REPRODUCTIVE BIOLOGY OF PRISTOLEPIS MARGINATA JERDON, 1849

Transcription

1 Eapen Jacob, Studies on the captive breeding and reproductive biology of two indigenous ornamental fishes of the Western Ghats. Ph. D Thesis, Mahatma Gandhi University, Kottayam, India CHAPTER 2 REPRODUCTIVE BIOLOGY OF PRISTOLEPIS MARGINATA JERDON, INTRODUCTION In order to obtain a comprehensive insight into the biology, population dynamics, applying monitoring, conservation and management programs of a particular species, its reproductive potentials should be investigated. Different reproductive parameters and indices such as number of produced eggs (fecundity), ova diameter, condition factor, stages of maturity, sex ratio, gonado-somatic index, modified gonadosomatic index (MGSI) have been used to determine reproductive status and spawning season of different fish species (Nikolsky, 1963; Biswas,1993, Dobriyal et l. 1999, Esmaeili 2009) 138

2 A complete knowledge of the reproductive system and the reproductive biology of fishes are essential to understand the reproductive strategy of any given species. Quite some work has been done on the fish reproductive system namely the gonads and their development. The notable of these early works are those of Yamamoto (1956 a), Yamamoto and Yamazaki (1961). The literature on the reproductive organs and reproduction was reviewed by Raven (1961), Hoar (1969), Nagahama (1983), West (1990) and Jalabert (2005). Recent works on the reproductive biology of the Western Ghts fishes involve those of Dasgupta (2002), Kurian and Inasu (2002), Goswami and Dasgupta (2004), Mercy et l (2004) and Divipala (2008). There has been little work carried out on the reproductive biology of fishes of the family Nandidae. Pal et al (2003) reported the artificial spawning of Nandus nandus. The only work on another species, Pristolepis malabaricus was that of Sherly (1983). Therefore, an in depth study on the reproductive biology of Pristolepis marginata was taken up in order to have baseline information for the captive breeding programme of the species. 2.2 MATERIALS AND METHODS Specimens of P. marginata Jerdon for the reproductive biology studies were procured directly from its natural habitats by means of caste net operations. The F1 generation individuals produced in the hatchery were also used for the studies. The collected specimens from different locations in Bharathapuzha river, approximately 150 km away from the College of Fisheries, Cochin, where the laboratory and hatchery facilities were established, were transported in polythene bags where water and medical oxygen were kept at a ratio of 1: 5. They were acclimatized to captive conditions in the 139

3 hatchery by gradual exchange of water. Fishes were kept in well oxygenated bio-filter attached glass aquaria as well as in cement cisterns. ph ranged from 7 to 7.5. Fishes were maintained under ambient temperature conditions ( C) Quantification of maturity stages A total of 160 female fishes (75-85 mm ) were used for the study of breeding biology. After measuring the length and weight of the individual specimen (Total length, Standard Length and weight) of the specimen the body was cut opened and the gonads were cleared of peritoneal covering and fat bodies if any. The ovaries were fixed in 5 % formalin and detailed investigations on quantification of ovary were carried out. Quantification is based on external evaluation of the gonad (Qasim, 1973). Macroscopic quantification characters used are: 1. Color and shape of the gonad 2. Space it occupies inside the body cavity 3. Texture of the gonad 4. Size of ova and blood supply A series of scales of ripeness have been worked out for each group of fishes for delimiting the different stages of the sexual cycle, based on a universal scale of six stages proposed by Nikolsky (1963), mainly for temperate species. In the present study, a modified five - stage key based on the reproductive strategy of tropical fishes was used. The most common five point maturity assessment method, ideal for most tropical fishes are as follows. Stage I- Immature virgins; stage II- maturing virgin; stage IIImature and ripening; stage IV- ripe and stage V- spent. In order to assess the oocyte 140

4 maturity, a piece of ovary from a representative freshly collected specimen was gently teased on a clean glass slide along with a drop of one percent formalin, the diameter of the egg was measured and oocyte frequency profile was constructed Size at first maturity Size at first maturity was computed separately for males and females. A total of 160 fishes were used for computing the Size at first maturity. It is the length at which 50% of the fish population is regarded to have attained maturity (Kagwade, 1968). The total lengths of all fishes collected for the study were grouped according to different length groups. The percentage occurrence of four stages (ripening, ripe, spent and partially spent) for females was found out. By plotting the percentage occurrence of mature fish against respective length classes (10 mm), maturation curve is obtained. From this maturation curve, the length at which 50 % of the fishes become mature can be demarcated Spawning frequency Ovaries belonging to all the stages of maturity were preserved in 5 % formalin for ova diameter measurements. The ova diameter measurements were done on a monocular microscope with 10 X magnification, after standardizing the calibration of the ocular micrometer using a stage micrometer. A mixed subsample was taken from different parts of the ovary to eliminate any error due to differential distribution of ova stocks in different parts of the ovary. The diameters so obtained are then represented 141

5 graphically. The spawning frequency of the fish is obtained by plotting the percentage frequency against different ova diameter classes (ova stocks). Ova diameter measurements were also taken from ovaries at different stages of maturity to study the progression and development of the ova following the method adopted by Clarke (1934), Hickling and Rutenberg (1936), De Jong (1939), Prabhu (1956) and Nair and Nair (1983) Fecundity Ripe ovaries of 16 fishes of size range 66 to 104 mm SL were used for the present study. During the initial quantification process itself those ovaries (stage V) were preserved in 5 % formalin. Later these samples were weighed after wiping with a blotting paper and length and width of each lobe of the ovary was measured. Average of the lengths of the two lobes has been taken as the length of the ovary. From a weighed ovary, a minimum of 4 samples were taken at random. Weight of the sub samples ranged between to g. Weighed sub samples were then kept in Gilson s fluid (Based on modified Simpson, 1959) with appropriate labels showing standard length, ovary weight, sub sample weight etc. It is then shaken vigorously and left for 24 hours. Gilson fluid hardened the eggs and helped to liberate them by breaking the ovarian tissue. The samples can be left for several months without any disadvantage. 142

6 Table 2.1 Gilson s Fluid Composition Component Quantity 60 % alcohol : 100ml 80 % Nitric acid : 15 ml Saturated formalin : 20 ml Water : 880 ml Glacial acetic acid : 18 ml Counting Fecundity was calculated by gravimetric sub sampling method. Since fecundity is defined as the total number of mature or maturing eggs present in a ripe ovary, all the yolked eggs were considered for the counts (Hickling and Rutenberg (1936). Sub samples kept in Gison s fluid were counted manually by taking in a watch glass. Average number of eggs was calculated for the sub samples. Then on the basis of the average count and average weight of the sub samples as well as the weight of the ovary, the total or absolute or individual fecundity was computed using the formula. Weight of the ovary X Average number of eggs per sub sample Fecundity = Average weight of the sub sample 143

7 Relative Fecundity Absolute fecundity calculated was expressed in terms of unit body weight, unit body length, unit ovary weight and unit ovary length. Fecundity - Length and Fecundity - Weight relationships A logarithmic transformation of absolute fecundity to body length (SL) and ovary length (OL) and absolute fecundity to body weight (W) and ovary weight (OW) can be expressed as follows, 1. Log F = Log a + b log L 2. Log F = Log a + b log OL 3. Log F = Log a + b log W 4. Log F = Log a + b log OW These lines can be fitted by the method of least squares, using standard statistical procedures for subsequent analysis Reproductive strategy The different reproductive strategies of a fish species can be explained based on the studies on different reproductive biological aspects like fecundity, ova diameter frequency studies. Study on captive breeding and breeding behavioural patterns also give picture of different reproductive strategies of a species (Balon, 1984). 144

8 2.3 RESULTS Classification of the maturity stages Female maturity stages (1) Stage I Immature virgins Young individuals that have not yet spawned. Differentiation of the gonads has just taken place. Ovaries in this stage are very small, triangular and translucent. They are usually translucent; pinkish occupying less the one fourth of the body cavity. Ova are not visible to the naked eye. (2) Stage II Maturing Virgin Oocytes yet to develop (virgin). Ova were not visible to the naked eye; only transparent immature stock was present. In maturing virgins ovaries were usually pinkish, translucent occupying about one third of the body cavity. The right lobe was slightly larger than the left lobe. Ova were still microscopic, irregular in shape with large nucleus and slight yolk deposition. Ova size was up to 350 µ, the majority of the ova being in the 132 to 264 µ. (3) Stage III : Mature and ripening Ovary was usually yellow or pale yellow occupying half to less than three fourth of the body cavity (process of vitellogenesis). Ovary usually turgid and ovarian wall transparent and ova were quite visible to the naked eye. Ovary increases in weight rapidly. The majority of the ova range between µ, the maximum size being around 650 µ. 145

9 (4) Stage IV : Ripening II Ovary was yellow or pale yellow in colour, occupying ¾ th or more of the body cavity. The right lobe of the ovary being visible through the extremely thin ovarian wall. The ovary was turgid but the ripe ova yet to run out when pressure was applied. (Vitellogenesis over, yet to ovulate). The ova stocks ranged between 6oo and 1450 µ, majority of ova being in the size range from 1100 to 1160 µ. (5) Stage V : Ripe Ovary was yellowish in colour. Ovarian wall was extremely thin and ova quite obvious. Occupied more than ¾ th or whole of the body cavity. Ovary was very turgid. Ovulation was complete. Slight pressure on the abdomen resulted in the extrusion of ripe ova. The size range of mature ova stock ranged from 990 to 1355 µ, the mode being in the size range 1320 to 1355 µ. Maximum size of the ova was around 1800 µ. (6) Stage VI: Partially spent Ovary was slightly flaccid, pale yellowish in hue not as stout as ripe ovaries but still retaining a large number of ripe ova (100 to 1400 µ) after one or two batches of them have been spawned. Ovaries may contain remnants of disintegrating ripe ova (residual or atretic ova) Size at first maturity The length (Standard length, SL) at first maturity was determined by analyzing the data relevant to all mature fishes (Stage II and above) were examined. The 146

10 percentage occurrence of the four stages (ripening I and II, ripe and partially spent) for females and three stages (ripening, ripe and spent) for males were plotted against length classes for both males and females. While the first mature females appeared in mm length group (15 %), the first mature males appeared only in the length class mm (17 %). 58 % of the females matured before 50 mm where as 56 % of the males matured only on reaching a standard length of 70 mm. All female fish were mature reaching a length of 90 mm SL and all male fish on reaching a length of 120 mm SL. The size at first maturity for females is 54 mm SL (50-60 mm length group) and 72 mm (SL) for males (70-80 mm length group) Distribution of ova in the ovary Ova diameter measurements were taken from anterior, middle and posterior regions of the ovary for understanding the pattern of oocyte distribution. Growth of ova in the ovary To eliminate the possibilities of slight differences in ova stock distribution between the regions of the ovary, mixed sub samples were taken for the ova diameter measurements to study the growth of ova in the ovary. Stage I is characterized by virgin stock alone % of the ova are represented in the diameter class µ and only 2.8 % is represented above 100 µ but usually this stock falls below 150 µ. The ova stock constituting the virgin stock biomass is generally 147

11 not yolked. The eggs were transparent each with a large nucleus and generally shapeless (Plate 2.2 fig.1). In the second stage, maximum size of the ova is 400 µ. Yolk deposition is found to occur in ova above 150 m (Plate 2.2 fig.2). During stage III, there are two modes, one at µ and the other at µ diameter class. In this stage leading model is positioned at µ constituting about 35 % of the total ova stocks. 65 % is constituted by the immature stock (Plate 2.2 fig.3). IV stage is characterized by the presence of two modes. They are positioned at ova diameter classes µ, µ and the leading mode is positioned at µ constituting about 58 % of the total ova stocks (Plate 2.2 fig.4). V stage is characterized by maximum no of ripe eggs. Leading mode (ripe stock) is positioned at µ. Maximum size of the ova present is in this stage is 1782 µ. In the secondary mode ripening stock is seen in diameter class µ. The immature stocks are presented as 2 distinct modes at diameter classes µ and µ respectively. Ripe stock constitutes 70 % of the total ova stocks constitutes 20 % of the total ova stocks. Immature stocks form only 10 % (Plate 2.2 fig.5). Stage IV is characterised by the backward shift of the leading mode due to the release of fraction of oocytes at the posterior end of the ovary. Ripe stock constitutes about 50 % of the total ova stocks. The percentage occurrence of ova diameter-class of ripening stock has further strengthened occupying about 30 % of the total ova stocks. The fall in 148

12 the ripe stock from the ripe ovary to the partially spent ovary that does not undergo atresia (Plate 2.2 fig.6). Table 2.2 Ova dia. class Percentage frequency of different maturity stages I II II IV V VI

13 The process of maturation of the ripening and maturing stocks occur fast because these batches of eggs have undergone half the process of maturation and hence only take half the time taken by the immature stock to attain full maturity. These newly ripe ova are extruded during the subsequent spawning processes, which occur with a very short period thus the same individual spawning more than once during a year. Captive studies have also shown that P. marginata is a protracted spawner, spawning in 3 or 4 times a year. Maturation and spawning of the fish in captivity are highly influenced by exogenous diet given as well as the ambient temperature conditions Fecundity Results of the fecundity counts are given in Table 2.3. Absolute fecundity ranged from 1102 to 4965 in fishes of sizes range mm SL. The relative fecundity values ranged from 166 to 517 per cm body length and 51 to 150 per g body wt. Relative fecundity values also ranged from 622 to 2089 per cm ovary length and 760 to 1365 per g ovary wt. The relationship between fecundity and a) SL, b) wt of fish, c) length of ovary and d) wt of ovary were found out and are represented in table 2.3 and illustrated in Plate 2.1 Fig. 1,2,3,4. 150

14 Table 2.3 Absolute fecundity observed in Pristolepis marginata Sl.No. Standard length (mm) Body weight (gm) Ovary weight (mg) GSI Fecundity

15 Table 2.4 Relative fecundity observed in Pristolepis marginata Sl. No Total fecundity Fecundity(per cm body length) Fecundity (per gram body weight) Fecundity (per mg ovary weight)

16 Table 2.5 Relationship of fecundity with different parameters Sl. No. 1 Variant (x) Standard length (SL) Constant (a) Regression coefficient (b) Correlation coefficient (R) Body weight (g) Ovary weight (mg) Ovary length Ova diameter studies Ova diameter measurements were taken from anterior, middle and posterior regions of the ovary for understanding the pattern of distribution of ova in the different regions of the ovary as well as to eliminate the possible differences in occurrences of ova in the ovary. The distribution of ova stocks in the anterior, middle and posterior regions of a ripe ovary are presented in table 2.2. It is evident that the gross picture of the ova stock distribution from the three regions remains almost the same except for the slight variations in the percentage composition of the mode. However, the ripe mode shifts slightly in the posterior region ( µ) indicating the presence of increased number of larger ova in this region as compared to the anterior and middle regions. It was also noted that ova above 200 µ in diameter are yolked generally. Fall in the ripe stock from the ripe ovary to the partially spent ovary that does not undergo atresia clearly points to the batch or functional spawning habit of P. marginata. Secondary 153

17 or ripening mode assumes greater importance in partially spent and spent recovering ovaries which help in the recovery of the ovary to the ripe condition in a short period leading to the same individual spawning again during the protracted spawning season of the species. Spawning Frequency Ova diameter frequency studies have been carried out and they are illustrated in the Plate 2.2. It is apparent from the figures that there are three batches of eggs representing the ripe, ripening and immature stocks. Ripe stock is having a sharp mode quite distinctly separated from the preceding batches. Ripe stock is destined to spawn immediately. Immature ripening stock and immature stock are not as separated from each other as the ripe stock. As the ripe stock of eggs is eliminated by spawning, the batches represented by the ripening and immature stock undergo rapid maturation to replace the ripe and ripening stocks respectively. Simultaneously a new stock of immature stock also gets ready for the next breeding season Gonadosomatic index (GSI) In the present study the highest GSI value recorded was in the case of a female fish of standard length of 9.3 cm and body weight of 29.3 g. The least 6.87 was met with standard length of 9.3 cm and body weight of 39 g. 154

18 2.3.7 Reproductive strategy Studies on different aspects of reproductive biology such as fecundity, ova diameter analysis and gonosomatic index revealed that the candidate fish species P. marginata is an asynchronous spawner i.e., a continuous development and release of gametes in the gonads are evident. The fish showed affinity towards a pebbly substratum or the presence of stones stimulated the fish to start the spawning activities. So the fish could be included in the group of lithophils. The fish is also found to be nest builder and highly territorial. In general, P. marginata is an iteroparous species, i.e., they spawn more than once during their lives and gonochoristic, i.e.,their sexes are separate and exhibit external fertilization with parental care. A summary of reproductive strategies shown by P. marginata is shown in table 2.6. The fish possessed an asynchronous type ovary i.e., oocytes of all stages of development are present without dominant populations. The species could be categorized as a batch spawner i.e., eggs are recruited and ovulated from the population of yolked oocytes in several batches over a protracted period during each spawning season. 155

19 Table 2.6 Reproductive strategies of Pristolepis marginata Sl. No. 1 Component of breeding system Number of breeding opportunities Reproductive strategy Iteroparous (Multiple breeding season) 2 Type of spawning Batch spawner 3 Mating system Promiscuous (both sexes with multiple partners during breeding season) 4 Gender system Gonochoristic 5 Secondary sexual characteristics Evident only during ripe stage 6 Spawning site preparation Nest builder 7 Place of fertilization External 8 Embryonic development Oviparity 9 Parental care Strong Parental Care 10 Spawning substratum Lithophil Ecological classification: Guarders Reproductive guild (Balon 1975) Ethological classification: Nest spawners Morphotype : Lithophils. 156

20 2.4 DISCUSSION When life history traits are utilized for evolving breeding protocols, it is essential to monitor such characteristics among wild stocks so that such information can be used for both commercial and conservation applications (Ponniah and Lal, 2000). A clear understanding of reproductive biology of fishes is of vital importance not only for successful natural fishery management but also for planning aquaculture strategies (Bindu, 2006). Studies on the size at first maturity, fecundity, spawning season and spawning frequency meant for the determination of breeding cycles with regard to population are essentially meant for elucidating both short and long term variations in the production of the broods which are finally recruited to the population as exploited stocks (Begenal, 1966; Joshi and Khanna, 1980). Under captive conditions size at first maturity, spawning frequency and spawning season shows a slightly different picture, as these physiological parameters are highly dependent on the external diet and ambient conditions provided. Hitherto, no such study has been reported on biology of the common catopra, Pristolepis marginata. Hence in the present study an attempt was made to understand different reproductive biological features, especially in captive conditions, for the purpose to disclose the secrets behind its real spawning acts to produce them in big numbers, to utilize in trade and at the same time save the species from extinction threats. It is well known that within a species and among populations surviving at different geographic locations, there occur major variations in life history patterns. Such variations in reproduction, growth, and life history fitness traits are of 157

21 crucial significance for assessing the long term variability of the fish species (Ponniah and Lal, 2000) Size at First Maturity In the present study, the size at first maturity of P. marginata is found to be around 54 mm SL (50-60 mm) for females and 72 mm SL (70-80mm) for males. The males mature only after reaching a much larger size. The largest female obtained during the present study was 10.4 cm SL and males above this size were very common. The largest male obtained being 15.5 cm SL. This is a kind of protective adaptation. Captive studies have shown that male of the species showed that this species is a guarder and builds nest with pebbles. The male alone is involved in this act of nest building and it exhibits aggressiveness and strong territoriality when it is sexually mature Ridley (1978) has classified male caring fishes into two types of mating systems, first, those in which male remains with the eggs (male site-attached with or without territoriality), and second in which male carries the eggs with him. Pristolepis marginata can be included in the first group in which the males remain with the eggs (site attached) with territoriality (Anna Mercy et al, 2003). The present study indicated that as the male remained with the eggs until the larvae become free swimming. Territoriality is to improve the probability of an individual offspring s survival to ensure reproductive success. In egg laying fishes in which the males are territorial during the breeding season, the males are often larger than the females (as in salmon). This is usually the case when the male protects the offspring, so that larger size is a protective adaptation (Nikolsky, 1963). However, in most fishes with sexual differences in size, it 158

22 is the female that is larger or at least achieves a larger size. This has also been suggested by Keenleyside (1991). Size at first maturity in Etroplus suratensis (guarder, lithophil) was found to be 100 mm (TL) in females and 119 mm (TL) in males (Prasadam, 1971; Keshava et al., 1988). Size at first maturity in Pristolepis malabaricus Gunther, a nonguarder have been determined to be 77 mm in males and 81 mm in females (Sherly, 1993) Distibution of ova in the ovary Gross picture of distribution of ova stocks in the anterior, middle and posterior regions of ripe ovary remains almost same except for the slight variations in the percentage composition of the node. Similar observations were made in Chanda comersoni (Nair and Nair, 1983), Sphyraena jello (Krishna Das, 1992), Pristolepis malabaricus (Sherly, 1993), Priacanthus hamrur (Sivakami et al., 2001) Spawning frequency Based on the distribution of ova stocks, P. marginata was found to release eggs more than once during an extended period of spawning. Ova diameter frequency studies revealed that this species comes under the category C of Karekar and Bal s classification (1960), characterized by spawning more than once (multiple spawner) during a protracted spawning season. It can be placed under the category II of Quasim and Qayyum (1961). These fishes show protracted spawning season with individual spawning more than once. Pristolepis malabaricus, which spawn in a slightly estuarine habitat, also shows protracted spawning period, which coincides with the monsoon season. It is classified under category B of Karekar and Bal (Sherly, 1993). On the 159

23 contrary, Etroplus suratensis spawn throughout the year with two peaks, February to May and October to November (Prasadam, 1971; Keshava et al., 1988). In P. marginata, ovary possessed a batch of ripe stock and intermediate ripening stock in between the ripe and immature stocks. Ovary of P. marginata can be classified as asynchronous, in which oocytes in all stages of development are present at the same time. The oocyte size frequency distribution is continuous except in ripe ovaries, where there may be a clear separation between the ripe and yolked oocytes. Similar pattern of oocytes size frequency distribution was observed by Wallace and Selam (1981) in sticklebacks. The present results were further strengthened by captivity studies in which fishes were reared for over two years and they were found to show protracted spawning activity, in which fishes, during a single spawning activity extrudes eggs in three or four batches Fecundity Estimation of fecundity of fish is a prerequisite in successful breeding programme as fecundity of a species is an indicator of reproductive potential of the species. Bagenal and Braum (1978) had reported that fecundity in fish species characteristically varied among individuals of the same size and age. Fagade et al. (1984) had also suggested that fecundity variation may be due to differential abundance of food. In the present study, absolute fecundity of P. marginata ranged from 1102 to 4965 in fishes whose size ranged from 66 to 104 mm SL. In repeat spawners, absolute fecundity ranged from 1957 to 4344 (80 to 104 mm SL). These values are comparable to 160

24 the fecundity of Etroplus suratensis. Fecundity of Etroplus ranged from 600 to 6000 (Bhaskaran, 1946; Prasadam, 1971; Keshava et al., 1988). On the contrary, the fecundity in a non-guarder, P. malabaricus ranged from 7814 to 19,000 in fishes whose size ranges from 9.7 cm to 12 cm SL (Sherly, 1993). Studies on fecundity shown that repeat spawners have larger fecundity than recruit spawners. The number of eggs released increases with age and size ranging from 1102 in first time spawner to 4344 for the oldest fish observed. Fecundity shows strong positive correlation with standard length and weight of the fish. It shows strong correlation with weight of the ovary while least correlation is shown between fecundity and ovary weight. Similar relationships were observed in P. malabaricus (Sherly, 1993) and E. suratensis (Prasadam, 1971; Keshava et al., 1988). Lower fecundity found in P. marginata may be explained on the basis of Ksection (Mac Arthur and Wilson, 1967). K-section is correlated with low fecundity and slow development. K-strategies tend to be multiple spawners who allocate a greater proportion of reserve to the non-reproductive effort such as territoriality, nest building, brooding and care of young, so tend towards greater efficiency Gonadosomatic index (GSI) Gonadosomatic index (GSI) is a reliable indicator on gonadal maturity; as naturally the weight of the gonad increases with maturity and when it spawns, there is a reduction in the weight of the gonad on account of the release of gametes (devlaming et al., 1982). GSI is the indicative of fish spawning in temperate and tropical regions 161

25 (Bouain and Sian, 1983; Biswas et al.,1984). In natural habitats maximum GSI is the sign of the breeding season of the fish (Rao, 1993; Hamza, 1980; Sharma, 1987). While in captive conditions it could be reckoned as the time of spawning regardless of the breeding season. Because, the physiological response of a species can be altered by providing changes in environmental parameters in captive conditions (Munro et al., 1990). Yoneda et al., (2001) stated that females of some species has allocation as high as 25 % of the body weight to gonads. In the case of P.marginata, this was found to be 13 %. Female Reproductive system The present study reveals that the reproductive organs of P. marginata are built on the usual percoid plan with paired ovaries, short oviduct opening out by the urinogenital opening, the peculiarity being that the right lobe is conspicuously larger than the left lobe. Similar observations made by Bennington (1936) on Betta splendens,james (1946) on Lepomis macrochirus, Aravindan and Padmanabhan (1972) on Tilapia mossambica, Sherly (1993) on Pristolepis malabaricus, Sivakami et al., (2001) on Priacanthus hamrur, Goswami and Dasgupta (2004) on Nandus nandus. Male reproductive system Asymmetry is shown in size and form of the two lobes of the testis of P. marginata. Male reproductive organs are pinkish white during the younger stages and creamish white on reaching the ripe stage. Partially spent testis appears dirty white with 162

26 blood shot. Same observations were made in Lepomis macrochirus (James, 1946), in P. malabaricus (Sherly, 1993), in Nandus nandus (Goswami and Dasgupta, 2004) Reproductive strategy A complete knowledge of the reproductive system and the reproductive biology of fishes are essential to understand the reproductive strategy of any given species. As far the natural habitat of P. marginata is concerned, the different reproductive strategy adapted by the species is well suited to thrive on the particular environment. The wide spectrum of reproductive strategies supports a diversity of adaptive processes by which species have adapted and populated the environment. In addition to aiding in assessing fecundity, understanding these mechanisms of reproduction could also lead to greater comprehension of the underlying mechanisms of variable fish recruitment (Murua and Saborido-Rey, 2003). Reproductive potential is a measure of the capacity of a population to produce viable eggs and larvae, and can be considered as the main outcome of a reproductive strategy (Murua and Saborido-Rey, 2003). P. marginata was found to possess an asynchronous type of ovary, because oocytes of all stages of development are present without dominant populations. The ovary appeared to be a random mixture of oocytes, at every conceivable stage. Such ovaries may be found in iteroparous species, with protracted seasons and where yolk accumulation, and hence oocyte development, relies mostly on the food available in the environment at that moment (Hunter and Leong, 1981). Because the fish recruited and ovulated the yolked oocytes in several batches over a protracted period during each spawning season, it can be classified as a batch spawner. Batch spawning can be seen as 163

27 a strategy to release eggs over a long period of time increasing the survival probability of offspring (Lambert and Ware, 1984). Balon (1975) has proposed thirty two guilds to encompass all 28,000 living species of fishes. The complexity of reproductive modes and strategies in fishes is reflected in the construction of an ecological and ethological classification of reproductive systems. As per his classification Pristolepis marginata belongs to the Ecological classification of Guarders, ethological group of Nest builders and morphotype of lithophils 164

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