed into somewhat flattened, whitish-yellow and relatively solid objects when fish progresses through different maturity stages. The ovaries, on the other hand, resembled immature testes during immature stages. At the onset of maturity, they become progressively enlarged in length and girth and tend to be somewhat yellowish.
Results showed that the cell ratios were similar at the developmental stage of the ovary (February to June) with high numbers of nucleolar cells. Numerous previtellogenic and maturing cells were observed during February and March (Fig 2). The maximum number of post-vitellogenic and mature cells was present at the advent of spawning season
i.e. from June to August. The presence of numerous post-ovulatory follicles with numerous early and late perinucleolar cells signified the degenerative or spent stage from September to December. In most freshwater teleosts, ovarian development has been classified asynchronous or asynchronous, based on the growth pattern of the oocytes (
Scott, 1987). Apart from these two growth stages,
Blazer (2002) mentioned grouped synchronous growth where two groups of oocytes are observed; one developing and the other one in the previtellogenic resting stage. The study revealed that oogonia proliferated from the germ mother cells of the ovigerous fold and is found in clusters. The period from October to January was observed as the period of gonadal recrudescence of oogonia. Primary oocytes were recruited and oogenesis began during this period. The growth phase of oocytes was reported from February to May. Primary oocytes developed rapidly with the incorporation of yolk. Yolk vesicles contained endogenetically synthesized lipids, glycoprotein, and increased the space for incorporation of yolk protein synthesized exogenously
(Selmen et al., 1986). Zona radiata and follicular cell layers as zona granulose and theca appeared in this stage. Zona granulose and theca cells played important role in the synthesis and incorporation of yolk precursors and were the site for steroid hormone synthesis
(Lubzens et al., 2010). The germinal vesicle breakdown (GVBD) and its migration towards the periphery was the major phenomenon of oocyte maturation. Yolk globules coalesced and formed a translucent yolk mass. Hydration diluted the cytoplasmic content resulting in translucent appearance and maximum size of oocytes (
Foucher and Beamish, 1980). The breeding season of
M. pancalus was predicted to be between June to August owing to the ovary attaining maximum size and the presence of mature follicles in this period. GSI also reached its peak during this period. The absence of mature and maturing follicles in September marked the completion of the spawning period. During the post-spawning phase (September-October) the post-ovulatory follicles and atretic follicles were found with residual primary oocytes. Similar cytomorphological changes in gonads have also been reported in different fish species
(Chakraborty et al., 2007; Sivakumaran et al., 2003). Mature follicles became atretic with the withdrawal of gonadotrophin (
Pant, 1968;
Hunter et al., 1985). Oogenesis and oogonial proliferation began after the completion of the degeneration of yolky oocytes for recovery and repeat the cycle.
Like female gonads, male gonads too changed during the annual reproductive cycle in fishes. Observations by
Sathyanesan (1959),
Schulz et al., (2010) and
Verma (2013) indicated that teleost testis exhibits similar observation in structure, spermatogenic pattern and maturation. Development of sperm or milt in fishes thus comprises of multiplication stage, growth and maturation stages (
Nagahma, 1983;
Schulz et al., 2002). In the present investigation, histological assessment of male gonads of
M. pancalus revealed five different maturity stages that showed conformity with earlier studies of
Zaki et al., (1995), Assem (2003) and
Abujam and Biswas (2020) (Fig 3). Mid spermatogenesis stage was observed from February to May. The spawning stage that had started in June continued until August. The advent of the spent stage or culmination of spawning season was observed in the latter half of September (
Abujam and Biswas, 2020). In the spermatogonial stage, the testes were simple and comprised of numerous seminiferous tubules or lobules which were encased in a dainty peritoneum and had a fairly thick tunica. In the primary spermatocyte stage, the tunica was thick, and the lobular structure was available and entrenched. Histological observations of the spermatid phase revealed they had lobules filled with sperm. A couple of secondary spermatocytes adjacent to spermatids were additionally present. Microscopically, very few sperm were seen in the lumen of the lobules.
