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DIET OF THE GIANT AFRICAN MARGINATA SWAINSON, 1821, IN : PREFERENCES AND ESTIMATE OF DAMAGES

Eric Ag o n g n i k p o 1, Mamadou Ka r a m o k o 1 & Atcho Ot c h o u m o u 1*

Résumé. — Régime alimentaire de l’achatine Swainson, 1821, en Côte d’Ivoire : préférences et estimation des dégâts occasionnés. — Un inventaire des plantes sauvages consom- mées par Archachatina marginata a été établi et testé en laboratoire. Les juvéniles et les adultes ont été nourris avec les quatre plantes les plus comestibles durant l’expérience de préférence alimentaire. Les com- posantes chimiques de ces plantes ont également été déterminées. Les dégâts provoqués par les escargots ont été estimés par la quantité journalière consommée, en l’occurrence le nombre moyen de feuilles jeunes et adultes consommées. La diversité végétale a été relevée et des observations de terrain conduites dans la forêt de l’Université d’Abobo-Adjamé (Abidjan, Côte d’Ivoire). A. marginata est apparue consommer diverses plantes sauvages mais préférer Clerodendrum paniculatum et qui offrent les meilleures teneurs en phosphore, sodium et potassium. Les plantes les plus consommées furent Laportea aestuans, Phaulopsis falcisepela et Palisota hirsuta. Cet escargot apparaît comme un déprédateur de la flore de forêt secondaire qui influe sur la diversité floristique de cette forêt.

Summary. — An inventory of wild plants consumed by the edible giant snail Archachatina marginata was compiled and tested in the laboratory. Juvenile and adult snails were fed on the four most edible plants during a dietary preference experiment. The chemical components of these plants were also determined. Damage caused by these snails was estimated according to daily consumed quantity, thus mean number of young and adult leaves consumed per day by these snails. Plant diversity was also noted and field observa- tions conducted within the forest of the University of Abobo-Adjamé (Abidjan, Ivory Coast). A. marginata consumed miscellaneous wild plants but prefers Clerodendrum paniculatum and Laportea aestuans which offered the best phosphorus, sodium and potassium contents. The plants most consumed were Laportea aestuans, Phaulopsis falcisepela and Palisota hirsuta. This snail is a secondary forest flora pest and has an influence on the plant biodiversity in this forest.

Animals select different food types according to a scale of preference. These diet choices may affect many plants, causing a disturbance of their development and their annual cycle (Cates & Orians, 1975 ; Lubchenko, 1978 ; Nicotri, 1980), and for this reason, in ecology and conservation of flora, the diets of are of great importance. Some of were recently introduced into the Ivory Coast. Achatina fulica, Bowdich, 1720 (Waitkuwait, 1987), was found in the secondary forests and the fallows of the south (De Winter, 1988). This species has since 1989 gradually extended its distribution to the west, mainly due to its success- ful proliferation (De Winter, 1989 ; Otchoumou et al., 2003). Otchoumou (2005) showed that

1 Laboratoire de Biologie et de Cytologie Animales, Unité de Formation et de Recherches en Sciences de la Nature, Université d’Abobo-Adjamé, 02 BP 801 Abidjan 02. Côte d’Ivoire. E-mail : [email protected] * Corresponding author. E-mail : [email protected]

Rev. Écol. (Terre Vie), vol. 65, 2010.

­– 319 ­– A. fulica is a degraded forest flora pest and is adverse to vegetal biodiversity in the Ivory Coast. At last, A. fulica was considered in the regions where it was introduced as a pest for green fod- ders, food crops and degraded forest plants (Craze & Mauremootoo, 2002 ; Raut & Barker, 2002 ; Otchoumou, 2007). Archachatina marginata, Swainson, 1821 originating from neigh- bouring was located primarily in the islands « Ehoutilés » close to Adiaké (Aboisso). In the light of this fast expansion, especially in the secondary forests, questions should be asked whether these exotic snail species will later not have a detrimental effect on the green fodder and wild plants of these forests. It is even possible that these snails will colonize the dense rainy forests of the Ivory Coast. However, in the Ivory Coast, the damages caused by A. marginata on the flora of its biotope remain to be elucidated. In the present paper, we : — present an inventory of the wild plant species consumed by A. marginata in the forest of the University of Abobo-Adjamé, — examine the preferred plant species consumed by these snails, — estimate the damage on the flora of the studied forest.

Material and Methods

Plants and study site

The leaves of 15 plants collected in the forest of the University of Abobo-Adjamé were used during this study. This forest covers a surface of approximately 5 ha. It belongs to the group of dense rainy forests with a high rainfall (approximately 2000 mm of rainfall/year), with an average temperature of 26°C and a relative humidity of approximately 90 %. The characteristic tree species of the dense part of this forest were : Carapa procera (Meliaceae), Mallotus oppositifolius (Euphorbiaceae), Hunteria eburnea (Apocynaceae) ; the shrub species were : Celtis spp. (Ulmaceae), Trichilia monadelpha (Meliaceae), Rauwolfia vomitoria(Apocynaceae) and the herb was Palisota hirsuta (Commelinaceae). The characteristic species of the degraded part were Cecropia peltata (Moraceae), Rauwolfia vomitoria (Apocynaceae) for shrubs, Palisota hirsuta (Commelinaceae) and Laportea aestuans (Urticaceae) for herbs (Otchoumou et al., 2005).

Snails

Archachatina marginata was the only snail species used for the experiments. The reproducers (adult snails) were collected in the south-east forests of the Ivory Coast and the juveniles born in captivity in the laboratory. The juvenile snails had an average shell length and live weight of 21.8 ± 0.41mm and 2.3 ± 0.05g and the reproducers an average shell length and live weight of 83.5 ± 1.13 mm and 95.0 ± 4.14g respectively.

Methods

In the dietary inventory experiment, 9 juveniles and 9 reproducers (18 snails) were selected and distributed into six containers with 3 animals/container at the biomasses of 3.10-3g/cm3 and 50.10-3 g/cm3. The breeding containers of the juvenile snails were plastic trapezoids with base square (L x l x H = 15.5x14.5x12 cm) and those of the reproducers were parallelepipeds (L x l x H = 21x19.5x14 cm). The containers had drilled lids which adhered to their circumference. During the experiment, a stock of snails with the same live weights and shell lengths subjected to the same conditions of breeding permitted to replace dead snails in order to preserve the starting biomasses. Leaves of vegetable species (5 g for the juveniles and 25 g for the reproducers), were proposed to the snails during 5 days after a 2 days fast. At the end of that period, the refusal of the diet was weighed. A pilot diet with the same weight subjected to the same conditions allowed an estimate of plants desiccation. A daily watering of the litter was carried out. The animals were weighed at the beginning and the end of the 5 days, as well as the diet refusals. For the dietary preference, the leaves of the four plants best consumed during the dietary inventory were proposed pairwise (2.5 g per plant for the juveniles and 12.5 g per plant for the reproducers) to the snails according to the same protocol and under the same experimental conditions. The chemical composition of the plants best consumed was determined in order to apprehend snail’s preferences. To estimate the abundance of the plants in the forest, plants species were counted in six sample areas of 100 m2 installed randomly in the forest. The damages caused by A. marginata on the plants best consumed were estimated using the average food intake, the average leave weight and the average number of leaves per young and adult plants in relation with snail’s aestivation periods.

­– 320 ­– Chemical analysis of leaves Dry matter Leaves samples were subjected to desiccation and the loss of mass determined by weighing. At least 50 g of the sample was mashed without variation of moisture. A mashed sample of 5 g was introduced into a tarred container and heated at 103°C during four hours in a drying oven. The mass of dry matter obtained was expressed as a percentage of the sample.

Ashes A sample of 5 g was weighed and put in a crucible of incineration. This crucible was gradually heated until carbonization of the matter on a hotplate, and then introduced thereafter into a muffle oven at 550°C ± 5°C until obtaining white ashes. The crucible was immediately weighed after cooling. The weight of the residue was calculated by deduction of the tare and expressed as a percentage of the sample.

Calcium A sample of 1g of ashes was treated by the hydrochloric acid and the calcium was precipitated in the form of calcium oxalate. The formed oxalic acid was titrated by a potassium permanganate solution. One ml of potassium permanganate corresponds to 2.004 mg calcium. The result was expressed as a percentage of the sample.

Total phosphorus One g of sample was mineralized by dry way and put in acid solution. The solution was treated by the Vanado molybdic reagent. The optical density of the yellow solution thus formed was measured with the spectrophotometer at 430 nm. The quantity of phosphorus was given compared to a calibration curve and expressed as a percentage of the sample.

Sodium and Potassium Ten g of the sample were incinerated at 450°C during 3 hours and the ashes put in solution in the hydrochloric acid. The sodium and the potassium contents of the solution were determined by photometry of flame at 763 nm using a calibration curve and were expressed as a percentage of the sample.

Rough cellulose The sample was treated successively by ebullient solutions of sulphuric acid and potassium oxide of determined concentrations. The residue separated by filtration on asbestos, washed, dried, weighed and calcined at 900°C and reweighed. The crude cellulose content expressed as a percentage of the sample was given by the following formula : (a-b) 100/3 with a = loss of weight in calcination during dosage, b = loss of weight in calcination during dummy trial.

Rough proteins The rough proteins were determined according to the method of Kjeldahl. The sample was mineralized by wet process. The acid solution was alkalized by a solution of sodium hydroxide. The released ammonia was pulled by distillation and collected in a quantity of sulphuric acid whose excess was titrated by a caustic solution of sodium hydroxide. One ml of sulphuric acid 0.1N corresponding to 1.4 mg of nitrogen, one multiplied the quantity of nitrogen obtained by 6.25. The result was expressed as a percentage of the sample.

Total lipids The total lipids were determined according to the method of Folch et al. (1957). It consisted in a homogenization of tissues by a mixture of chloroform and methanol in proportions 2/1 and the extract washed by addition of water (0.2 % in volume). The mixture was separated in two phases. The phase supernatant was the extract of pure total lipids.

Expression of the results The snail’s dietary inventory, dietary preference and damages were estimated using the following parameters :

Food intake (g/day) (FI)

F.I. (g/day) = Qs - Qf / d with Qs = starting quantity of plants (g) ; Qf = final quantity of plants (g) ; d = duration (day).

The percentage daily food intake (DFI) DFI ( %) = 100 (FI) /W with W = average weight of the snail (g).

­– 321 ­– The ecological growth yield (EGY) EGY ( %) = 100 (ΔW) / FI with ΔW = increased in body weight in a time Δt.

Relative density of distribution (plants/m2)(RD) RD (plant/m2) = (total plant species / number of sample areas) x 100.

Average number of damaged leaves (Leaves/day) (NDL) Quantity of consumed leaves (g/day) / Average leave weight (g)

Average number of damaged plants (Plants/days) (NDP)

NDP = NDL / ANL with NDL = Number of damaged leaves ; ANL = Average number of leaves per young and adult plant.

Statistical analyses The average food intake was subjected to a statistical analysis (analysis of variances according to HSD (honest significant difference) of Tukey test (p < 0.05).

Results

Dietary inventory The quantities consumed the snails of the 15 plants proposed to them are indicated in table I. The four species best consumed, in decreasing order of the quantities consumed, by juvenile snails were Clerodendrum paniculatum, Laportea aestuans, Phaulopsis falcisepala and Ficus exasperata vahl. The reproducers’ four plant species best consumed, in decreasing order of the quantities, were C. paniculatum, L. aestuans, Palisota hirsuta and Sterculia tra- gacantha. The statistical analysis in the two groups of snails indicated a significant difference (p < 0.05) between snails average food intakes of the four plant species best consumed. Thus, leaves of C. paniculatum and L. aestuans were the most appreciated whatever the group.

Di e t a r y p r e f e r e n c e In juvenile snails, the leaves of L. aestuans and C. paniculatum taken individually were preferred compared to the leaves of P. falcisepala which were preferred to those of F. exaspe- rata vahl (Tab. II). Among all proposed couples, C. paniculatum / P. falcisepala was the most consumed (3.25 g) while the less consumed (1.44 g) was L. aestuans / P. falcisepala. In repro- ducers, the leaves of C. paniculatum and L. aestuans taken individually were preferred to those of P. hirsuta and S. tragacantha. Among all proposed couples, C. paniculatum / P. hirsuta was the most consumed (13.21 g) while the less consumed (8.9 g) was P. hirsute / S. tragacantha. The statistical analysis in the two groups of snails indicated a significant difference between average food intakes (3.25 and 1.44 g ; 13.21 and 8.9 g). The best couple of plants for the juve- nile snails was L. aestuans / P. falcisepala (EGY = 14.3 %) while that of the reproducers was L. aestuans / S. tragacantha (EGY = 12.8 %).

Ch e m i c a l a n a ly s i s o f b e s t c o n s u m e d p l a n t s The principal component of the leaves was water (more than 70 %) (Tab. III). The highest potassium, sodium, phosphorus, protein, lipid and cellulose contents were observed in C. pani- culatum and in L. aestuans which were the most preferred plant species. Thus, couples of plants remaining the best diet for snails (L. aestuans / P. falcisepala and L. aestuans / S. traga- cantha) showed the highest calcium contents.

­– 322 ­– 0 0 0 0 0 0 0 0 0 0 0 0 %)

( 175.062 15.901 17.922

– Ecological growth yield %)

1.35 0.05 0.82 0.076 0.021 3.319 5.056 0.632 3.441 0.201 1.931 0.739 0.807 0.247 0.082 Reproducers percentage of Average daily Average food intake ( 0.05).

e e de a de b de c cd de cde e de e cde

ferent (P < ferent (P 1.814 ± 0.116 0.073 ± 0.003 0.020 ± 0.007 0.300 ± 0.137 4.792 ± 0.000 0.556 ± 0.045 0.190 ± 0.045 0.704 ± 0.065 0.510 ± 0.062 0.738 ± 0.286 0.082 ± 0.033 0.221 ± 0.036 0.081 ± 0.017 0.757 ± 0.201 3.620 ± 0.094 Food intake (g/day) 0 0 0 0 0 %)

( 33.4 25.6 21.57 14.71 44.21 25.123 41.258 22.549 22.383 22.035 Archachatina marginata Ecological growth yield I %) able T 2.54 1.79 1.67 9.311 3.501 1.717 1.132 5.188 1.384 8.197 2.464 3.291 0.621 3.019 14.054 Juveniles percentage of Average daily Average food intake (

a a a a a a a a a a a a a a a Inventory of wild plants consumed by 0.085 ± 0.020 0.040 ± 0.018 0.028 ± 0.009 0.351 ± 0.016 0.134 ± 0.031 0.226 ± 0.052 0.069 ± 0.016 0.040 ± 0.010 0.199 ± 0.370 0.038 ± 0.009 0.068 ± 0.017 0.047 ± 0.013 0.016 ± 0.005 0.030 ± 0.001 0.082 ± 0.026 Food intake (g/day) Mean values marked with the same letter and in column are not significantly dif Plant species sceptum (Zingiberaceae) Afromomum Cananga odorata (Annonaceae) peltata (Moraceae) Cecropia paniculatum (Verbenaceae) Clerodendrum Ficus exasperata vahl (Moraceae) Laportea aestuans (Urticaceae) Lophira alata (Ochnaceae) Palisota hirsuta (Commelinaceae) Phaulopsis falcisepala (Acanthaceae) (Rubiaceae) Psydrax subcordata Pueraria phaseloides (Fabaceae) Rauwolfia vomitoria (Apocynaceae) brachystachys (Verbenaceae) Sarcophrynium Solanum torvum (Solanaceae) tragacantha (Sterculiaceae) Sterculia

­– 323 ­– 0 0 0 0 0 0 0 0 ( %) 5.208 9.743 14.351 12.884 Ecological growth yield 2.248 1.812 1.606 2.072 1.629 1.665 10.011 14.826 17.299 22.323 15.094 20.695 percentage of Average daily Average food intake ( %) ferent (P < 0.05). ferent (P Preference level Clerodendron paniculatum Clerodendron Phaulopsis falcisepala Ficus exasperata vahl Phaulopsis falcisepala Ficus exasperata vahl Ficus exasperata vahl Palisota hirsuta tragacantha Sterculia tragacantha Sterculia Laportea aestuans Palisota hirsuta tragacantha Sterculia Laportea aestuans > paniculatum > Clerodendron paniculatum > Clerodendron Phaulopsis falcisepala > paniculatum > Clerodendron paniculatum > Clerodendron Palisota hirsuta > Laportea aestuans > Laportea aestuans > Laportea aestuans > paniculatum > Clerodendron Laportea aestuans > 1.39 63.39 45.85 41.85 23.46 68.06 34.82 27.83 26.07 45.27 48.31 39.36 54.15 58.15 76.54 31.94 65.18 72.17 73.93 51.69 60.64 98.61 54.73 36.61 Proportion ( %) able II T a a ab c a a bc bc ab b b bc Total (g) Total 8.9 ± 1.15 9.07 ± 1.9 Food intake 1.79 ± 0.2 11.93 ± 0.9 11.93 3.25 ± 0.25 1.44 ± 0.43 2.05 ± 0.05 2.63 ± 0.32 2.24 ± 0.22 13.21 ± 1.61 12.15 ± 1.88 10.64 ± 2.67 Wilds plants preferentially consumed by Archachatina marginata plants preferentially Wilds 5.5 ± 0.5 4.6 ± 0.46 5.5 ± 0.7 5.5 ± 1 S-total (g) 1.11 ± 0.09 1.11 5.14 ± 0.81 1.36 ± 0.04 0.94 ± 0.04 1.37 ± 0.02 0.84 ± 0.05 1.79 ± 0.1 8.86 ± 0.41 8.61 ± 0.02 3.32 ± 0.82 2.32 ± 0.6 1.89 ± 0.01 0.84 ± 0.12 6.58 ± 0.2 3.57 ± 0.45 1.42 ± 0.18 0.02 ± 0.01 6.65 ± 0.44 0.82 ± 0.08 1.42 ± 0.1 Mean values marked with the same letter and in column are not significantly dif couple of Plants Palisota hirsuta Phaulopsis falcisepala Laportea aestuans/ Ficus exasperata vahl paniculatum/ Clerodendron Ficus exasperata vahl/ Phaulopsis falcisepala paniculatum/ Clerodendron paniculatum/ Clerodendron tragacantha Sterculia tragacantha Sterculia Laportea aestuans/ Clerodendron paniculatum/ Clerodendron Ficus exasperata vahl tragacantha Sterculia Palisota hirsuta/ Laportea aestuans/ Palisota hirsuta Laportea aestuans/ Phaulopsis falcisepala paniculatum/ Clerodendron Laportea aestuans Clerodendron paniculatum/ Clerodendron Laportea aestuans l i r r s s J e e e e c n p o u v d u R

­– 324 ­– 1.00 1.20 1.30 0.53 0.30 Potassium 0.40 0.80 0.89 0.67 0.70 Sodium 0.22 0.18 0.33 0.10 0.13 Total Total Phosphorus 5 6.4 0.19 0.15 0.46 Calcium Components 22 Ash 9.44 10.25 23.23 13.33 ( % DM) 8.64 0.83 4.67 7.85 10.83 Cellulose able III T 2.19 2.07 4.37 3.51 1.40 Total Total lipids 21.51 23.50 32.22 23.52 22.23 Rough proteins 25 Dry 17.4 29.70 29.85 23.85 matter Chemical composition of the wild plants best consumed by Archachatina marginata 75 ( %) 70.30 70.15 82.06 76.15 Water Water Clerodendron paniculatum Clerodendron Ficus exasperata vahl Laportea aestuans Phaulopsis falcisepala tragacantha Sterculia Plants species

­– 325 ­– Re l at i v e d e n s i t i e s a n d d i s t r i b u t i o n o f p l a n t s p e c i e s p r o p o s e d t o t h e s n a i l s Table IV summarizes the inventory and the relative density of distribution of the plants of the forest of the University of Abobo-Adjamé, proposed to the snails. One notices a clear abundance and a regular distribution of P. hirsuta (0.141 plant/m2) in the whole forest while S. tragacantha (0.028 plant/m2), P. falcisepala (0.016 plant/m2) and F. exasperata vahl (0.011 plant/m2) were observed in the slightly degraded parts of the forest. C. paniculatum and L. aestuans were distributed in the most degraded part of the forest. Plants such as Sarcophrynium brachystachys (0.148 plant/m2) and Cananga odorata (0.128 plant/m2) whose leaves were less consumed were relatively abundant in the forest.

Sn a i l d a m a g e s in t h e s h o rt a n d m e d i u m t e r m s The juvenile snails damaged more the leaves of P. falcisepala (6.2 leaves/day/snail and 0.34 young plant/day against 0.19 adult plant/day) and those of L. aestuans (2.9 leaves/day/ snail and 0.24 young plant/day against 0.15 adult plant/day) (Tab. V). Reproducers damaged more L. aestuans (12.5 leaves/day/snail and 1.04 young plants/day against 0.65 adult plant/ day) and P. hirsuta (6.1 leaves/day/snail and 0.55 young plant/day against 0.16 adult plant/ day). Whatever the plant species suggested and snails ages, the number of young plants dam- aged were higher than that of adult plants damaged.

Table IV Density of distribution of some plants in the forest of the University of Abobo-Adjamé

Identification transects

Plant species n° 1 n° 2 n° 3 n° 4 n° 5 n° 6 Total Density/m2

Afromomum sceptum (Zingiberaceae) 1 1 1 0 1 8 12 0.020

Cananga odorata (Annonaceae) 20 15 10 0 25 7 77 0.128

Cecropia peltata (Moraceae) 0 2 5 0 0 2 7 0.011

Clerodendrum paniculatum (Verbenaceae) 0 0 0 0 0 0 0 0

Ficus exasperata vahl (Moraceae) 0 0 2 5 0 0 7 0.011

Laportea aestuans (Urticaceae) 0 0 0 0 0 0 0 0

Lophira alata (Ochnaceae) 0 0 6 0 5 0 11 0.018

Palisota hirsuta (Commelinaceae) 3 9 29 13 6 25 85 0.141

Phaulopsis falcisepala (Acanthaceae) 0 10 0 0 0 0 10 0.016

Psydrax subcordata (Rubiaceae) 11 0 7 0 5 0 23 0.038

Pueraria phaseloides (Fabaceae) 0 0 0 0 2 0 2 0.003

Rauwolfia vomitoria (Apocynaceae) 9 2 5 18 3 0 37 0.061 Sarcophrynium brachystachys 56 21 0 0 12 0 89 0.148 (Verbenaceae) Solanum torvum (Solanaceae) 10 0 0 2 0 6 18 0.03

Sterculia tragacantha (Sterculiaceae) 0 7 3 4 3 0 17 0.028

­– 326 ­–

11 3.05 5.44 NAP 83.95 17.72 90.67 106.56 361.84 45 305 NYP 17.19 21.52 99.69 132.92 189.44 572.92 2 years l Q 275 495 990 1595 3410 1595 6875 3355 5.5 1.53 2.72 8.86 NAP 41.97 53.28 45.34 180.92 8.59 22.5 NYP 10.76 66.45 94.72 49.84 152.5 1 year 286.46 damages l Q 495 1705 137.5 247.5 797.5 797.5 3437.5 1677.5 Estimated 0.01 0.01 0.15 0.19 0.03 0.65 0.16 0.02 NAP 0.03 0.04 0.24 0.34 0.18 1.04 0.55 0.08 NYP 1 day l Q 0.5 0.9 2.9 6.2 2.9 6.1 1.8 12.5 V able T 90 91 19 32 90 19 37 90 Adult plant number of leaves = Number of adult plants. 11 16 23 12 18 16 12 22 Average Average Young plant Young on some plants of the forest of the University of Abobo-Adjamé of the University on some plants of the forest Damages of Archachatina marginata = Number of young plants ; NAP Plants species Clerodendron paniculatum Clerodendron Ficus exasperata vahl Laportea aestuans Phaulopsis falcisepala Clerodendron paniculatum Clerodendron Laportea aestuans Sterculia tragacantha Sterculia Palisota hirsuta NI J U RE VE LES ERS PRO D U C Ql = Quantity of leaves ; NYP

­– 327 ­– Table VI Variance analysis of food intake

Food intake SC effect dl effect MC SC error dl error MC error F P

Juveniles 1.67402 14 0.1195 2.6408 30 0.0880 1.3583 0.2332

Reproducers 83.4152 14 5.9582 4.4633 30 0.1487 40.0481 0.0000

Significant effect at P < 0.05.

discussion The leaves of the 15 plants proposed to the two groups of snails were consumed. This result is in agreement with that of Hodasi (1979) who indicated that snails feed on miscella- neous wild plants. Our results show that Archachatina marginata prefers Clerodendrum panic- ulatum and Laportea aestuans what is in agreement with the results obtained by Otchoumou et al. (2003, 2005) in the Achatinidae. Finally, the Achatinidae snails consume a varied range of wild and cultivated plants. Thus, Achatina fulica and Archachatina ventricosa prefer Cecropia peltata and Laportea aestuans, shrubs of secondary anthropized forests. Memel et al. (2008), studying the distribution of terrestrial snails in a tropical rainforest, showed that these snails were present in secondary forest with much human activity, an opened canopy and a tenuous litter. In this forest, it resulted that there are several preferential microhabitats for Achatini- dae : these are the litter, the soil, the leaves, the herbs, the branches and the dead or living tree trunks. Each species or group of snails has a preferential microhabitat, which would reduce dietary competition. Achatina fulica, Archachatina marginata and Archachatina ventricosa prefer higher location (herbs, leaves) (Memel et al., 2009). The choice of the microhabitat would thus be related to the qualitative and quantitative diversity of food (plants) available and conversely. Our results show that the leaves of C. paniculatum and L. aestuans were appreciated by both juvenile and reproducer snails. This dietary behaviour suggests that leaves of these plants have particular textures and anatomical characters. This observation is also in agreement with that of Rousselet (1979) who reported that snails search particularly young and tender leaves of plants. The average food intake of juvenile snails varied from 1.44 g/day/snail to 3.25 g/day/ snail for the couples of plants proposed. This result agrees with that of Otchoumou et al. (1989-1990) with (Linné, 1758). The differences observed on levels of food intakes seemed to depend on the palatability of the couples proposed. The principal component of the leaves best appreciated was the water (Tab. III). In their natural environment snails find most of the water necessary to their hydration in consumed green fodders. The highest rates of phosphorus, sodium, potassium, cellulose, proteins and lipids are observed in Clerodendrum paniculatum and Laportea aestuans which are consumed whatever the age. We think that these nutrients would be responsible for this dietary prefer- endum. Indeed the proteins will be used for the building of the basic structures of the body while the lipids and the cellulose would provide energy necessary to the activity of the snail. Thus, contrary to other pulmonate molluscs, the Achatinidae snails digest cellulose in simple carbohydrates using enzymes contained in salivary glands (Sea et al., 2008). The Achatinidae snail is so provided with an array of enzymes which can also digest substances indigestible for many animals. With the help of symbiotic bacteria it can even digest cellulose ; in fact it would even be capable of that in its own. However, the choice or the refusal of a plant could be related to the nutrients that it contains but also to secondary metabolites such as sterols and terpenes which compose it and which would play a repulsive action. The highest potassium, sodium and phosphorus contents were observed in C. panicu- latum and L. aestuans who were preferred whatever the snails’ ages. We thought that these

­– 328 ­– minerals would be responsible for this dietary preference. The best diets of the juvenile snails (L. aestuans / P. falcisepala) and reproducer snails (L. aestuans / S. tragacantha), according to the ecological growth yield, presented the best calcium contents (Tab. III). This mineral would be an essential element for snail eggs and shell building (Bonnet et al., 1990). How- ever, in natural environment, snails would be supplied with calcium in the soil (Johannessen & Solhoy, 2001). The most abundant plants in the dense part as well as in the degraded part of the forest were respectively P. hirsuta, C. odorata and S. brachystachys. The most damaged plant spe- cies were respectively, L. aestuans, P. falcisepala and P. hirsuta. That was revealed by field observations. We think that the damage on these plants would be due to the fact that they were herbaceous with small size and thus would be exposed to the snail attacks. However, A. margi- nata caused fewer damages on C. paniculatum, F. exasperata vahl and S. tragacantha. Indeed, according to their big sizes (4 to 20 m), these plant species could truly be subjected to snail attacks only when young plants (Otchoumou et al., 2005). At the end of this study, it is advisable to retain that A. marginata consumes miscellaneous wild plants and prefers tender leaves. One notes that the dietary preferences of the snails and the best plant combinations were related to the chemical components of the leaves. This study also revealed that the damages of this invasive snail could constitute a true danger for all the flora of the forest and it remains a potential enemy for the plants biodiversity expansion.

References

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