1. Introduction

Archachatina marginata snails is one of the species of African land snails called Giant African land snails [12, 23]. It could be black and white skinned [12]. The black skinned is popular and appreciated as a valuable source of animal protein in Nigeria and beyond. It constitutes an important component of the food for numerous rural dwellers, especially in the rainforest zones [2]. Although the white skinned snails has the same nutritional attributes as the black-skinned, they are being discriminated against by some people because of tabbos and superstitious beliefs in some communities, others associate them with certain gods or deities; hence its domestication has been discouraged by some snail farmers [12].

Snail meat has been consumed by humans worldwide since prehistoric times [10, 11]. Majority of the developing and underdeveloped countries of the world especially Africa is currently in trouble of massive reduction in per capital income and food production, especially within the last few decades [7]. The food deficient situation is intensified with protein deficiency when compared to the availability of calories [4].

The rapid growth of human population has not only led to higher demand for animal protein but has called for increase efficiency in feedstuff utilization by livestock. The implication of the alarming increase in population, however, is that many people require snail meat in their diet because of its importance in improving the activities of the hormones and enzymes and its improvement of the defense mechanism of the body [1]. Nigerians are presently interested in the production of highly nutritive, medicinally by-product utilizers like snail [8].

The average citizen cannot afford most of the conventional animal protein sources like: goat, beef, pork and mutton. These major protein sources are currently threatened by high cost of feed, persistent drought, diseases, primitive animal husbandry techniques and low productivity of local animal breeds [22]. Snail meat is a high quality food that is rich in protein, low in fat and a source of iron calcium, magnesium and zinc [1]. Snail meat is high in protein (88.37%) and irons (45-50mg/kg), low in fat and contains almost all the amino acids needed by humans [4]. A recent study has also shown the glandular substance in edible snail meat cause agglutination of certain bacteria which could be of value in fighting a variety of ailments including whooping cough [4]. Mackerras and Sandars [25] stated that rat lungworm (Angiostrongylus cantonensis), was found to be carried normally by unidentified species of garden slugs. Fromming [26] also emphasized that the coprophagous habits of slugs strongly implicates them in spreading the diseases of humans and plants. But these items are only suggestive in the case of A. fulica. Mead [27] stated that under no circumstances can there be found any grounds for the fears of [28] who believe that these snails could spread the several dread fluke diseases of humans.

This study was designed to compare the growth performance and body parameters of black-skinned and white-skinned A. marginata and to develop regression equation for predicting their body weight using quantitative traits based on number of whorls.

2. Materials and Methods

2.1. Study Area, Snail Species and Management of Experimental Animals

The study was conducted at the Botanical garden of the University of Calabar, Calabar, Nigeria. The description of the area and climate were as prescribed in Okon et al [14, 17]. Sixty grower snails (Archachatina marginata var. saturalis) were used. These consisted of 30 snails each of the black- and white- skinned ectotypes of A. marginata. The weight of the snails ranged from 11.65 to 30.99 g for the two ectotype. The snails were selected based on active appearance and no injury on the foot and/or shell from a base population.

2.2. Study Procedure

Experiment I: The snails were grouped into two treatments for growth performance.

Experiment II: The selected snails were grouped into six treatments (number of whorls – 2, 3 and 4) with three replicates; black-skinned and white-skinned ectotypes in a completely randomized design. The snails were fed with pawpaw leaves for the period of 16 weeks. The snails were managed in wooden cage compartments kept outside under trees shade. Each cell of the cage compartments measured 40 cm (length) x 40 cm (width) x 30 cm (height) and housed two snails per cell.

2.3. Data Collection and Analysis

Data collected on growth performance included body weight (BWT), shell length (SHL), shell width (SHW), shell mouth length (SML) and shell mouth width (SMW). Weight was measured using a ScoutTM Pro electronic scale with 0.01 g sensitivity, while measurements of the length and width were done using Vernier caliper. These data were subjected to analysis of variance using SPSS package. T-test statistical tool was used to compare means of measured parameters between the breeds (experiment 1). Multiple regression function was used in predicting snails body weights from morphometric traits of the two ectotypes of snails studied.

3. Results and Discussion

Table 1 shows the results of the description of sampled population expressed as Mean ± standard error of mean and T-test for each morphometric measurement. The black-skinned snails had the best mean weight gain (13.98 g) while white-skinned snails the least mean weight gain (8.77 g). The T test analysis shows that there was significant difference (P<0.05) in the length increase of the snails between black and white-skinned snails. Black-skinned snails had the best mean width increase of (3.35cm) while white-skinned snails recorded (2.48cm). Shell mouth length increase and mouth width increase were higher for black-skinned snails. However, the T-test analysis shows that there was no significant difference (P>0.05) in the SMLI and SMWI. There were large disparities which were significantly different (p<0.01) between the mean body weights between these ectotypes of snails; 25.63 0.19 g for black skinned snails while 39.76 ± 1.46 g for white skinned snails. Although, all other measured morphometric traits of black skinned snails were bigger and longer than those of white skinned snails. The results indicated that black skinned snails gain more weight and other morphometric traits than the white skinned snails, as this was confirmed by the test of significance of the difference (t-test) between the two ectotypes.

Table 1. T-Test between black-skinned and white-skinned snails A. marginata

The results of this study is not in agreement with the results obtained by Okon et al. [16] with A. marginata for mean body weight of 120.30±2.10 g for large sized snails with 4 whorls, 197.37±3.25 g for extra-large sized snails with 4 whorls in Odukpani Local Government Area (L.G.A.) and also 122.97±2.53 g for large sized snails with 4 whorls and 167.35±3.15 g for extra-large sized snails with 4 whorls in Yakurr L.G.A. For A. marginata snails from Ogoja L.G.A., Okon et al. [16] obtained mean body weight values of 107.26±2.22 g for medium sized snails with 4 whorls, whereas for small sized and large sized A. marginata snails with 4 whorls from Ikom L.G.A., Okon et al. [16] obtained mean body weights of 77.33±1.00 g and 166.25±3.16 g respectively.

Similarly, the results of mean body weights (Table 1) in this study did not agree with the higher mean body weight (BDW) value of 138.60 g for A. fulica snails with 4 whorls by Okon et al. [15]. The BDW results were also lower than the 127.20 g and 48.85 g for mean body weights for A. achatina and A. fulica snails with 4 whorls respectively reported recently by Etim [5]. The mean BDW of 182.00 g and 65.05 g for A. achatina and A. fulica snails with 5 whorls also reported by Etim (5) were higher and heavier than the 25.63 g and 39.76 g for black skinned and white skinned snails obtained in this study. Etta et al. [6] also reported a higher mean BDW of 137.50 g for A. fulica snails with 4 whorls. In another study, Ibom et al. [9] had earlier reported higher mean body weights of 93.70 g, 109.70 g and 73.00 g for A. fulica snails from central agro-ecological zone, northern agro-ecological zone and southern agro-ecological zone of Delta state respectively. The difference in BDW here could be attributed to the age and size differences of snails used; body weight ranges as well as number of whorls on the shell of the snails.

Morphometric traits measured showed that body length (BHL) was not significantly (P > 0.05) different. White skinned snails had the longest body (4.51 mm) while the shortest shell length was obtained for black skinned snails (3.23 mm). The value obtained were higher than the range of 2.79 - 3.15mm reported by Ibom and Okon [12] for A. marginata and lower than 10.44 mm reported by Etta et al. [6] but comparable with the 3.34 - 3.80mm in Okon et al. [15] for A. achatina. Shell width (SHW) was not significantly (P > 0.05) different among the snails, although the highest value was obtained for black skinned snails while white skinned snails recorded the least. The results for average shell width obtained were lower than the 14.1 mm mean shell width reported in Amubode [3] and the range of 11.09-11.15cm recorded in Omole [24]. The difference here might be due to the size used by the authors. The results obtained shell mouth length was lower than the 5.291 mm while the width was higher than the 2.990 mm for APL and APW respectively, reported by Etta et al. [6].

The prediction equations to estimate body weight from quantitative traits measurement for black-skinned and white-skinned A. marginata snails based on number of whorls are presented in Table 2. In both ectotypes, A. marginata with 2 numbers of whorls had the highest R². All regression coefficients (R²) were found to be significant (P<0.001) with high R² values (0.680 – 0.867 for black-skinned A. marginata and 0.736 – 0.899 for white-skinned A. marginata). This confirmed that body weight of A. marginata can be predicted with confidence from the quantitative traits measurements irrespective of the number of whorls.

Table 2. Body weight prediction equations for black and white-skinned A. marginata

The equations indicated that these quantitative traits, namely; body length, body width, shell ‘mouth’ length and shell ‘mouth’ width best predicted body weight for A. marginata, as there were little or no differences between the actual and predicted live weights of these snails using multiple regression equations. However, these results of prediction do not agree with that of Okon et al. [13] and Olawoyin and Ogogo [21] for A. marginata snails because of age differences and number of quantitative traits used in the equation. Okon et al. [13] could not closely predict body weights of A. marginata snails using shell length and shell width. Also Olawoyin and Ogogo [21] reported shell length as a better predictor of body weight for growing snails. Thus, using more than two quantitative traits in the prediction equation may likely give a better and more reliable result. This might be attributed to the effects of age and size of snails, number of whorls on the snail shells and number of traits involved in the prediction as Okon et al. [13] used juvenile snails with 2 to 3 whorls and two traits (shell length and shell width) in their study, while Olawoyin and Ogogo [21] used growing snails with 3 to 5 whorls but only one trait (shell length) in their prediction.

High percent coefficients of determination (R² %) of 86.7% and 89.9% obtained in this study for black and white skinned A. marginata with 2 whorls indicated that variations in body weights of the two ectotypes of snails with 2 whorls of the shell used can be explained by changes in the number of quantitative traits and methods of statistical analyses used in the prediction. Hence, Okon et al. [13] noted that methods of statistical analysis could also affect the results obtained. Okon et al. [18, 20] using simple correlation analysis for a single trait obtained very high coefficient of determination (R²), whereas Okon et al. [13] results that involved multiple correlation analysis used two traits (shell length and shell width) in the equations obtained low R² values. But for this study, using multiple regression analysis, multiple quantitative traits (shell length, shell width, shell “mouth” length, and shell “mouth” width) from snails with multiple [2] whorls, the coefficients of determination obtained were very high.

The comparison of the actual body weight and predicted body weight from linear regression equations are presented in Table 3. The actual and predicted body weights were more or less similar which confirmed the fact that body weight can be predicted from quantitative traits measurements with accuracy. This is in line with earlier works [19, 13, 23] who indicated that there were no significant differences between actual and predicted body weights.

Table 3. Comparison between actual and predicted body weights for black and white-skinned A. marginata

4. Conclusions

From the results of this study, the black-skinned A. marginata gain more weight than the white-skinned A. marginata. Prediction of body weight using multiple regression analysis accurately predicted the body weight using quantitative traits. These quantitative traits (shell length, shell width, shell “mouth” length and shell “mouth” width) predicted the body weight of both black and white skinned A. marginata snail with 2, 3 and 4 whorls very accurately. Thus, these quantitative or phenotypic traits of the two ectotypes of snail studied could be chosen to differentiate snails.

ACKNOWLEDGEMENTS

The authors are grateful to the Department of Animal Science, University of Calabar, Calabar, Cross River State for providing a conducive environment to embark on the research.