A BIOMETRIC STUDY OF SOME ASPECTS OF THE BIOLOGY OF MACROGNATHUS ACULEATUM. (PISCES: MASTACEMBELIDAE)

Trabajo recibido el 17 de junio de 1978 y aceptado para su publicación el 6 de septiembre de 1978.

RESUMEN

INTRODUCCIÓN

Macrognathus aculeatum, the common mudeel inhabits the muddy bottom of ponds and ditches in India. Its body size does not exceed 28 cm in length and 8.5 cm in girth at the level of the confluence of the dorsal and caudal fins. Inspite of its morphological and ecological peculiarities, this fish has not been subjected to comprehensive investigation because of its non-availability in all localities throughout the year. The availablity of this species of fish not far away from Gaya (some ponds in Masaurhi, Poonpoon and Dinapore areas in Patna district in Bihar state) encouraged the present workers to undertake a biometric study to elucidate some aspects of its biology. In the present investigation, growth of different body parameters in relation to standard body lengt, length-weight relationship in breeding and non-breeding seasons, relationship of body length and weight with gonad length and weight, relative condition factor and fecundity hav, been taken into account and an attempt has been made to comment on the morphological, ambiguities of this species which are detrirríental. to its interests in the struggle for existence and which make it an unsuitable variety for commercial exploitation.

Live specimens of M.aculeatum were caught from shallow ponds and brought to, the laboratory in open plastic containers containing pond water. They were, kept in glass aquaria for 3 - 4 days for proper acclimatization prior to any investigation. The measurements of length and breadth were made in cm and weight recorded in g from seven groups of fishes separated on the basis of their body length and weigth. Each group included 10 - 40 fishes. The following techniques were adopted for the assessment of various relationships and measurements of parameters:

1) Length/wt. relationship: Le cren (1951), Sekharan (1968), Pathak (1975),

2) Growth of body parameters: Banerjee (1973),

3) Relative conditon factor: Le Cren (1951), Ramakrishnaiah (1972),

4) Fecundity: Sinha (1975), and

5) Body wt./gonad wt. and body length/ gonad length relationship: Sinha (1975).

The mean values of standard body length and length of various body parameters in seven groups of M.actileatum are recorded in Table 1. The relationship between standard body length and the length of various body parameters was established by calculating from the data in Table 1 the equations for the best fitting linear regression lines (Y = a + bX, where X and Y are independent and dependent variables respectively, and the intercept a and the regression coefficient b, are two constants) and the degree of correlation was estimated by determining the product moment correlation coefficient (r). The straight line equations and r values are presented in Table 2 and the growth patterri of various body parameters in relation to standard body length is shown in Fig. 1. It would appear that the fastest growing body parameter is the region between the snout and the ventral fin (b = 0.704), followed in de creasing order by snout to dorsal fin (b = 0.648), pectoral fin to anus (b = 0.3933), breadth at dorsal fin (b = 0.3003), snout to pectoral fin (= 0.1948) and head length (b = 0.1715). The r values indicate a higlily significant functional relationship.

The length-weight data gave a good fit with the equation log Y = log a + b log X. This relationship varies in winter and summer months. In winter, the weight of the fish increases by an exponent 2.39, in summer by 3.07 (Table 3).

The mean values of testis and ovary length in relation to standard body length and those of testis and ovary weight in relation to body weight are presented in Table 4. The regression equations of these relationships are recorded in Table 5. The body length/gonad length relationship is quite significant and it shows a linear trend. It is observed that the length of ovary increases more rapidly (b = 0.3481) than that of testis (b = 0.2306) in relation to body length. In the same manner, ovary weight increases more rapidly than testis weight in relation to body weight, but a straight line relationship can be expressed on a log. log plot (Table 5).

The relative condition (K_n) was calculated w by using the formula K_n= W/W', where w is the observed weight and w´ the calculated weight from length/weight relationship equation. The man values of K_n for male and female fishes of different length groups are recorded in Table 6 and graphically presented in Fig. 2. The peaks observed at 15.5 cm in male and nearly 18 cm in female fishes (Fig. 2) are indicative of the time at which sexual naturity is first attained. Histological examination of gonads confirmed this finding.

Ovaries taken from not less than 50 fish divided into seven groups measuring 15.1 - 21.1 cm in length and weighing 13.7 -"39.9 g were used for the study of fecundity. The relationship between fecundity and fish length is found to be non-linear and the high value of the exponent (3.98, Table 5) and r (0.793) indicate that fecundity increases at a rate proportional to the fourth power of body length and also that the two parameters are significantly related. The relationship between fecundity and fish iveight is also non-linear and the exponent (b = 1.80) indicate a less strong relationship in comparison to that betiveen fecundity and body length. There appears to be a significant relationship between fecundity and ovary weight (r - 0.8601), but the low value of b (0.4910) is indicative of poor fecundity of this species of fish (210 - 1828, Table 7).

An examination of Fig. 1 would reveal that the fastest growing body parameter is the region between the snout and the ventral fin. Next to it are the regions between snout and dorsal fin and hat between pectoral fin and anus. The increase in breadth at the level of dorsal fin in relation to increase in standard body length occurs at a slow rate and the slowest growing parameters are head length and the region between snout and pectoral fin. From a geometrical point of view, this pattern of growth involving almost an equal rate of growth of the dorsal and ventral aspects of the anterior half of the fish along with a slow rate of growth in breadth explains why the animal possesses a narrow and laterally compressed body, suitably adapted for burrowing into mud.

The length-weigth relationship varies in winter and summer months (Table 3). The difference between the regression coefficients of male and female fishes was not found to be significant.. Hence the entire length-weight data were pooled into a single equation. In summer, this relatinship followed the cube law, the weight increasing in proportion to the cube of length. The cube law relationship is of common ocurrence in fishes as reported by several workers (Sekharan, 1968; M. Sinha, 1972; Ra- 1972; Rao and Rao, 1972; A. L. Sinha, 1973; jhingran, 1974; Pathak, 1975) and the 'b' value (3.07) determined in this study confirms the findig (b = 3.36) of Ojha (1973; quoted by A. L. Sinha, 1973) for M. aculeatum.

The ovary shows greater increase in length and weight in relation to body length and weight respectively than does the testis. The values of K_m (Le Cren, 1951) or relative conditon factor plotted against body length of both male and female fishes in Fig. 2 show the inflection of the curves at 15.5 cm in male and near about 18 cm in female. This suggests that some important physiological change occurs when the fishes attain this size. These figures correspond to just above 15 g and 22 g in weight. Histological examination of testis and ovary of several fishes has revealed that they become sexually mature when they attain length and weight of the magnitudes given above.

Fig. 1. Regression of different body measurements on standard body length in M. aculeatum. 1. snout to ventral. 2. snout to dorsal. 3. pectoral fin to, anus 4. breadth at dorsal fin 5. snout to pectoral fin and 6. head length.

A knowledge of fecundity is of considerable importance in fish population studies and piscicultural practices. It has been studied in a number of species of fishes to establish its relationship with length, weight and age (Hickling, 1940; Simpson, 1951; Pitt, 1954; Jhingran, 1961; Ramakrihnaiah 1972; Sinha, 1975; David and Rahman, 1975). The fecundity of M. aculeatum is very low (210 - 1828) and the reason for it appears to be the small size of the ovary which can be accommodated within the anatomical limits of the fish.

The present study has led the authors to believe that nature has not been kind o M. aculeatum in the course of its struggle for existence. This species has acquired its bizarre and incongruous form to be able to live in mud. lt has not been a wise step so far as its fecunditY and productivity are concerned. The pattern of growth of various body parameters has resulted in a narrow and laterally compressed body with a highly reduced perivisceral cavity, the major part of which is occupied by heart, liver, gut and air bladder. Consequently little room is left for the development of ovaries. This explains the reason for the low fecundity of this species of fish. Further, it may be added that the sex ratio in any population of this species is is 7 males: 3 females Assuming that the mortality rate in young stages of the fish is the same as occurs in other species of fresh water fish, it should not be difficult to comprehend that breeding results in the production of rela- fewer productive females. This explains why this species is not commonly available in good numbers in all localities throughout the year. The fear of being devoured by larger species has taken it to mud, but safety and absence of competition in this habitat have pre- it from any further evolution. The result is that Macroanathits is represented by a single species, M. aculeatum. The shape, size, weicultureght and low fecundity render this species a commercially poor variety and any attempt for its and breeding for commercial exploitation would be an exercise in futility.

Fig. 2. Relative condition factor (Kn) at different body lengths of M. aculeatum.

TABLE 1 MEAN VALUES OF LENGTH (CM) OF BODY PARAMETERS IN SEVEN GROUPS OF M. ACULEATUM..

TABLE 2 STRAIGHT LINE EQUATIONS FOR THE RELATIONSHIP BETWEEN DIFFERENT BODY PARAMETERS AND STANDARD BODY LENGTH IN M. ACULEATUM

TABLE 3 REGRESSION EQUATIONS FOR log STANDARD BODY LENGTH/log BODY WEIGHT RELATIONSHIP FOR

TABLE 4 MEAN VALUES OF LENGTH (cm) AND WEIGHT (g) OF BODY, TESTIS AND OVARY IN SEVEN GROUPS OF MALE AND FEMALE M. ACULEATUM

TABLE 5 STRAIGHT LINE EQUATIONS AND CORRELATION COEFICIENTS (r) FOR BODY LENGHT/ GONAD LENGHT, BODY WT./ GONAD WT., BODY LENGHT/FECUNDITY, BODY WT./FECUNDITY, AND OVARY WT./ FECUNDITY RELATIONSHIPS IN M. ACULEATUM.

TABLE 6 BODY LENGTH, BODY WEIGHT AND RELATIVE CONDITION FACTOR (kn) OF M. ACULEATUM

TABLE 7 MEAN VALUES FOR OVARY WEIGHT, OVA/g BODY WEIGHT AND FECUNDITY AT DIFFERENT BODY WEIGHTS OF M. ACULEATUM.

One of us (B.N.P.) is grateful to the Vice Chancellor, Magadh University, Bodh Gaya and the authorities of University Grants Commission, New Delhi, for financial assistance, a part of which was utilised during the present investigation.

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