Body Weight , Carapace Length and Width Relationship and Condition Factor of the Mud Crab Scylla serrata ( Forskal ) in Mangrove Ecosystem

The relationship of length-weight and width-weight of the carapace and the relative condition factor of mud crab Scylla serrata from Coringa Wildlife Sanctuary (Andhra Pradesh, India) was the subject of present study. Significant difference in both males and females was observed between carapace length-weight and carapace width-weight, with a linear relationship. The regression values of both genders were found to be statistically significant. The regression equations calculated for lengthweight were W = 0.00000178 L 3.1139 (r = 0.95) for males, W = 0.00000520 L 2.8056 (r = 0.94) for females and for genders combined it was W = 0.0000297 L (r = 0.94). In case of carapace width-weight relationship of males, females and genders combined the regression equations were W = 0.00000121 CW 3.0426 (r = 0.92), W = 0.00000178 CW 2.775 (r = 0.93) and W = 0.00000204 CW 2.9210 (r = 0.92) respectively. The male crabs showed positive allometric growth, whereas females had negative allometric growth. Analysis of covariance confirmed remarkable difference between males and females in the growth pattern. The mean relative condition factor (Kn values) of both males and females and of the pooled genders ranged from 0.680 (April) to 1.029 (November). A gradual raise in Kn values was observed from smallto bigsized crabs in both genders. Peak values were observed in 12.0-13.9 cm size group in November.


Introduction
Mud crab, Scylla serrata (Forskal) is the only species of the family Portunidae inhabiting the mangroves of the tropical latitudes particularly Indo-Pacific (Hill, 1975;Overton et al., 1997) and subtropical environments of China and Japan (Sakai, 1976;Dai and Yang, 1991), having tolerance to a wide range of environmental variations.It ranks first among crabs because of its large size, high meat quality, delicate taste (Rattanachote and Dangwatanakul, 1992).The species is widely distributed in the Coringa mangroves of Gautami-Godavari (India) estuarine system and commands a very good market demand for local consumption and export.
The length-weight or width-weight relationships are important prerequisites to know the variations in weight from the known length or width indicating feeding, growth, breeding etc., and to estimate the population size of a stock for the purpose of its exploitation (Atar and Secer, 2003).According to Mohapatra (2008) the carapace width-weight relationship, condition factor and relative condition factors in mud crabs are important for biological studies like catch data, yield recruit, model analysis and so on.Individual variation in length-weight relationship has been considered as the general condition of the organism to analyze the condition factor or k-factor or prepondral index (Le Cren, 1951).
Many studies contributed on these lines on Scylla serrata and added important data to those of Lalitha Devi (1985) who studied the relationship between width and weight in crabs of Kakinada region, Andhra Pradesh.Poovachiranon (1992) has made biological studies of the mud crab S. serrata of mangrove ecosystem in the Andaman Sea.Length-weight relationship of S. serrata in the Sundarban mangroves has been studied by Ali et al. (2004).Davis (2004) has studied the relationship between carapace width and mass in mature females of S. serrata in the sub-tropical Umalazi estuary.Ewel (2007) has investigated mean carapace width of S. serrata at different locations in Kosrae.Allometric relationship between carapace width and weight in mangrove crabs (S. serrata) on Kosrae of Macronesia has been reported by Bonine et al. (2008).Studies on carapace width and weight relationships, relative condition factor of mud crabs S. serrata and S. tranquebarica have been made by Mohapatra et al. (2010) in Chilka lagoon.(Snedecor and Cochran, 1968).
Similarly, the relationship between carapace width and body weight was also studied.

Condition factor (K) and relative Condition factor (Kn)
The condition factor is computed by the formula: K = W×10 5 /L 3 where K= condition factor, W= weight (g) and L= length (cm).
This equation is based on the ideal form where in formula for the length-weight relationship is w = aL b , where w is the weight, L is the length and 'a' and 'b' are constants.When b = 3, the animal will obey the cube law and it will follow the isometric growth pattern (Le Cren, 1951).If it is less than 3, the animal is exhibiting allometric growth and when b >3 the animal is with faster growth.The effect of length on K however can be eliminated by computing the relative condition factor based on the empirical lengthweight relationship and is calculated by the formula Kn = W/w¯, where, Kn = Relative condition factor, W= observed weight and w¯ = calculated weight.
The relative condition factor 'Kn' was calculated for each individual crab by dividing the observed weight by calculated weight.The mean 'Kn' values were estimated in different months during the study period and also against different size classes.The 'Kn' values were estimated separately for both genders.The regression values of carapace length and body weight and carapace width and body weight were estimated.
Analysis of covariance (F-test) revealed significant difference at 5% level in males, females and genders combined (Table 1).The exponent 'b' value in males was 3.1139, so the pattern of growth was positive allometry.In case of females it was 2.8056 indicating that the growth was of negative allometry.Moreover, the crabs were sexually dimorphic i.e., males were larger to females.It was supported by the 'b' value.When the genders combined were studied, it was 2.9891, indicating almost all isometric growth.

Width-weight relationship
The regression values showed a straight-line relationship between the width and weight (Fig. 2).The parabolic equations derived for males, females and genders combined were as follows: Thirunavukkarasu and Shanmugam (2011) have estimated the carapace length-weight and carapace widthweight relationships of the mud crab S. tranquebarica.Laura et al. (2016) have studied the size-distribution, growth pattern and condition factor of mangrove crab S. serrata in the coastal waters of Western Seram, Indonesia.Wahio et al. (2016) have assessed the size distribution, length-weight realationship and size at the onset of sexual maturity of the orange mud crab (S. olivacea) from Malaysian waters.
The present study aimed to assess the relationship between length-weight, width-weight and relative condition factor in different months over a period of twenty four months of the study and in different size groups of males and females of S. serrata in Coringa mangroves (India).

Materials and Methods
Crab samples were collected every month from the main landing center Pedavalasala village (16° 46' 45.93"N; 82° 15' 47.05" E) where all the crab catches in Coringa mangroves were brought and pooled.Sampling was carried out for about twenty four months.The average values of crab samples collected in two months of 2007 and 2008 were used for the study.A total of 693 males and 782 females with more than 20 mm in size inter moult condition were taken for the study following Kunju (1978).The carapace length (CL) was measured across the carapace between the ninth antero-lateral teeth and carapace width (CW) from tip of anterior margin of the carapace to the anterior margin of first abdominal segment was taken to the nearest 0.1cm using a divider and scale.The total weight of the crab was determined using a top pan balance to the nearest point (±0.1 g).Males and females were grouped into 6 size-classes based on carapace length.They were 2.0-3.9 cm, 4.0-5.9cm, 6.0-7.9 cm, 8.0-9.9 cm, 10.0-11.9cm and 12.0-13.9cm.The relationship was derived by the formula of Le Cren (1951): W = cL n , where W= weight (g), L= length (cm) and 'c' and 'n' are constants.
The logarithmic expression was log W= log c+ nlog L, for the determination of the length-weight relationship in crabs as compared to the cube law expressed as K = W/L 3 , where K= Growth coefficient, W=Weight (g) and L= Length (cm).
The relationship between carapace length and total weight was exponential.Therefore logarithmic form of the expression loge W= loge a + b loge L was used for determining the relationship between length and weight, where W (g) is the weight, L (mm) is the length and 'a' and 'b' are constants representing the intercept and the slope (regression coefficient).The expression loge denotes the natural logarithm.The linear equation was converted into the exponential form i.e.
W=aL b , where W= Weight (g), L = Length (mm), 'a' = a constant equivalent to 'c' and 'b' = a constant to be determined empirically.
Analysis of covariance (F-test) revealed significant difference at 5% level in males, females and genders combined (Table 2).The exponent 'b' value was more in males i.e., 3.0426 so the pattern of growth was positive allometry.In case of females it was 2.775 indicating negative allometry.

Relative condition factor (Kn)
The mean Kn values were estimated for males, females and genders combined separately every month and also in different size groups.

Month-wise mean Kn values
Males showed a low value of 0.760 in April and a high value of 1.046 in November followed by December with 1.01.In case of females, the values ranged from 0.755 in April to 1.015 in November.When the values of genders combined were studied, the highest value was recorded in November (1.037) and the lowest in April (0.764) (Fig. 3).

Size-group wise mean Kn values
The average Kn values gradually increased with the increasing size of the crabs.In males, the highest value of Kn was observed in 12.0-13.9cm size class with 1.248 and the lowest value of 0.595 was in 4.0-5.9cm size group.A gradual increase was noticed from 4.0-5.9cm size class to 12.0-13.9341 cm.In females, the peak value (1.378) was noticed in the size group of 12.0-13.9cm and a low value (0.703) in 4.0-5.9cm size group.Regarding the genders combined, a peak value was found at 12.0-13.9cm and the lowest value in 4.0-5.9cm size group (Fig. 4).

Discussion
The overall size of crab S. serrata recorded in the present study ranged from 3.1 cm to 13.4 cm with a mean carapace length of 7.99 cm in males, and 3.6 to 13.0 cm with an average of 7.74 cm in females.The mean weight of male crab was 165 g and that of female was114 g.An overall study brings to light that the females have shown less weight to males of the same size class.The model size class observed in the two years was 8.0-9.9 cm carapace length in both males and females.Lalitha Devi (1985) has reported size ranging from 15 mm to 129 mm in S. serrata in the backwaters of Kakinada.Mohapatra et al. (2006) have reported the similar carapace width in S. serrata, from 20-181 cm, in Chilika lagoon.A little bigger size of 46-215 mm was reported in S. serrata from Korapuzha estuary in Kerala (Sarada, 1997).
A gradual decrease in the size of the crab i.e., from 7.87 cm to 7.84 cm in carapace length and in weight from 142.34 g to 133 g has been noticed in the present study.It may be due to the environmental impact or over-exploitation of small sized crabs.The average size of S. serrata recorded in West Bengal coast was 90.05 mm carapace width and 62 mm carapace length by Bandyopadhyay (2008).Nandi and Pramanik (1994) had observed 180 mm carapace width and 140 mm carapace length.In Andaman Islands the average carapace width recorded was 112 mm and the carapace length was 80 mm (Poovichiranon, 1992).Mohapatra et al. (2006) had noticed that the carapace width varyied between 21 mm to150 mm and body weight between 5 g to 720 gm in Sundarbans.The largest size of male and female S. serrata reported by Mohapatra (2006) from Chilka lagoon of Orissa, India, was 181 mm/0.83kg and 159 mm/0.68 kg respectively.The carapace length ranging from 50 to 125 mm with a peak at 75 mm for male and 70 mm for female crabs have been observed by Fondo et al. (2010) in S. serrata.According to them, the female crabs are smaller in size than the male crabs.The difference mean carapace length for male and female i.e., 85 mm and 81 mm observed by them is not statistically significant.A positive allometric growth for males and negative for females have been observed by Fondo et al. (2010).Similar growth pattern has been recorded in the present study also with exponent 'b' value 3.1139 for males and 2.8056 for females, indicating the positive and negative allometric growth pattern respectively.
The length frequency distribution population has shown a unimodal size distribution and majority of the crabs caught were of small and medium size class only.The logarithmic relation between weight and carapace length and also between weight and carapace width indicated a linear relationship signifying the increase in weight with increase in length or width.The calculated 'b' values have shown a clear sexual dimorphism between males and females with larger males than females of the same group.This is in correlation with the findings of Poovachiranon (1992) in Andaman Sea, Khan and Alam (1992) in Bangladesh and Zafar et al. (2006) in Southern coastal region of Bangladesh, Fond et al. (2010) in Kenya, Mahopatra et al. (2010) in Chilka lake etc., where higher 'b' values for male crabs than female crabs of S. serrata were observed.Warner (1997) opined that in true crabs, the males are generally larger than females.There is positive correlation between carapace length and total weight, and carapace width and total weight of the crabs of the study area.Mohapatra et al. (2010) has also observed a positive correlation between the length and weight in both the species of Scylla in Chilka lagoon of Orissa.Thirunavukkarasu and Shanmugam (2011) observed the 'b' values of length-weight and width-weight relationships to be 3.1408 and 2.8291 and 3.2718 and 3.0202 for male and female crabs respectively in S. tranquebarica.
The value of relative condition factor ranged from 0.65 to 1.37 in the overall study period and it has shown a significant difference with crab size and month of investigation.However, crabs of bigger size showed higher value.The relative condition factor in the present study was higher in males than the females.This is in correlation with the findings of Mohapatra et al. (2010) in S. serrata of Chilika lagoon.The relative condition factor of males was recorded a peak in November and there after decreased slowly in April and May.The females have shown the peak kn value in the month of November followed by December.Higher Kn value in the month of November reflects the peak breeding season of S. serrata in Coringa mangroves in the winter season i.e., in the last quarter of the year (October to December) and the juveniles are more abundant in the first half of the year.Pillai et al. (2002) reported that the growth rate has been decreased during April-May due to the increased salinity.This may be the reason for the low relative condition factor in the months of April and May in the study area.Afterwards, a gradual increase in growth and reaching peak in winter months has been observed.Fluctuations in the relative condition factor at different size groups in both genders has been noticed in mud crab S. serrata by Ali et al. (2004) and according to them the Kn values ranged from 0.41 to 1.53 and 1.037417 to 1.00746 in male and female respectively.Generally seasonal variation of relative condition factor is more or less related to the sexual cycle of the species (Le Cren, 1951).
As far as the sized-based relative condition factor, it has been found to be high in larger size groups.The Kn values are low in the size group of 4.0-5.9cm size class in both male and female, in both years.Similar observation has been made in S. serrata and S. tranquebarica of Chilika lagoon by Mohapatra et al. (2010).Oluwatoyin et al. (2013) have found the condition factor k for male (66.52), female (67.64) and for the entire population 66.89 in Callinectes pallidus and also found a significant difference (P > 0.01) in condition factor between the genders.Laura et al. (2016) have found the average values of condition factor for mangrove crabs identical in the coastal waters of Indonesia.

Conclusions
The male crabs showed positive allometric growth, whereas females had negative allometric growth.Analysis of covariance confirmed remarkable difference between males and females in the growth pattern.The mean relative condition factor (Kn values) of both males and females and of the pooled genders ranged from 0.680 (April) to 1.029 (November).A gradual raise in Kn values was observed from small-to big-sized crabs in both genders.Peak values were observed in 12.0-13.9cm size group, in November.

Fig. 1 .
Fig. 1.Scatter diagram showing logarithmic relationship of carapace length-weight relationship of S. serrata in males, females and genders combined

Fig. 2 .
Fig. 2. Scatter diagram showing logarithmic relationship of carapace width-weight relationship of S. serrata in males, females and genders combined

Fig. 3 .
Fig. 3. Month-wise condition factor of S. serrata in males, females and genders combined

Table 1 .
Comparison of regression lines of length-weight relationship in males, females and combined of S. serrata

Table 2 .
Comparison of regression lines of width-weight relationship in males, females and genders combined of S. serrata Degrees of freedom; ∑ X 2 , ∑XY, ∑Y 2 = corrected sum of squares and products; S.S= Sum of Squares; M.S.S= Mean Sum of Squares