Growth Characteristics of Edible Land Crab Cardisomaarmatum (Brachyura: Gecarcinidae) from Kribi Mangroves, Cameroon

: The land crab Cardisoma armatum is traditionally overexploited for consumption issue in Cameroonian mangroves. However, the exploitation of this natural resource ignores sustainable management. Hence, the study aims to assess the availability and growth patterns of C. armatum in Kribi mangroves throughout some of their morphological parameters. Several individuals were captured inside traps baited once each two months from March 2019 to March 2020. Then, wellbeing, growth patterns and size at the first sexual maturity were assessed based on sex, seasons or maturity stages. Despite the fact that the condition factor (K) value overall remained below 1, females and matures individuals were in a better wellbeing compared to males and immature. In the CW-BW relationships, the allometry coefficients (b) were below 3 for both sexes, suggesting a negative allometry growth patterns which means that crabs are lighter than their body weights. In the CW-CL and CW-BH relationships, males showed a positive allometry (b>1) while females a negative allometry (b<1). Fifty percent maturity of females (CW 50 =58.87±0.14 mm) was significantly lower than that of males (CW 50 =62.67±0.08 mm). Immature and mature crabs were recorded throughout the sampling period indicating that this species breeds throughout the year with a peak between September and November.


Introduction
Ecological roles of crabs in terms of structure and function in most aquatic ecosystems are multiples [1,2]. They constitute one of the most important members of estuarine food chain [3]. Some crabs may be useful as biological indicators to assess the impacts of human activities on mangroves in the current degradation of these ecosystems [4] while the land crabs in particular are the major drivers of tree recruitment in tropical coastal forest ecosystems, and their conservation should be included in management plans of these forests [5]. Gecarcinid crabs are often large in size and commonly referred to as land crabs, based on the terrestrial habits shown by adults of most its species [6]. They comprise 24 species distributed among six genera, including Cardisoma among C. armatum Herklots, 1851 is a tropical species known [7,8].
Human consumption of crabs has increased tremendously worldwide [9]. Usually, crabs are prominent sources of protein and of essential macro and micro element for local populations [10,11]. In Cameroon in particular, edible crabs of mangroves are one that remain very little known unlike freshwater and marine crabs. However, in some West Africa countries mangrove crab C. armatum is known forms an important part of the diets for several coastal communities which ignores sustainable management of crab resources [12,13]. Besides, this species also strongly suffers from impacts caused by depletion of their habitat for urbanization [4], (Brachyura: Gecarcinidae) from Kribi Mangroves, Cameroon which affects it abundance and might even lead to local extirpation [14]. Currently, C. armatumis primarily vegetarians but may show an opportunist omnivorous diet. The aforementioned urbanization transformed vast mature stands of Rhizophora spp. trees into highly degraded areas with a dramatic change in botanical assemblages [15] thus causing the lack of food.
Facing the significant demographic growth of populations, the food and nutritional needs are increasing. Therefore, Cameroon chosen some development programs to boost growth and jobs to maintain long-term food security and balance [16]. Among these programs, the promotion of aquaculture, including crab farming is prioritizing. However, this new income-generating activity is nevertheless confronted with the precariousness of techniques and an absolute ignorance of the biology of species. Faced with these shortcomings, preliminary works on the reproduction, growth, morphological and structural characterization of the new mangrove fisheries resources with high aquaculture potential such as C. armatum are data necessary as a prerequisite for aquaculture activities.
To assess the ecological condition of the species in the mangrove habitat, the size-frequency is one of most widely used method for growth pattern [17]. For that, several measurements frequently made on crabs concern the weight, width and length of carapace, the abdomen and the chelipeds for estimating the population size and the growth pattern [18]. Additionally, the relative condition factor (K) that was derived from the size was determined for evaluating general health, habitat conditions [19,20]. Considering their nutritional, ecological and economic importance, the current study aims to assess the availability and growth patterns of C. armatum crabs in the wild based on their morphological parameters in order to improve and promote knowledge and conservation of this species in Cameroon mangroves.

Site Description
The survey was carried out in the Kribicityaround the back mangroves of Nziou (2°58'35"-2°58'44"N and 9°54'59"-9°55'07"E) ( Figure 1). Wood harvesting for urban settlement and infrastructures and sand extraction are the most important factors of mangrove degeneration. The climate is of a typical equatorial regime with four seasons well individualized, marked by high and stable temperatures of about 28.7 °C. Heavy annual rainfalls reach 3,000 mm and the tidal regime reaches 1.5 m in the spring tide [21]. Rhizophoraspp. are largely the dominant species of the flora.

Data Sampling
Specimens were collected once each two months from April 2019 to March 2020 during the low tide at day time into the traps baited distributed around their holes. Additional crabs were captured randomly inside sampling area. Captured crabs were immediately transported in individual plastic bags and stored for one hour in a deep freeze in the laboratory. Then, they were thawed, washed and separated by sex ( Figure 2). Crab carapace width (CW) and length (CL), body height (BH) and abdomen size (AW and AL) were measured using caliper (0.01 mm accuracy). Body weight was determined using digital scales to the nearest 0.01g. The size at first maturity (CW 50 ) which is the one reached by 50% of mature individuals of both sexes for the first time was assessed according to Giresse et al. [22]. Then, all individuals were dissected in order to identify macroscopically the gonadal development stages according to Lawal-Are [23].

Data Analysis
A Chi-squared test (χ 2 ) was performed to detect significant deviations from a 1:1 sex ratio [24] as follows: The condition factor (K) was calculated as follows: K=condition factor (g.mm -3 ); BW=body weight (g); CW=carapace width (mm) The population structure was analyzed for both sexes according to the CW size classes. The one-way ANOVA test was used to compare morphometric variables and condition factor (K) of individuals among the sex, months, seasons or stages of maturity. Morphometric variables were described by the proportionality index (I): Determination model of the relationship between the widths of the carapace and other morphological parameters using the equation according to [25]: Y aCWb = a=Constant; CW=carapace width (mm); b=allometry coefficient The above equation may be a linear regression equation algorithm: ln BW=ln a + b ln CW or Y=a + b X Growth pattern related to value of allometry coefficient (b) was defined. Then, b value was comparing with the theoretical value 1 or 3 according to linking of the two parameters. If b=0, isometry growth; b>1 (3), positive allometric growth; b<1 (3), negative allometry [26]. The significance of b was tested using Student's test at 5%. CW 50 will be determined from the equation of the sigmoid curve of evolution of mature individuals' percentages (stages two to five) according to the carapace width (in mm). This curve is obtained by logistic transformation according to [27] and is given by the formula: x

Sex Ratio
Sex ratio was identified as illustrated in Table 1. In all, 111 crabs were collected 51 females (45.95%) and 60 males (54.05%). Sex was globally unbalanced (1:0.85) in favor to males but the global Chi-squared test confirms that the difference was not significant (p>0.05). Females were abundant than males only in September (male/female ratio 1:1.5) and November 2019 (M/F 1:1.4). In conformity with [28] survey which reported the cumulative sex ratio of C. armatum showed that males are higher than female and found no significant difference between the number of males and females. Also, [29] reported the calculated sex-ratio in C. armatum is not different from the theoretical sex-ratio but that the males are in high proportion (M/F 1:0.97).

Condition Factor K
Globally, Condition factor (K) ranged from 0.22 to 0.95 g.mm -3 (mean 0.55±0.18 g.mm -3 ) (Figure 3a, b). K of immature ranged from 0.22 to 0.64 and from 0.32 to 0.95 (mean 0.57±0.20) for matures individuals. Throughout the entire sampling period, (K) was below 1 means that crab is not wellbeing according to [30]. Therefore, K values obtained were very low compared to those of [28] which recorded the highest values of K for C. armatum (30.7) indicating favorable environmental conditions such as habitat and food availability. Like in fish, variations of K may be indicative of food abundance, adaptation to environment and gonadal development [31]. The mean K of females (0.60±0.24) was significantly (p<0.001) higher compared to males (0.52±0.09) suggests that females showed a better condition (welfare) than males. K higher for females was previous reported with a significant growth rate [32]. In contrast, in some brachyuran crabs, males show their K higher than females because K is strongly influenced by ecological factors, gonad development, rate of feeding (or growth), degree of parasitism and may vary among seasons, species and populations [33,34]. The one-way ANOVA test showed the strongly different and significantly K condition factor among the sampling periods (p<0.001). K was not significantly higher (p>0.05) during dry season (mean 0.56±0.26) compared to rainy season (mean 0.54±0.11). K of matures was significantly higher (p<0.001) compared to immature.

Relationships Between Carapace Width and Other Parameters
The mean values of morphological parameters were illustrated in Table 2. Globally, Carapace width (CW) ranged from 46 to 93 mm. Carapace length (CL) varied from26 to 80 mm. Body weight (BW) varied from 71.9 to 400 g. Body height (BH) varied from 26 to 55 mm. Abdomen length (AL) varied from 25 to 57 mm. Abdomen width (AW) varied from 10to 42 mm. The maximum carapace length in this study was 80 mm which is low compared to 95 mm recorded in some previous studies in Table 3. This is probably due to the poor abundance of trees and the low richness in the mangrove ecosystem in the study area, in contrary to the findings of [35].  The correlation coefficients (r) obtained for the CW and BW of males, females and combined were positive and nearly to 1 (0.70< r < 0.90; p<0.001) indicating that a high degree of correlation was evident between CW-BW. Relative growth between carapace width and some others metrics variables were summarized in Tables (4)(5)(6)(7)(8). The (b) values of CW-BW relationship were below 3 which imply a negative allometry growth patterns for both sex, similar to findings for the same species in Nigeria by [35]. In the CW-CL and CW-BH relationships, males showed a positive allometry (b >1) while females a negative allometry (b<1). For CW-AL relationships, males and females showed a negative allometry (b<1) while for CW-AW relationships, both of them showed a positive allometry (b >1). Globally, the negative allometric growth suggests that crabs have a relatively slow growth rate and tend to be thinner [36] and for female crabs in particular means slow rate of growth which is scientifically attributed to the great amount of energy invested in the reproductive process at the expenses of growth [37]. The relationships between carapace width and body weight were illustrated in Figure4 below.  Table 5 presents the correlation matrix between the morphological variables in C. armatum. Various others factors may be responsible of the b value variations in the CW-BW relationships such as temperature, salinity, food (quantity, quality and size), sex, and time of year and stage of maturity [38,39]. Ovigerous females are usually heavier than non-ovigerous in many species of crabs [40,41]. The overall low symmetrical or isometric growth of b values was less than 3 and it's due to the recruitment stock in biomass which is invariably due to the peculiarity of coastal dwelling land crabs to show irregular recruitment pattern with uncertainty of returning to a small land mass after the planktonic larval phase [42]. Observations of higher b value of C. armatum were linked to the higher population and standing stock biomass and condition indices [43].

Sizes at First Sexual Maturity
The 111 individuals collected were grouped into 17 class sizes with class number interval (3 mm). The minimum carapace widths at sexual maturity were 58.2 mm for males and 55.7 mm for females. Fifty percent maturity (CW 50 ) was attained at 62.67±0.08 mm in males and 58.87±0.14 mm in females and according to a Chi-squared test, CW 50 in females is significant low compared to in males (p<0.05). The CW 25 and CW 75 were 59.01 mm and 66.33 mm respectively for males and CW 25 =56.19 mm andCW 75 =61.55 mm for females. The CW 50 in C. armatum in both males and females were close to that reported for the same species by [44] C. armatum was few documented, but reported for some other crab species like Carcinus aestuarii (male=43 mm and female=34 mm) and Scylla serrata (male=75 mm and female=70 mm) [45,46]. Many factors may affect the size at sexual maturity in a crab population such as the availability of food, predation, density and as well as differences in the duration and the proportion of molting crab [47][48][49].

Conclusion
This study contributes to improve the baseline data about growth pattern of C. armatum crab in Cameroon. Overall, the condition factor value remained below 1; indicate that C. armatum does not show the good wellbeing in this natural area. Almost all CW-BW relationships showed a negative allometric growth which suggests that crabs have a relatively slow growth rate and tend to be thinner; might be attributed to the degradation of their environmental conditions. Here, the size at first sexual maturity of C. armatum is very close to those reported for the same species in other country. According to the low abundance of C. armatum recorded in the study area, the current exploitation of this crab represents a threat to the survival of this species.