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Metal concentrations in Helix pomatia, Helix aspersa and Arion rufus: a comparative study
Environmental Pollution 115 (2001) 205–208 www.elsevier.com/locate/envpol
Metal concentrations in Helix pomatia, Helix aspersa and Arion rufus: a comparative study C. Menta *, V. Parisi Natural History Museum, University of Parma, Via Farini, 90, 43100 Parma, Italy Received 11 March 2000; accepted 1 February 2001
‘‘Capsule’’: Mucus secretions in slugs could serve as a vehicle for elimination of some metals. Abstract In this study we evaluated the differences between concentrations of copper, iron, zinc, manganese, lead and cadmium in three terrestrial gastropods: Helix pomatia, Helix aspersa and Arion rufus, collected in a semi-rural location in Northern Italy. Metal concentrations in the foot and in the digestive gland were measured. In the hepatopancreas, copper and zinc did not differ significantly in the three species; the levels of copper were also similar in the foot. In comparison to Helix sps., A. rufus demonstrated lower concentrations of manganese and cadmium in the hepatopancreas and higher concentrations in the foot. In the slug the mucus produced in the foot could represent an efficient elimination mechanism of some elements. We also investigated correlations between the trace element content in the soft tissues and the mollusk size (weight of the body and height of the shell). # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Metals; Helix pomatia; Helix aspersa; Arion rufus; Bioaccumulation
1. Introduction Different species of terrestrial invertebrates exhibit different dispositions to accumulate and eliminate essential and nonessential trace metals. Adults of the species Porcellio scaber, Tetrodontophora bielanensis, Julus scandinavius and Deroceras reticulatum exposed to food and soil contaminated by lead, cadmium or zinc differed in their metal accumulation strategies (Graef et al., 1997). Species of terrestrial gastropods, as in the case of earthworms, isopods and spiders, collected in an unpolluted forest, accumulated larger amounts of cadmium in comparison to other invertebrate groups (Dallinger, 1993). Higher concentrations of lead, copper, cadmium and zinc have been detected in tissues of metal-exposed snails in the field (Martin and Coughtry, 1982) and in the laboratory (Dallinger and Wieser, 1984; Berger et al., 1993) as well as in slugs from polluted woodlands and mining areas (Greville and Morgan, 1989, 1990). Typically some organs and tissues are involved in metal accumulation while others are not. * Corresponding author. Tel.: +39-0521-236465. E-mail address: [email protected] (C. Menta).
Metal-exposed terrestrial snails and slugs store cadmium, zinc and lead in the midgut gland (Dallinger, 1993) and concentrations in the digestive gland of Helix pomatia, exposed to lead through its food, reached levels 100 times higher than those in gastropods unexposed to metal contamination (Beeby and Richmond, 1991). In Arion ater the highest concentrations of magnesium, phosphate, cadmium and zinc were deposited in the digestive gland (Ireland, 1979). The capacity to store essential and toxic trace elements in tissues is explicable as a strategy for metal detoxification in terrestrial invertebrates in order to reduce water loss (Hopkin, 1989). Numerous studies have contributed to an understanding of bioaccumulation and metals distribution in the bodies of terrestrial gastropods, but these studies do not always provide an understanding as to whether the differences in these capacities in different species are related to the availability of these elements in the area from which the sample is drawn or to the animals’ species-specific internal regulating mechanisms. In this study we offer a comparison between the accumulation of metals in H. pomatia (L.), Helix aspersa (Mu¨ller) and Arion rufus (L.) terrestrial gastropods, all of
0269-7491/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0269-7491(01)00110-5
206
C. Menta, V. Parisi / Environmental Pollution 115 (2001) 205–208
which are rarely found in the same habitat, H. pomatia and A. rufus being distributed throughout the alpin arc while H. aspersa, a more rural species, is diffused in the Mediterranean region and the European Atlantic Coast.
soft tissues and shell height and body weight (after data transformation in ln) was evaluated with regression lines.
3. Results 2. Method The adult samples of H. pomatia, H. aspersa and A. rufus were taken (July, 1997) in a semi-rural location near Cremona (Northern Italy), from a small site between partially cultivated parcels of land. There are no caves or mines in this area. A probable source of pollution is an elevated state road 200 m from the sampling area. The specimens were rinsed in dionized water and deprived of food for three days in order to evacuate their guts. They were then weighed (g; H. pomatia: 20 8.9; H. aspersa: 7.8 2.3; A. rufus: 14 2.9) and the shell heights (mm) of the Helix sps. were measured (H. pomatia: 24 0.018; H. aspersa: 21 0.027). Then the digestive gland and foot were removed from each sample. Soft tissues were dried in a microwave oven (Milestone 1200). A sample of 0.3 g of soft tissue was mineralized by adding 3 ml HNO3 suprapure and 0.5 ml H2O2 (36% at 110 volumes) and then placed in a microwave oven (Milestone 1200) for 24 minutes. Copper, iron, manganese and zinc were measured by flame atomic absorption spectrophotometry (AAS; VarianSpectrAA-40) and cadmium and lead were determined by graphite furnace AAS (Perkin Elmer 305/B) without background correction. The AAS was calibrated using standard bovine reference material. Concentrations were obtained by direct comparison with standard solutions. Data were analysed using a one-way analysis of variance and the difference between means compared using Student’s t. We used a 1% level of significance in all tests. The relationship between trace element content in the
Table 1 indicates the mean and the standard error in the concentrations of copper, zinc, iron, manganese, lead and cadmium measured in the three species. The samples did not indicate any correlation between their weight and shell height and the concentration of metals accumulated. The tissue metal analysis of garden snails H. pomatia and H. aspersa confirmed that the highest concentrations of zinc, iron, manganese, lead and cadmium were deposited in the hepatopancreas, (in all cases differences between the two soft tissue concentrations were significant: P<0.01) while the concentration of copper was not significantly different than that found in the foot. In both snails and in A. rufus copper values reached comparable levels in the two body sections (Table 1). Iron and cadmium were deposited in the digestive gland (in both cases P<0.01) while lead and zinc seemed to be accumulated in the hepatopancreas, but the differences between the two soft tissue concentrations were not significant. Moreover, in the slug the concentration of manganese was different in the two soft tissues (P<0.01): it reached the highest value in the foot while the digestive gland seems not to store this metal. A comparative study of metal concentrations in H. pomatia and H. aspersa showed that copper, zinc, iron, manganese and cadmium were not significantly different in the hepatopancreas of the two species. On the contrary iron and manganese differed in the foot: iron was more concentrated in H. pomatia (P<0.01) while manganese reached a higher value in H. aspersa (P<0.01). The two species demonstrate different lead concentrations in the hepatopancreas. Indeed, in H. aspersa
Table 1 Means (11 values) and standard errors of dry weight (g) and metal concentrations (mg g 1) in soft tissues of Helix pomatia, Helix aspersa and Arion rufus H. pomatiaa Digestive gland Dry weight Cu Zn Fe Mn Pb Cd a b c d
H. aspersab Foot
Digestive gland
0.4600.006
1.1200.011
0.1210.003
55.6815.49 187.3237.31 385.9692.97 210.7832.25 10.652.95 13.402.63
39.496.82 86.688.79 107.5911.51 11.820.79
34.695.32 384.6964.11 620.35159.32 299.1669.31 23.043.77 15.905.77
A. rufusc Foot
Digestive gland
0.1750.005 49.966.90 70.515.04 72.515.03 21.842.26
Height of shell (mm) 24 0.02; weight of body (g) 208.9. Height of shell (mm) 210.03; weight of body (g) 7.8 2.3. Weight of body (g) 142.9. DL, detection limit (detection limit for lead and cadmium were respectively, 0.15 mg g
1
and 0.1 mg g 1).
Foot
1.5010.030
5.9030.120
35.504.70 283.49125.15 922.39168.87 92.015.61 5.12 1.96 4.20 0.48
34.923.19 171.9414.17 99.217.69 394.5466.03
C. Menta, V. Parisi / Environmental Pollution 115 (2001) 205–208
the lead concentration was twice as high as that in H. pomatia. A comparative study of Helix sps. and A. rufus showed that copper and zinc had similar levels of accumulation in the digestive gland. The slug showed less manganese and cadmium concentrations in this organ (for both metals P<0.01). The foot of the slug had higher concentrations of manganese, zinc and cadmium as opposed to its hepatopancreas (in all cases P<0.001).
4. Discussion It is well known that the metabolism and bioaccumulation of trace elements by an organism is different in young individuals than in adults. Thus it is not surprising that, generally speaking, a relationship between a species’ individual member size and an accumulated concentration of metals can be observed in immature samples. We maintain that the absence of such a relationship was evidenced in our study due to the fact that we analysed only adult individuals. This condition permitted us to eliminate an ‘individual age’ variable and rendered the analyzed sample maximally homogeneous and thus better suited for our study. In this study the three species typically indicated that zinc, cadmium, lead and iron were accumulated in the digestive gland, an organ that plays a crucial role in the animals’ nutritional physiology, but that copper was not stored there. The species H. aspersa when exposed to copper seems not to indicate a clear preponderance for storing this metal in its digestive gland (Menta, 1999), while H. pomatia is capable of eliminating it through efficient intestinal and digestive gland excretions, thus maintaining an almost constant level in its tissues (Dallinger, 1993). Similar copper concentrations in the soft tissues may in part be attributed to hemocyanin (Dallinger and Wieser, 1984). In this comparative study, the hepatopancreas of the three species showed similar concentrations of copper and zinc. The reason for this result may be explained by the presence of similar processes of absorption and excretion, the function of which makes all three capable of maintaining concentrations of the two elements at comparable levels. A comparison of H. aspersa and A. rufus samples taken from the same location demonstrated a similar copper and zinc accumulation in the digestive gland (Menta, 1999). Our collected data demonstrate that the concentration of manganese in the foot of A. rufus is 10 times higher than that found in Helix sps. This species’ remarkable capacity for accumulating manganese has been previously reported in a study conducted by Cavalloro and Ravera (1966), in which the authors noted that the concentration of this element in the slug was four times greater than the values measured in the vegetation
207
which comprised its diet. The authors stated that manganese enters the animal not only through its diet but also by way of the epithelium, noted for its permeability to solutions and specific material. According to our results such a secondary route does not seem adequate enough to explain the notable differences in concentration measurements of manganese that we found in the slug and the snails. Ireland (1979) suggests that A. rufus secretes manganese by way of a mucus secretion, basing this finding on the fact that elevated concentrations of this element have been discovered in this secretion and have not been observed in association with digestive gland granules as has occurred in the case of H. aspersa. In the latter case, H. aspersa was injected with manganese and the digestive gland granules were reformed as concentric layers of calcium magnesium pyrophosphate. The mineralization process apparently requires the formation of a nucleus of calcium and magnesium pyrophosphate before the manganese can be mineralised within the vacuole (Mason and Simkiss, 1982). Relative to the observed differences and taking into consideration an analogous mode of mineralization found in vertebrates, in which manganese seems to be essential to normal bone formation (Reilly, 1991), we hypothesize that the different amounts of manganese in the foot of the slug as compared to that of the snail is related to the different quantity of calcium required by the latter for shell formation. According to our results, A. rufus seems capable of limiting the amount of cadmium in its digestive gland. The higher concentration of cadmium in its foot as compared to that of Helix sps. causes us to hypothesize that the slug has a capacity to relocate this toxic element in different parts of its body, a means not shared by the snail as part of its accumulation process. In a similar mode as that proposed by Ireland (1979) for the elimination of manganese, the mucus secretion could function as a vehicle for the elimination of cadmium. In A. ater cadmium was not associated with any specific organelles, but was bound to a cytosolic protein showing typical features of metallothioneins (Ireland, 1981). Similar results have been reported in the slug Arion lusitanicus (Dallinger et al., 1989). This condition makes the relocation of an element from the gland to the foot easier. Further in-depth knowledge regarding the toxic metal content of the mucus secretion in the epitheliums of the snail and slug could confirm the possible function of this secretion as serving, together with the feces, as a vehicle for the elimination of some metals.
Acknowledgements We wish to express our thanks to the Alimentary Science and Technology Institute of the University of
208
C. Menta, V. Parisi / Environmental Pollution 115 (2001) 205–208
Parma for its assistance in the analytical work involved in this study. References Beeby, A., Richmond, L., 1991. Adaptation of homeostatic mechanisms: lead assimilation and mineral metabolism in the snail Helix aspersa from a polluted ecosystem. In: Ravera, O. (Eds.), Terrestrial and Aquatic Ecosystems. Perturbation and Recovery. Ellis Horwood, New York, pp. 165–170. Berger, B., Dallinger, R., Felder, E., Moser, J., 1993. Budgeting the flow of cadmium and zinc through the terrestrial gastropod, Helix pomatia L. In: Dallinger, R., Rainbow, P.S. (Eds.), Ecotoxicology of Metals in Invertebrates. Lewis Publishers, London, pp. 291–313. Cavalloro, R., Ravera, O., 1966. Biological Indicator of Manganese54 Contamination in Terrestrial Environments. Nature 5029, 1259. Dallinger, R., 1993. Strategies of metal detoxification in terrestrial invertebrates. In: Dallinger, R., Rainbow, P.S. (Eds.), Ecotoxicology of Metals in Invertebrates. Lewis Publishers, London, pp. 245–289. Dallinger, R., Janssen, H.H., Bauer-Hilty, A., Berger, B., 1989. Characterization of an inducible cadmium-binding protein from hepatopancreas of metal-exposed slugs (Arionidae, Mollusca). Comp. Biochem. Physiol. 92C, 355. Dallinger, R., Wieser, W., 1984. Patterns of accumulation, distribution and liberation of Zn, Cu, Cd, and Pb in different organs of the land snail Helix pomatia L. Comp. Biochem. Physiol. 79C, 117.
Graef, S., Berkus, M., Alberti, G., Kohler, H.R., 1997. Metal accumulation strategies in saprophagous and phytophagous soil invertebrates: a quantitative comparison. Biometals 10, 45–53. Greville, R.W., Morgan, A.J., 1989. Concentrations of metals (Cu, Pb, Cd, Zn, Ca) in six species of British terrestrial gastropods near disused lead and zinc mine. J. Mol. Stud. 55, 31. Greville, R.W., Morgan, A.J., 1990. The influence of size on the accumulated amounts of metals (Cu, Pb, Cd, Zn and Ca) in six species of slugs sampled from a contaminated woodland site. J. Mol. Stud. 56, 355. Hopkin, S.P., 1989. Ecophysiology of Metals in Terrestrial Invertebrates. Elsevier Applied Science, London and New York. Ireland, M.P., 1979. Distribution of essential and toxic metals in the terrestrial gastropod Arion ater. Environ. Pollut. 20, 271–278. Ireland, M.P., 1981. Uptake and distribution of cadmium in the terrestrial slug Arion ater (L.). Comp. Biochem. Physiol. 68A, 37. Mason, A.Z., Simkiss, K., 1982. Sites of minerals deposition in metalaccumulating cells. Exp. Cell Res. 139, 383–391. Martin, M.H., Coughtrey, P.J., 1982. Biological Monitoring of Heavy Metal Pollution: Land and Air. Applied Science Publishers, London. Menta, C., 1999. Heavy metals concentrations in terrestrial gastropods — natural and experimental conditions. Acta Naturalia 35, 43–48. Reilly, C., 1991. Metal Contamination of Food. Elsevier Applied Science, London.
Metal concentrations in Helix pomatia, Helix aspersa and Arion rufus: a comparative study C. Menta *, V. Parisi Natural History Museum, University of Parma, Via Farini, 90, 43100 Parma, Italy Received 11 March 2000; accepted 1 February 2001
‘‘Capsule’’: Mucus secretions in slugs could serve as a vehicle for elimination of some metals. Abstract In this study we evaluated the differences between concentrations of copper, iron, zinc, manganese, lead and cadmium in three terrestrial gastropods: Helix pomatia, Helix aspersa and Arion rufus, collected in a semi-rural location in Northern Italy. Metal concentrations in the foot and in the digestive gland were measured. In the hepatopancreas, copper and zinc did not differ significantly in the three species; the levels of copper were also similar in the foot. In comparison to Helix sps., A. rufus demonstrated lower concentrations of manganese and cadmium in the hepatopancreas and higher concentrations in the foot. In the slug the mucus produced in the foot could represent an efficient elimination mechanism of some elements. We also investigated correlations between the trace element content in the soft tissues and the mollusk size (weight of the body and height of the shell). # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Metals; Helix pomatia; Helix aspersa; Arion rufus; Bioaccumulation
1. Introduction Different species of terrestrial invertebrates exhibit different dispositions to accumulate and eliminate essential and nonessential trace metals. Adults of the species Porcellio scaber, Tetrodontophora bielanensis, Julus scandinavius and Deroceras reticulatum exposed to food and soil contaminated by lead, cadmium or zinc differed in their metal accumulation strategies (Graef et al., 1997). Species of terrestrial gastropods, as in the case of earthworms, isopods and spiders, collected in an unpolluted forest, accumulated larger amounts of cadmium in comparison to other invertebrate groups (Dallinger, 1993). Higher concentrations of lead, copper, cadmium and zinc have been detected in tissues of metal-exposed snails in the field (Martin and Coughtry, 1982) and in the laboratory (Dallinger and Wieser, 1984; Berger et al., 1993) as well as in slugs from polluted woodlands and mining areas (Greville and Morgan, 1989, 1990). Typically some organs and tissues are involved in metal accumulation while others are not. * Corresponding author. Tel.: +39-0521-236465. E-mail address: [email protected] (C. Menta).
Metal-exposed terrestrial snails and slugs store cadmium, zinc and lead in the midgut gland (Dallinger, 1993) and concentrations in the digestive gland of Helix pomatia, exposed to lead through its food, reached levels 100 times higher than those in gastropods unexposed to metal contamination (Beeby and Richmond, 1991). In Arion ater the highest concentrations of magnesium, phosphate, cadmium and zinc were deposited in the digestive gland (Ireland, 1979). The capacity to store essential and toxic trace elements in tissues is explicable as a strategy for metal detoxification in terrestrial invertebrates in order to reduce water loss (Hopkin, 1989). Numerous studies have contributed to an understanding of bioaccumulation and metals distribution in the bodies of terrestrial gastropods, but these studies do not always provide an understanding as to whether the differences in these capacities in different species are related to the availability of these elements in the area from which the sample is drawn or to the animals’ species-specific internal regulating mechanisms. In this study we offer a comparison between the accumulation of metals in H. pomatia (L.), Helix aspersa (Mu¨ller) and Arion rufus (L.) terrestrial gastropods, all of
0269-7491/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0269-7491(01)00110-5
206
C. Menta, V. Parisi / Environmental Pollution 115 (2001) 205–208
which are rarely found in the same habitat, H. pomatia and A. rufus being distributed throughout the alpin arc while H. aspersa, a more rural species, is diffused in the Mediterranean region and the European Atlantic Coast.
soft tissues and shell height and body weight (after data transformation in ln) was evaluated with regression lines.
3. Results 2. Method The adult samples of H. pomatia, H. aspersa and A. rufus were taken (July, 1997) in a semi-rural location near Cremona (Northern Italy), from a small site between partially cultivated parcels of land. There are no caves or mines in this area. A probable source of pollution is an elevated state road 200 m from the sampling area. The specimens were rinsed in dionized water and deprived of food for three days in order to evacuate their guts. They were then weighed (g; H. pomatia: 20 8.9; H. aspersa: 7.8 2.3; A. rufus: 14 2.9) and the shell heights (mm) of the Helix sps. were measured (H. pomatia: 24 0.018; H. aspersa: 21 0.027). Then the digestive gland and foot were removed from each sample. Soft tissues were dried in a microwave oven (Milestone 1200). A sample of 0.3 g of soft tissue was mineralized by adding 3 ml HNO3 suprapure and 0.5 ml H2O2 (36% at 110 volumes) and then placed in a microwave oven (Milestone 1200) for 24 minutes. Copper, iron, manganese and zinc were measured by flame atomic absorption spectrophotometry (AAS; VarianSpectrAA-40) and cadmium and lead were determined by graphite furnace AAS (Perkin Elmer 305/B) without background correction. The AAS was calibrated using standard bovine reference material. Concentrations were obtained by direct comparison with standard solutions. Data were analysed using a one-way analysis of variance and the difference between means compared using Student’s t. We used a 1% level of significance in all tests. The relationship between trace element content in the
Table 1 indicates the mean and the standard error in the concentrations of copper, zinc, iron, manganese, lead and cadmium measured in the three species. The samples did not indicate any correlation between their weight and shell height and the concentration of metals accumulated. The tissue metal analysis of garden snails H. pomatia and H. aspersa confirmed that the highest concentrations of zinc, iron, manganese, lead and cadmium were deposited in the hepatopancreas, (in all cases differences between the two soft tissue concentrations were significant: P<0.01) while the concentration of copper was not significantly different than that found in the foot. In both snails and in A. rufus copper values reached comparable levels in the two body sections (Table 1). Iron and cadmium were deposited in the digestive gland (in both cases P<0.01) while lead and zinc seemed to be accumulated in the hepatopancreas, but the differences between the two soft tissue concentrations were not significant. Moreover, in the slug the concentration of manganese was different in the two soft tissues (P<0.01): it reached the highest value in the foot while the digestive gland seems not to store this metal. A comparative study of metal concentrations in H. pomatia and H. aspersa showed that copper, zinc, iron, manganese and cadmium were not significantly different in the hepatopancreas of the two species. On the contrary iron and manganese differed in the foot: iron was more concentrated in H. pomatia (P<0.01) while manganese reached a higher value in H. aspersa (P<0.01). The two species demonstrate different lead concentrations in the hepatopancreas. Indeed, in H. aspersa
Table 1 Means (11 values) and standard errors of dry weight (g) and metal concentrations (mg g 1) in soft tissues of Helix pomatia, Helix aspersa and Arion rufus H. pomatiaa Digestive gland Dry weight Cu Zn Fe Mn Pb Cd a b c d
H. aspersab Foot
Digestive gland
0.4600.006
1.1200.011
0.1210.003
55.6815.49 187.3237.31 385.9692.97 210.7832.25 10.652.95 13.402.63
39.496.82 86.688.79 107.5911.51 11.820.79
34.695.32 384.6964.11 620.35159.32 299.1669.31 23.043.77 15.905.77
A. rufusc Foot
Digestive gland
0.1750.005 49.966.90 70.515.04 72.515.03 21.842.26
Height of shell (mm) 24 0.02; weight of body (g) 208.9. Height of shell (mm) 210.03; weight of body (g) 7.8 2.3. Weight of body (g) 142.9. DL, detection limit (detection limit for lead and cadmium were respectively, 0.15 mg g
1
and 0.1 mg g 1).
Foot
1.5010.030
5.9030.120
35.504.70 283.49125.15 922.39168.87 92.015.61 5.12 1.96 4.20 0.48
34.923.19 171.9414.17 99.217.69 394.5466.03
C. Menta, V. Parisi / Environmental Pollution 115 (2001) 205–208
the lead concentration was twice as high as that in H. pomatia. A comparative study of Helix sps. and A. rufus showed that copper and zinc had similar levels of accumulation in the digestive gland. The slug showed less manganese and cadmium concentrations in this organ (for both metals P<0.01). The foot of the slug had higher concentrations of manganese, zinc and cadmium as opposed to its hepatopancreas (in all cases P<0.001).
4. Discussion It is well known that the metabolism and bioaccumulation of trace elements by an organism is different in young individuals than in adults. Thus it is not surprising that, generally speaking, a relationship between a species’ individual member size and an accumulated concentration of metals can be observed in immature samples. We maintain that the absence of such a relationship was evidenced in our study due to the fact that we analysed only adult individuals. This condition permitted us to eliminate an ‘individual age’ variable and rendered the analyzed sample maximally homogeneous and thus better suited for our study. In this study the three species typically indicated that zinc, cadmium, lead and iron were accumulated in the digestive gland, an organ that plays a crucial role in the animals’ nutritional physiology, but that copper was not stored there. The species H. aspersa when exposed to copper seems not to indicate a clear preponderance for storing this metal in its digestive gland (Menta, 1999), while H. pomatia is capable of eliminating it through efficient intestinal and digestive gland excretions, thus maintaining an almost constant level in its tissues (Dallinger, 1993). Similar copper concentrations in the soft tissues may in part be attributed to hemocyanin (Dallinger and Wieser, 1984). In this comparative study, the hepatopancreas of the three species showed similar concentrations of copper and zinc. The reason for this result may be explained by the presence of similar processes of absorption and excretion, the function of which makes all three capable of maintaining concentrations of the two elements at comparable levels. A comparison of H. aspersa and A. rufus samples taken from the same location demonstrated a similar copper and zinc accumulation in the digestive gland (Menta, 1999). Our collected data demonstrate that the concentration of manganese in the foot of A. rufus is 10 times higher than that found in Helix sps. This species’ remarkable capacity for accumulating manganese has been previously reported in a study conducted by Cavalloro and Ravera (1966), in which the authors noted that the concentration of this element in the slug was four times greater than the values measured in the vegetation
207
which comprised its diet. The authors stated that manganese enters the animal not only through its diet but also by way of the epithelium, noted for its permeability to solutions and specific material. According to our results such a secondary route does not seem adequate enough to explain the notable differences in concentration measurements of manganese that we found in the slug and the snails. Ireland (1979) suggests that A. rufus secretes manganese by way of a mucus secretion, basing this finding on the fact that elevated concentrations of this element have been discovered in this secretion and have not been observed in association with digestive gland granules as has occurred in the case of H. aspersa. In the latter case, H. aspersa was injected with manganese and the digestive gland granules were reformed as concentric layers of calcium magnesium pyrophosphate. The mineralization process apparently requires the formation of a nucleus of calcium and magnesium pyrophosphate before the manganese can be mineralised within the vacuole (Mason and Simkiss, 1982). Relative to the observed differences and taking into consideration an analogous mode of mineralization found in vertebrates, in which manganese seems to be essential to normal bone formation (Reilly, 1991), we hypothesize that the different amounts of manganese in the foot of the slug as compared to that of the snail is related to the different quantity of calcium required by the latter for shell formation. According to our results, A. rufus seems capable of limiting the amount of cadmium in its digestive gland. The higher concentration of cadmium in its foot as compared to that of Helix sps. causes us to hypothesize that the slug has a capacity to relocate this toxic element in different parts of its body, a means not shared by the snail as part of its accumulation process. In a similar mode as that proposed by Ireland (1979) for the elimination of manganese, the mucus secretion could function as a vehicle for the elimination of cadmium. In A. ater cadmium was not associated with any specific organelles, but was bound to a cytosolic protein showing typical features of metallothioneins (Ireland, 1981). Similar results have been reported in the slug Arion lusitanicus (Dallinger et al., 1989). This condition makes the relocation of an element from the gland to the foot easier. Further in-depth knowledge regarding the toxic metal content of the mucus secretion in the epitheliums of the snail and slug could confirm the possible function of this secretion as serving, together with the feces, as a vehicle for the elimination of some metals.
Acknowledgements We wish to express our thanks to the Alimentary Science and Technology Institute of the University of
208
C. Menta, V. Parisi / Environmental Pollution 115 (2001) 205–208
Parma for its assistance in the analytical work involved in this study. References Beeby, A., Richmond, L., 1991. Adaptation of homeostatic mechanisms: lead assimilation and mineral metabolism in the snail Helix aspersa from a polluted ecosystem. In: Ravera, O. (Eds.), Terrestrial and Aquatic Ecosystems. Perturbation and Recovery. Ellis Horwood, New York, pp. 165–170. Berger, B., Dallinger, R., Felder, E., Moser, J., 1993. Budgeting the flow of cadmium and zinc through the terrestrial gastropod, Helix pomatia L. In: Dallinger, R., Rainbow, P.S. (Eds.), Ecotoxicology of Metals in Invertebrates. Lewis Publishers, London, pp. 291–313. Cavalloro, R., Ravera, O., 1966. Biological Indicator of Manganese54 Contamination in Terrestrial Environments. Nature 5029, 1259. Dallinger, R., 1993. Strategies of metal detoxification in terrestrial invertebrates. In: Dallinger, R., Rainbow, P.S. (Eds.), Ecotoxicology of Metals in Invertebrates. Lewis Publishers, London, pp. 245–289. Dallinger, R., Janssen, H.H., Bauer-Hilty, A., Berger, B., 1989. Characterization of an inducible cadmium-binding protein from hepatopancreas of metal-exposed slugs (Arionidae, Mollusca). Comp. Biochem. Physiol. 92C, 355. Dallinger, R., Wieser, W., 1984. Patterns of accumulation, distribution and liberation of Zn, Cu, Cd, and Pb in different organs of the land snail Helix pomatia L. Comp. Biochem. Physiol. 79C, 117.
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