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Agricultural Science Digest, volume 42 issue 2 (april 2022) : 210-216

Investigations on Gonadosomatic Index and Gonad Histology of Barred Spiny eel Macrognathus pancalus Hamilton, 1822 from Upper Assam, India

Rimle Borah1, Jyotirmoy Sonowal1,*, Akash Kachari2, Nipen Nayak1, Shyama Prasad Biswas1
1Freshwater Biology Research Laboratory, Department of Life Sciences, Dibrugarh University, Assam, India.
2Department of Zoology, North Lakhimpur College, Lakhimpur-787 031, Assam, India.
Cite article:- Borah Rimle, Sonowal Jyotirmoy, Kachari Akash, Nayak Nipen, Biswas Prasad Shyama (2022). Investigations on Gonadosomatic Index and Gonad Histology of Barred Spiny eel Macrognathus pancalus Hamilton, 1822 from Upper Assam, India . Agricultural Science Digest. 42(2): 210-216. doi: 10.18805/ag.D-5271.
Background: Macrognathus pancalus or barred spiny eel is a highly valued ornamental and food fish found in the Indian subcontinent. Due to the burgeoning population and their associated impacts, the population of the species is in rapid decline that necessitates time-bound intervention to conserve the species. The current investigation was undertaken to study the gonadosomatic index and gonad histology of Macrognathus pancalus collected from upper Assam, India.

Methods: 500 samples of Macrognathus pancalus were collected from different water bodies such as wetlands, ponds, paddy fields, etc. from upper Assam, India during 2018-2019. Monthly samplings were carried out to evaluate the gonadosomatic index. The gonads were dissected out, measured and subsequently preserved for histological studies. Histological sections of preserved samples were prepared by employing accepted methodologies. The sections were then photographed using Leica DM 750 and gonadal staging was ascertained. 

Result: Gonadosomatic index studies revealed that it peaked in August in both males (3.45±0.18) and females (8.85±0.35), thereby indicating its spawning season. Minimum GSI values were observed during January in males (0.39±0.04) and in December in females (1.04±0.15) indicating the culmination of the breeding period and the advent of the preparatory phase. Macroscopic and microscopic examination unearthed five different phases of gonadal maturation. Gonadal staging through microscopic and macroscopic technique showed synchronicity with GSI values. 
The barred spiny eel or striped spiny eel, Macrognathus pancalus Hamilton, 1822, is a common fish species belonging to the family Mastacembelidae (Talwar and Jhingran 1991). It is a sturdy fish inhabiting a variety of habitats including ‘beels’/wetlands, small rivers, streams, canals, inundated fields, river plains and estuaries of Pakistan, Nepal, Bangladesh and India (Rahman, 1989; Talwar and Jhingran, 1991; Galib et al., 2009). The species commands considerable market value as both food and ornamental fish in domestic and international trade (Suresh et al., 2006; Abujam and Biswas, 2011; Raghavan et al., 2013). Their entire demands, however, are met through wild catches from their native habitats (Suresh et al., 2006). It has been observed that along with unregulated catching practices, the impact of different anthropogenic activities including habitat modifications, construction of dams, invasive species introduction, overexploitation, etc. has had a profound impact on population dynamics of many freshwater fishes around the world including M. pancalus (Maitland, 1995; Abujam and Biswas, 2011). The persistence of these problems highlights time-bound interventions to prevent further declination of natural stocks.
In recent years, aquaculture practices have shown tobe a viable alternative to boost productivity in many extant fishes through the adoption of effective management and conservation strategies. Successful implementations, in turn, have proved fruitful in reducing pressure on the depleted stocks as well as the sustainable utilization of aquatic resources (Diana, 2009; De Silva, 2012). Nonetheless, efficacious aquaculture ventures require thorough studies on taxonomy, distribution, feeding biology, reproductive dynamics, etc. of target species (Maitland, 1995). Among them, the identification of spawning chronologies is a major contributing factor in determining life history information and the environmental variability of fish populations (Brewer et al., 2008). Traditionally, numerous methodologies have been employed to ascertain reproductive success in fishes where the gonadosomatic index (GSI) stands out as the most frequent indices to understand the reproductive traits in different groups of fishes. Also, it has been observed that histological interpretation of gonadal stages is effective to identify reproductive timing in fishes, especially of those that have multiple spawning or low reproductive investments (McAdam et al., 1999). Detailed studies on these aspects often determine the success of aquaculture practices. Many authors have carried out studies on sexual dimorphism, reproductive strategies, feeding habits, induced breeding, etc. up to some extent in M. pancalus (Swarup et al., 1972; Serajuddin and Ali, 2005; Abujam and Biswas, 2020; Borah et al., 2020). However, a detailed study on the histology of gonads of M. pancalus is still lacking. With this backdrop information, morpho-anatomical indices and validation through histological techniques were incorporated for the analysis of gonads and to ascertain the reproductive traits in M. pancalus which will supplement existing information to develop future management and conservation strategies.
Sampling and Gonadosomatic index (GSI)
Samples of M. pancalus (n= 500, 209 males, 291 females) were collected from different water bodies (ponds, wetlands, streams, etc.) of Dibrugarh district, Assam, India, on monthly basis during 2018-2019. Analyses of specimens were carried out in Freshwater Biology Research Laboratory, Department of Life Sciences, Dibrugarh University, Assam. Measurements of fish length (nearest cm) and weight (nearest g) were precisely carried out before dissections of gonads. After segregating the samples into males and females through external examination, the gonads were dissected, weighed, and subsequently preserved in 10% formalin for further studies. Determination of reproductive traits and spawning season was carried out through monthly observation of GSI using the following equation:
Gonadal histology
Male and female gonads of M. pancalus were dissected out from fresh specimens every month for histological preparation. The dissected gonads were immediately cleared and subsequently fixed in alcoholic Bouin’s solution for 24 hours for further processing. They were then passed through graded alcohols (ethanol) for dehydration and finally embedded in paraffin blocks. Embedded tissues were sectioned at 4-5 µm thickness. The sections were then cleared in xylene solution and then stained with haematoxylin and eosin stain following Lal (2009). Four to five slides prepared from each tissue sample were photographed using the Leica DM 750 microscope. Morphological characteristics of both ovaries and testes were examined according to colour, structure, and types of cells. Female gonads were examined according to the mature oocyte, presence of most dominant oocyte type, and atretic follicle whereas the stages in male gonads were identified according to spermatogonium, spermatocyte, spermatids and spermatozoa numbers. The maturity stages were classified following Pathak et al., (2012). The microscopic data were also arranged according to the five maturity stages from the macroscopic scale.
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.            

Fig 1: Monthly variation of GSI of M. pancalus studied in upper Assam region, India

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.

Table 1: Macroscopic and histological characteristics of ovary of five developmental stages of M. pancalus.


Table 2: Macroscopic and histological characteristics of testes at five developmental stages of M. pancalus.

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.

Fig 2: Six microscopic maturity stages of female gonad of M. pancalus.

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.

Fig 3: Six microscopic maturity stages of testes of M. pancalus.

In summary, the present investigation elucidated the GSI and histology of gonads of M. pancalus. Based on morpho-anatomical and histological endeavours, it was observed that the species was a single spawner with a prolonged breeding season. The spawning season started at the advent of the monsoon i.e. May onwards and culminates at the end of the monsoon (September onwards). Histological interventions showed synchronicity of Gonad development with that of gonad development stages. Comparison with other studies on the species revealed the impact of physiological parameters on Gonad development of M. pancalus. The study provides new information that may fill the existing lacunas to formulate future conservation strategies.
The first author is grateful to the University Grants Commission (UGC/NET-JRF Fellowship), New Delhi, India for providing financial support to carry out the work. The second and last author also acknowledges the Department of Biotechnology (DBT), Govt. of India, for providing financial support for the construction of rearing setups under DBT Twinning Project-North Eastern Region, Government of India. The authors further acknowledge the Head of Department, Department of Life Sciences, Dibrugarh University for providing necessary facilities under DST-FIST (Department of Science and Technology- Fund for Improvement of Science and Technology Infrastructure in Universities and Higher Educational Institutions) to carry out laboratory works.

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