Length–weight and chelae length–width relationships of the crayfish Procambarus clarkii under culture conditions

Abstract

Length–weight (body length; L _B versus wet weight; W _WT) and chelae length–width (L _Ch versus W _Ch) relationships were determined for male and female crayfish (Procambarus clarkii) under culture conditions. The length–weight relationships for males, females, and total individuals were described as W _WT = 0.014 × , W _WT = 0.020 × , and W _WT = 0.017 × , respectively. This relationship was significantly different (p < 0.001) between males and females. Compared to females, males were heavier and their tail length was shorter (p < 0.001). No statistical difference was observed in mean L _Ch, cheliped length (L _Cp), and W _Ch between the right and left sides within a gender (p > 0.05), but significant differences were detected between males and females (p < 0.001). The regressions for L _Ch versus W _Ch for males and females were W _Ch = 3.083 ×  and W _Ch = 0.284 × , respectively. This regression differed significantly between males and females; males’ chelae were wider than those of females (p < 0.001). L _Ch, L _Cp, and W _Ch were highly related to L _B; males had longer L _Ch and L _Cp and wider W _Ch than females. Individuals without chelipeds were measured to determine the relationship between the length and body weight (W _BW). The L _B–W _BW relationships for males and females were described as W _BW = 0.074 ×  and W _BW = 0.033 ×  respectively. Females exhibited positive allometric growth (b > 3), and males showed negative allometric growth (b < 3). No statistical difference was observed in mean W _BW between males and females when L _B was used as a covariate (p > 0.05).

Keywords:

• length
• weight
• chelae length-width
• crayfish
• Procambarus clarkii

Introduction

The red swamp crayfish (Procambarus clarkii) is one of the most commonly farmed freshwater species in inland China. However, culture technology is still very poor and must be improved to increase production of this species. Although environmental factors that affect crayfish growth, such as water temperature, water quality, nutrition, light intensity, photoperiod, and density, have been investigated (McClain et al. 1992; Enin 1994; Nyström 1994; Jover et al. 1999; Paglianti and Gherardi 2004; Ramalho et al. 2008; Yue et al. 2009), little is known about basic morphometrics of P. clarkii under culture conditions.

Measuring individual body length and weight is a basic procedure in the scientific study of a species. The length–weight relationship of a species often is used to calculate the standing stock biomass (Martin-Smith 1996; Froese 1998) and condition indices, to analyze ontogenetic changes (Safran 1992), and to differentiate sexes (Yang and Chen 2003). It can also be used to analyze condition factor in prawns (Enin 1994; Araneda et al. 2008). Therefore, understanding the relationship between length and weight might have important implications for the culture and management of aquaculture species. In prawns, chelae length and width are important factors in aggressive behavior and play a significant role in determining competitive outcomes. Crayfish species compete for limited resources such as food, shelter, and space (Söderbäck 1991; Garvey and Stein 1993; Nakata and Goshima 2003; Mazlum and Eversole 2005); thus, morphometric relationships such as chelae length and width may be very important. Only a few studies have examined chelae length–width relationships (Garvey and Stein 1993; Eversole et al. 2006; Mazlum et al. 2007).

Prior to this study, no detailed information about length–weight and chelae length–width relationships was available for P. clarkii under culture conditions. Therefore, the objective of this study was to determine the length–weight relationships, chelae length–width relationships, and sexual dimorphism of cultured P. clarkii.

Materials and methods

To determine the length–weight relationships for this species, 678 P. clarkii were taken from culture ponds located at the Zhougang Field Station, Nanjing Fisheries Research Institute, Jiangsu, China. Length and weight measurements were made on all 678 crayfish. Of these samples, we selected 159 crayfish randomly to determine chelae length, chelae width, and cheliped length. All the measured individuals had the full complement of walking legs and no visible body deformations. Those crayfish with damaged or regenerated chelae were not used. Wet weight (W _WT) was based only on individuals preserving a full complement of appendages. Another 62 individuals without chelipeds were measured to determine body length (L _B) and body weight (W _BW). Individuals were separated by sex.

The crayfish were placed on filter paper for several minutes to remove excess water and then were weighed with an accuracy of 0.01 g. We measured L _B (from the postorbital margin to the tip of the telson); tail length (from the margin of the first abdominal segment to the tip of the telson; L _T); chelae length (from the tip of the propodus to the carpal joint; L _Ch); and cheliped length (from the tip of the propodus to the basis; L _Cp) with an accuracy of 0.1 cm. Chelae width (greatest palm width; W _Ch) was measured with an accuracy of 0.01 cm.

Regression analysis was performed to determine the relationship between L _B and weight (W _WT or W _BW) for each sex using the following equation: W _WT (or W _BW) = a. Ten models of L _Ch versus W _Ch were tested to find the regression that best fit this relationship. Analysis of covariance (ANCOVA) was used to compare the length–weight and chelae length–width regressions between males and females. It was also used to compare L _T, L _Ch, L _Cp, W _Ch, W _WT, and W _BW between males and females (employing L _B as a covariate). Differences between means were detected using the least significant difference test.

Results

Length–weight relationship

In total, 678 specimens (male = 337, female = 341) were examined for L _B and W _WT (Figure 1). The length–weight relationships for males, females, and total individuals were described as follows: W _WT = 0.014 × , W _WT = 0.020 × , and W _WT = 0.017 × , respectively. Males, females, and total individuals exhibited positive allometric growth (b > 3). The length–weight regression was significantly different between males and females (ANCOVA; p < 0.001).

Figure 1. Total length–weight relationship for Procambarus clarkii.

Chelae length–width relationship

There were no statistical differences in mean L _Ch, L _Cp, or W _Ch between the right and left sides (p > 0.05). Thus, the left cheliped was used to determine the chelae length–width relationship and the power equation best fit the data (Table 1). The regressions for the chelae length–width relationship for males and females were W _Ch = 3.083 ×  and W _Ch = 0.284 × , respectively (Table 2). The regressions differed significantly between males and females (ANCOVA; p < 0.001). Males’ chelae were wider than those of females, when L _Ch was used as a covariate (p < 0.001).

Table 1. Model summary and parameter estimates of chelae length–width relationship for females.

Equation	Model summary					Parameter estimates
	R ^2	F	df1	df2	Significance	Constant	b1	b2	b3
Linear	0.925	864.106	1	70	0.000	−0.222	0.445
Logarithmic	0.916	763.874	1	70	0.000	−0.353	1.366
Inverse	0.866	454.119	1	70	0.000	2.469	−3.834
Quadratic	0.927	434.897	2	69	0.000	−0.444	0.587	−0.022
Cubic	0.927	435.676	2	69	0.000	−0.385	0.523	0.000	−0.002
Compound	0.903	652.718	1	70	0.000	0.329	1.480
Power	0.935	1.005E3	1	70	0.000	0.284	1.232
S	0.927	889.173	1	70	0.000	1.310	−3.540
Growth	0.903	652.718	1	70	0.000	−1.112	0.392
Exponential	0.903	652.718	1	70	0.000	0.329	0.392
Note: W Ch, dependent variable; L Ch, independent variable.

Table 2. Parameters of the relationship between chelae length (L _Ch) and chelae width (W _Ch) of each sex.

Category	N	Parameters of the relationships
		A	b	R ^2
Male	87	3.083	0.847	0.955
Female	72	0.284	1.232	0.935
Note: N = number of crayfish.

Sexual dimorphism

The L _B of the 678 crayfish ranged from 1.8 to 11.1 cm, the L _T ranged from 1.1 to 6.1 cm, and the W _WT ranged from 0.18 to 83.43 g (Table 3). Males were heavier than females (ANCOVA; p < 0.001), and the L _T of females was longer than that of males (p < 0.001). Males had longer L _Ch and L _Cp and wider W _Ch than females of the same body length (ANCOVA; p < 0.001; Table 4).

Table 3. Descriptive statistics of tail length, body length, and wet weight.

Category	N	Tail length (cm)			Body length (cm)			Wet weight (g)
		Mean (SD)	Min	Max	Mean (SD)	Min	Max	Mean (SD)	Min	Max
Male	337	3.77 (0.87)	1.1	5.5	6.85 (1.70)	1.8	10.6	18.85 (15.75)	0.18	83.43
Female	341	3.91 (0.98)	1.4	6.1	6.99 (1.84)	2.4	11.1	17.50 (13.48)	0.42	67.38
Note: N = number of crayfish.

Table 4. Descriptive statistics of chelae length, cheliped length, and chelae width.

Category	N	Chelae length (cm)			Cheliped length (cm)			Chelae width (cm)
		Mean (SD)	Min	Max	Mean (SD)	Min	Max	Mean (SD)	Min	Max
Male	87	4.13 (0.92)	1.9	6.5	8.61 (1.58)	4.4	12.7	1.42 (0.36)	0.56	2.29
Female	72	3.38 (0.67)	1.7	4.6	7.41 (1.25)	4.3	9.7	1.28 (0.31)	0.57	1.91
Note: N = number of crayfish.

Length–body weight relationship of crayfish without chelipeds

The L _B–W _BW relationships for males and females were described as W _BW = 0.074 ×  and W _BW = 0.033 × , respectively. Females exhibited positive allometric growth (b > 3), and males showed negative allometric growth (b < 3). The length–weight regression differed significantly between males and females (ANCOVA; p < 0.001), but there was no statistical difference in mean W _BW between males and females, when L _B was used as a covariate (p > 0.05).

Discussion

Length–weight relationships vary among crayfish species according to sex, sexual stage, and ecological conditions (Lindqvist and Lahti 1983). These differences may be a reflection of a number of factors, including photoperiod, population density, food abundance, water level fluctuations, water temperature, and water quality (Huner and Romaire 1979; Chien and Avault 1983). Thus, understanding length–weight relationships might have important implications for culture and management of crayfish. In our studies, the length–weight relationships of P. clarkii males, females, and total individuals showed positive allometric growth (b > 3.0). Similar results were reported for Procambarus zonangulus (Romaire et al. 1977), Procambarus alleni (Acosta and Perry 2000) and Procambarus acutus acutus (Mazlum et al. 2007).

Sexual dimorphism appears to be common in freshwater crayfish (Rhodes and Holdich 1984; Mazlum et al. 2007). For example, we found that P. clarkii females had a longer abdomen than equal-sized males. A longer abdomen in females could be the result of carrying eggs under the abdomen. Particularly obvious sexual dimorphism in crayfish is largely due to the disproportionately rapid growth of chelae in males compared with females. Differences between male and female chelae lengths have been well documented in the crayfish literature since the work of Huxley (1881). The more robust nature of male crayfish imparts a distinct advantage in activities related to sexual reproduction (Stein 1976). In our study, the chelae length–width regression differed significantly between males and females. For equal length chelae, chelae were wider in males than in females. Furthermore, males had longer chelae and chelipeds and wider chelae than equal-sized females. These factors make males more aggressive and competitive than females.

In our study, the length–weight relationships showed that males were heavier than females of the same length. The longest male (10.6 cm) was shorter, but heavier than the longest female (11.1 cm). Furthermore, no statistical difference in mean weight was observed between males and females without chelipeds. Therefore, the difference in weight was mainly due to the chelae of males being larger than those of females. Rhodes and Holdich (1979) identified diphasic chelae growth in male and female Austropotamobius pallipes; male chelae grew significantly faster than female chelae after sexual maturity. This disparity is primarily due to the accelerated development of the chelae in sexually mature males, whereas chelae of females remain isometric throughout life.

Enin (1994) stated that when b = 3, growth is isometric and when b < 3 or b > 3, it is allometric. Wootton (1992) was more specific and stated that positive allometric growth occurs when organism weight increases more than length (b > 3) and negative allometric growth occurs when length increases more than weight (b < 3). The L _B–W _BW relationships for males and females in our study showed that for individuals without chelipeds, females exhibited positive allometric growth (b > 3) and males showed negative allometric growth (b < 3).

Procambarus clarkii originates from areas of high latitudes. It was introduced to China in the 1930s, and its distribution has expanded to many areas along the Yangtze River. In recent years, P. clarkii has become one of the most important freshwater products in China, and the market demands greatly exceed the supply. Thus, improved aquaculture techniques are needed to increase the production of this crayfish. Successful aquaculture farming requires both high yield and large size. As discussed above, we found that males were heavier than females of the same length, but this difference was largely due to the fact that the chelae of males were larger than those of females. Meanwhile, females had a longer abdomen than equal-sized males. A commercial consequence of this sexual dimorphism is that male crayfish yield relatively less abdominal meat than female crayfish (Huner et al. 1988; Huner 1993; Harlioğlu and Holdich 2001). In addition, bigger chelae make large individuals more aggressive and competitive, which may negatively affect the growth of small crayfish and thus reduce the yield and size. Therefore, controlling the proportion of males and removing large individuals from aquaculture tanks may be the key to successful aquaculture farming of P. clarkii. The bigger individuals (especially males) must be removed and sold every now and then during the culture season.

Acknowledgements

The authors thank Wen Wang for her valuable suggestion on the manuscript and Zhou Yang for help with software. This work was supported by Technology Development Foundation of Nanjing (No. 200905001). A project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.