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Seasonal changes in zinc, manganese, magnesium, copper and calcium content in the digestive gland of the slug Arion ater L.
Camp. Biochem. Physiol. Vol. 78A, No. 4, PP. 855-858, 1984
0300-9629/84 $3.00-I-0.00 0 1984Pergamon Press Ltd
Printed in Great Britain
SEASONAL CHANGES IN ZINC, MANGANESE, MAGNESIUM, COPPER AND CALCIUM CONTENT IN THE DIGESTIVE GLAND OF THE SLUG ARION ATER L. M. P. IRELAND Department of Zoology, University College of Wales, Aberystwyth, Dyfed, SY23 3DA, UK (Receives
7 December
1983)
Except for zinc there was an increase in the total metal content of the digestive gland from May to October. This was due essentially to an increase in the weight of the gland. 2. The drop in total metal content during November was probably the result of a decrease in the size of the digestive gland. 3. Except for copper in May and calcium in November there was an increase in the metal concentration from May to Nov~ber. In some cases it was not significant. 4. Variations in the metal content are discussed in relation to seasonal uptake and age.
Abstract-l.
INTRODUCTION changes in the mineral content of the liver of vertebrates has been investigated in mammals (Hyvarinen, 1972), fish (Hyvarinen and Valtonen, 1973), birds (Haarakangas et al., 1974) and amphibia (Pasanen and Koskela, 1974a). Similar mineral studies in gastropods using the digestive gland have been reported for Helix pomatia but are confined to calcium and magnesia phosphates (Barfurth, 1883; Burton, 1972). In the animals where seasonal variation has been studied the longevity exceeds one year. Age-related changes in the mineral content of vertebrate tissues have been investigated in many species with a particular emphasis on the divalent cations magnesium and calcium (Mori and Duruisseau, 1960; &&Broom and Weiss, 1983). Arion ater was selected for the present investigation because the life span is approximately one year, maximum oocyte size is attained in September and eggs are usually laid during October (Lusis, 1961; Smith, 1966; Parivar, 1978). Under these conditions a study could be made of the seasonal variation in mineral content which would be closely related to age changes. Seasonal
MATERIALS AND Arim
METHODS
(Llanbadarn churchyard) in the first week of each month investigated. The slugs were pIaced in glass vessels at a relative humidity of 100% and a temperature of 15°C under a 12j12 photoperiod. They were deprived of food for 5 days to expel any material from the gut. To prevent coprophagy, slug receptacles were cleaned out daily. After weighing the slugs they were decapitated and the digestive gland removed, rinsed in distilled water, moist dried and weighed. To obtain the drv weight,,-digestive eland material was dried at 105°C for 48 hr. Dried material was digested in concentrated nitric acid (Aristar) and metals estimated by atomic absorption analysis. Samples used to estimate calcium contained 0.65% lanthanum chloride to suppress phosphate interference. C.B.P. 78,4A-P
RESULTS
Body weight to digestive gland weight There is a gradual increase in the ratio of body weight to digestive gland weight from June to October. The peak occurred during September-Octo~r (Table 1) which is the time when the highest wet body weight and wet and dry digestive gland weight are recorded. Water content of digestive gland The lowest significant percentage of water in Ark digestive gland is in October which corresponds with the highest dry weight mass of the digestive gland in September-October. The highest significant water content is found in June-July (Table 2). Metal levels in digestive gland A summary of the total metal content of the digestive gland is shown in Table 2. Table 3 represents a summary of the metal concentration in the digestive gland of A. ater. Calcium
ater were collected at the same site in Aberystwyth
”
Data were analysed using a one-way analysis of variance and the difference between means compared using Duncan’s multiple range test. The 5% level of si~~~nce was used in all tests.
The dry weight calcium content is significantly higher in SeptemberU~~ober than in the other Table 1. Monthly
digestive gland water and ratio of digestive gland to whole body in Arion ater Water (%)
June July August September October November
80.0 79.2 76.5 74.4 70.8 77.1
Mean k SEM. N = 14.
855
f 2 i * + *
0.5 0.8 0.1 0.4 0.6 0.7
Body wt/Digestive gld wt (wet) 8.8 i 0.3 7.9 & 0.4 9.9 i 0.3 11.0+_0.3 12.4 + 0.6 9.8 k 0.5
856
M. Table 2. Monthly Zinc (fig) May June July August Seotember Odtober November
111.9* 189 f 363.1 k 306.8 & 475.7 + 576.8 + 577.0 &
13 24 53 37 37 51 65
P.
IRELAND
metal content
Manganese
(Fg)
4.5 + 0.4 10.5 i 1.3 13.6 k 1.5 19.6 f 2.2 29.8 + 3.2 31.9z2.1 18.4 + 2.4
of Arion am
Magnesium 303.6 798.6 772.1 2010.7 2457.1 2692.9 1545.7
(pg)
+ 29 5 140 k 123 k 132 + 149 z 132 & 185
digestive
gland
Copper 5.7 I 4.6 + 6.4 + 2.9 2 6.3 + 11.4z 10.9 *
Calcium
(pg)
2.42 4.64 2.28 8.81 19.76 35.10 11.03
1.0 0.9 1.1 0.4 1.5 1.6 1.5
(mg)
+ k i + + + f
0.29 0.57 0.44 1.74 2.12 2.70 1.60
Mean i_ SEM, N = 14. Table 3. Monthly Zinc May June July August September October November
1899.6 1681.2 2009.8 953.1 1326.7 1653.2 3571.2
2 k i_ * k + +
260 318 219 87 74 98 510
metal concentration
@g/g dry wt) in Arion nfer digestive
Manganese
Magnesium
72.8 86.7 80.3 62.7 81.8 93.1 101.5
5004.7 6207.1 4277.8 6472.9 6610.1 8040.3 8299.2
2 5.6 f 8.0 rt 6.1 & 6.2 & 6.1 * 5.0 + 11.9
k k + + + * +
488 913 342 332 505 501 659
Copper 96.8 31.3 37.4 8.9 18.4 31.8 57.9
k * * ct + k *
20 5.0 5.0 1.2 5.1 3.1 7.1
gland Calcium 38.63 39.89 28.01 28.86 53.75 95.07 59.81
(mg) + 3.6 * 4.4 k 3.2 & 4.2 + 3.6 * 9.5 f 5.3
Mean * SEM, N = 14.
months tested. This corresponds with the highest significant dry weight of digestive gland. The calcium concentration gradually increases from May to October, but in October it is significantly higher than any other month sampled. In November there is a highly significant drop in the dry weight content and concentration to similar values found in September. Magnesium
There is a gradual increase in the magnesium content of the digestive gland from May to October but reaching a significant peak in SeptemberOctober, followed by a significant drop in November, The dry weight magnesium concentration showed a gradual increase from May to November, May being significantly lower than October-November. Manganese
The dry weight manganese content is significantly higher in September-October than any other month sampled. This is followed by a significant drop in November. The dry weight manganese concentration showed a gradual but insignificant increase from May to November. Copper
The dry weight copper content of Arion digestive gland is significantly the highest in OctoberNovember. The copper concentration is significantly the highest in May but not significantly different in the other months sampled. Zinc
There is a gradual increase in the dry weight zince content from May to November. The zinc content is significantly higher in October-November than during May-June. The zinc dry weight concentration is significantly higher in November than any other month sampled. DISCUSSION
The increase in weight of the digestive gland of in relation to total body weight is not un-
Arion
expected since in numerous animals the size of internal organs is selected is related to body weight. The rat liver has been reported to show proportional changes in weight directly related to body weight (Yiengst et al., 1959). Except for November, there was a tendency for water content of the digestive gland of A. ater to fall from June to October. According to Pasanen and Koskela (1974b) the water content of Rana temporariu liver was lowest in September-October and reached a maximum in June, showing a seasonal variation. In the rat, the water content of the aorta, muscle and liver showed a marked decrease with age (Mori and Durisseau, 1960). The elevated water content of the digestive gland of mature Arion recorded in early November could be due to the extremely emaciated and swollen state of the slugs during this time of year when they are reaching the end of their life expectancy. The life span of A. ater is 1l-12 months and the fertile eggs hatch in October (Lusis, 1961; Smith, 1966). It is difficult to compare seasonal variations in the metal levels of Arion with published work on the shrew (Hyvarinen, 1972), R. temporaria (Pasanen and Koskela, 1974a) and fish (Hyvarinen and Valtonen, 1973) since breeding cycles and longevity are different but it is worth highlighting some of the more interesting comparisons. In Arion there was a general trend towards an increase from May to October in the digestive gland content of the five metals studied. This can be attributed to an increase in the weight of the digestive gland from May to October. In the frog an increase in the total liver content of copper, zinc, magnesium and calcium occurred beween May and October (Pasanen and Koskela, 1974a). Except for zinc and copper, there was a significant drop in the metal content of the digestive gland of A. ater in November compared with October. This may be supported by a descrease in the dry weight of the digestive gland in November. In the frog liver a similar drop in copper, zinc, magnesium and calcium occurred when Ram were wintering (November-March) compared with when they were entering their winter place
Seasonal
changes
in Zn, Mn, Mg, Cu and Ca in slug digestive
(September-October). During wintering the frogs did not eat and the liver size decreased (Pasanen and Koskela, 1974a). The high concentration of copper in Arion digestive gland during May compared with other months has also been reported for the liver of R. temporariu females (Pasanen and Koskela, 1974a). They associated the copper increase with a decrease in the size of the liver and that during spawning (May) copper accumulated in large quantities in the liver of female frogs. This is clearly not the case in A. ater since there was no drop in digestive gland weight during May and the male and female gametes were just beginning to differentiate (Parivar, 1978). In the soft tissues of the American oyster, Crassostrea virginica, there are elevated concentrations of zinc, copper and manganese in the summer. Zinc and copper reached a peak in early August and manganese reached a peak around the first of October. All three metals decreased to winter levels by November. It was suggested that the rapid decrease in zinc and copper during August-September may be related to gametogenesis. Manganese concentrations, on the other hand, were probably correlated with shell growth between July and November (Frazier, 1975). Although not significantly different using a one-way analysis of variance, there was a drop in the concentrations of copper and manganese during August in Arion. During August-September the oocytes of A. ater attain maximum size (Parivar, 1978). From May there is a rapid increase in the body weight of A. ater (Parivar, 1978). This may have resulted in the general increase in the metal’concentration of the digestive gland throughout the year. Since divalent cations are probably not absorbed across the intestine but across the foot epithelium (Ireland, 1982) the accumulation may be simply related to the time the slugs are in contact with the substratum. The concentration of magnesium and calcium in the digestive gland of Helix pomatiu declined markedly between mid-July and the end of September. It was tentatively suggested that the digestive gland supplied the calcium phosphate utilized by the thick winter epiphragm secreted by the mantle (Burton, 1972). According to Pollard (1973) H. pomatia as old as 10 years are not uncommon in natural populations. The change in the mineral concentrations during the autumnal months may be indicative of the snails going into winter quiescence and not termination of their life span. The high concentration of calcium in A. ater in October may be related to ovulation. Maximum oocyte size is attained in August-September and eggs are usually laid during October (Lusis, 1961; Smith, 1966; Parivar, 1978). The oocytes of A. ater contain high concentrations of calcium (Lusis, 1961; Tompa, 1976). In Helix blood calcium levels are elevated by 70% during egg-laying but the source of the calcium required was not reported (Tompa and Wilbut, 1977). The high concentration of calcium in the digestive gland of Arion in October could be indicative of calcium build-up, this extra calcium being the source of the metal utilized in eggs during calcification. The drop in the calcium content and concentration in the digestive gland in November could simply
gland
857
indicate a depletion after egg-laying, or transport of the calcium to other sites. Barfurth (1883) postulated that the digestive gland calcium in Arion was deposited in the skin during winter. An alternative explanation is that the slugs were reaching the end of their life expectancy and the response was age-related. In the rat no change occurred in the liver concentration of calcium but cardiac and skeletal muscle, together with skin calcium, showed a significant drop in concentration associated with ageing (Mori and Duruisseu, 1960; McBroom and Weiss, 1973).
REFERENCES
Barfurth D. (1883) Ueber den Bau und die Thatigkeit der Gasteropodenleber. Arch. mikrosk. Anat. EntwMech. 22, 473-524. Burton R. F. (1972) The storage of calcium and magnesium phosphates and of calcite in the digestive glands of the pulmonata (Gastropoda). Comp. Biochem. Physiol. 43A, 655-663. Frazier J. M. (1975) The dynamics of metals in the American oyster Crassostrea virgiizica. 1. Seasonal effects. Chesapeake Sci. 16, 162-171. Haarakangas H., Hyvarinen H. and Ojanen M. (1974) Seasonal variations and the effects of nesting and moulting on the liver mineral content in the house sparrow (Passer domesticus L.). Comp. Biochem. Physiol. 4lA, 153-163. Hyvarinen H. (1972) Seasonal changes in the copper content of the liver of the common shrew (Sorex aruneus) over a two year period. J. Zool. Lond. 166, 411416. Hyvarinen H. and Baltonen T. (1973) Seasonal changes in the liver mineral contents of Coregonus nurus (Pallas), sensu Svar&on, in the Bay of Bothnia. Camp. Biochem. Physiol. 45B, 875-881. Ireland M. P. (1982) Sites of water. zinc and calcium uutake and distribution of these metals after cadmium administration in Arion ater (Gastropoda: Pulmonata). Comp. Biochem. Physiol. 13A, 217-221. Lusis 0. (1961) Postembryonic changes in the reproductive system of the slug Arion ater refis L. Proc. zool. Sot. Lond. 137, 433-468. McBroom M. J. and Weiss A. K. (1973) A longitudinal and comparative study of the soft tissue calcium levels throughout the life-span of highly inbred rats. J. Geront. 28, 143-151. McBroom M. J. and Weiss A. K. (1983) Soft tissue potassium, magnesium, sodium and ash contents throughout the life-span of highly inbred rats. Comp. Biochem. Physiol. ISA, 529-533. Mori K. and Duruisseau J. (1960) Water and electrolyte changes in aging process with special reference to calcium and magnesium in cardiac muscle. Can. J. Biochem. Physiol. 38, 919-928. Parivar K. (1978) A histological survey of gonadal development in Arion ater L. (Mollusca, Pulmonata). J. mall. Stud. 44, 250-264. Pasanen S. and Koskela P. (1974a) Seasonal changes in calcium, magnesium, copper and zinc in the liver of the common frog, Rana temporaria L. Comp. Biochem. Physiol. MA, 27-36. Pasanen S. and Koskela P. (1974b) Seasonal and age variation in the metabolism of the common frog, Rana temporaria L. in Northern Finland. Comp. Biochem. Physiol. 47A, 635-654. Pollard E. (1975) Aspects of the ecology of Helix pomatia L. J. Anim. Ecol. 44, 305-329. Smith B. J. (1966) Maturation of the reproductive tract of Arion ater (Pulmonata: Arionidae). Malacologia 4, 325-349.
M. P. IRELAND
858
Tompa A. S. (1976) A comparative study of the ultrastructure and minerology of calcified land snail ;;ysgrgulmonata: Stylommatophora). J. Morph. 150, Tompa
A. S. and Wilbut K. M. (1977) Calcium
mobilisation
during reproduction in the snail Helix aspena. Nature, Lond. 210, 53-54. Yiengst M. J., Barrows C. H. and Shock N. W. (1959) Age changes in the chemical composition of muscle and liver in the rat. J. Geront. 14, 40@404.
0300-9629/84 $3.00-I-0.00 0 1984Pergamon Press Ltd
Printed in Great Britain
SEASONAL CHANGES IN ZINC, MANGANESE, MAGNESIUM, COPPER AND CALCIUM CONTENT IN THE DIGESTIVE GLAND OF THE SLUG ARION ATER L. M. P. IRELAND Department of Zoology, University College of Wales, Aberystwyth, Dyfed, SY23 3DA, UK (Receives
7 December
1983)
Except for zinc there was an increase in the total metal content of the digestive gland from May to October. This was due essentially to an increase in the weight of the gland. 2. The drop in total metal content during November was probably the result of a decrease in the size of the digestive gland. 3. Except for copper in May and calcium in November there was an increase in the metal concentration from May to Nov~ber. In some cases it was not significant. 4. Variations in the metal content are discussed in relation to seasonal uptake and age.
Abstract-l.
INTRODUCTION changes in the mineral content of the liver of vertebrates has been investigated in mammals (Hyvarinen, 1972), fish (Hyvarinen and Valtonen, 1973), birds (Haarakangas et al., 1974) and amphibia (Pasanen and Koskela, 1974a). Similar mineral studies in gastropods using the digestive gland have been reported for Helix pomatia but are confined to calcium and magnesia phosphates (Barfurth, 1883; Burton, 1972). In the animals where seasonal variation has been studied the longevity exceeds one year. Age-related changes in the mineral content of vertebrate tissues have been investigated in many species with a particular emphasis on the divalent cations magnesium and calcium (Mori and Duruisseau, 1960; &&Broom and Weiss, 1983). Arion ater was selected for the present investigation because the life span is approximately one year, maximum oocyte size is attained in September and eggs are usually laid during October (Lusis, 1961; Smith, 1966; Parivar, 1978). Under these conditions a study could be made of the seasonal variation in mineral content which would be closely related to age changes. Seasonal
MATERIALS AND Arim
METHODS
(Llanbadarn churchyard) in the first week of each month investigated. The slugs were pIaced in glass vessels at a relative humidity of 100% and a temperature of 15°C under a 12j12 photoperiod. They were deprived of food for 5 days to expel any material from the gut. To prevent coprophagy, slug receptacles were cleaned out daily. After weighing the slugs they were decapitated and the digestive gland removed, rinsed in distilled water, moist dried and weighed. To obtain the drv weight,,-digestive eland material was dried at 105°C for 48 hr. Dried material was digested in concentrated nitric acid (Aristar) and metals estimated by atomic absorption analysis. Samples used to estimate calcium contained 0.65% lanthanum chloride to suppress phosphate interference. C.B.P. 78,4A-P
RESULTS
Body weight to digestive gland weight There is a gradual increase in the ratio of body weight to digestive gland weight from June to October. The peak occurred during September-Octo~r (Table 1) which is the time when the highest wet body weight and wet and dry digestive gland weight are recorded. Water content of digestive gland The lowest significant percentage of water in Ark digestive gland is in October which corresponds with the highest dry weight mass of the digestive gland in September-October. The highest significant water content is found in June-July (Table 2). Metal levels in digestive gland A summary of the total metal content of the digestive gland is shown in Table 2. Table 3 represents a summary of the metal concentration in the digestive gland of A. ater. Calcium
ater were collected at the same site in Aberystwyth
”
Data were analysed using a one-way analysis of variance and the difference between means compared using Duncan’s multiple range test. The 5% level of si~~~nce was used in all tests.
The dry weight calcium content is significantly higher in SeptemberU~~ober than in the other Table 1. Monthly
digestive gland water and ratio of digestive gland to whole body in Arion ater Water (%)
June July August September October November
80.0 79.2 76.5 74.4 70.8 77.1
Mean k SEM. N = 14.
855
f 2 i * + *
0.5 0.8 0.1 0.4 0.6 0.7
Body wt/Digestive gld wt (wet) 8.8 i 0.3 7.9 & 0.4 9.9 i 0.3 11.0+_0.3 12.4 + 0.6 9.8 k 0.5
856
M. Table 2. Monthly Zinc (fig) May June July August Seotember Odtober November
111.9* 189 f 363.1 k 306.8 & 475.7 + 576.8 + 577.0 &
13 24 53 37 37 51 65
P.
IRELAND
metal content
Manganese
(Fg)
4.5 + 0.4 10.5 i 1.3 13.6 k 1.5 19.6 f 2.2 29.8 + 3.2 31.9z2.1 18.4 + 2.4
of Arion am
Magnesium 303.6 798.6 772.1 2010.7 2457.1 2692.9 1545.7
(pg)
+ 29 5 140 k 123 k 132 + 149 z 132 & 185
digestive
gland
Copper 5.7 I 4.6 + 6.4 + 2.9 2 6.3 + 11.4z 10.9 *
Calcium
(pg)
2.42 4.64 2.28 8.81 19.76 35.10 11.03
1.0 0.9 1.1 0.4 1.5 1.6 1.5
(mg)
+ k i + + + f
0.29 0.57 0.44 1.74 2.12 2.70 1.60
Mean i_ SEM, N = 14. Table 3. Monthly Zinc May June July August September October November
1899.6 1681.2 2009.8 953.1 1326.7 1653.2 3571.2
2 k i_ * k + +
260 318 219 87 74 98 510
metal concentration
@g/g dry wt) in Arion nfer digestive
Manganese
Magnesium
72.8 86.7 80.3 62.7 81.8 93.1 101.5
5004.7 6207.1 4277.8 6472.9 6610.1 8040.3 8299.2
2 5.6 f 8.0 rt 6.1 & 6.2 & 6.1 * 5.0 + 11.9
k k + + + * +
488 913 342 332 505 501 659
Copper 96.8 31.3 37.4 8.9 18.4 31.8 57.9
k * * ct + k *
20 5.0 5.0 1.2 5.1 3.1 7.1
gland Calcium 38.63 39.89 28.01 28.86 53.75 95.07 59.81
(mg) + 3.6 * 4.4 k 3.2 & 4.2 + 3.6 * 9.5 f 5.3
Mean * SEM, N = 14.
months tested. This corresponds with the highest significant dry weight of digestive gland. The calcium concentration gradually increases from May to October, but in October it is significantly higher than any other month sampled. In November there is a highly significant drop in the dry weight content and concentration to similar values found in September. Magnesium
There is a gradual increase in the magnesium content of the digestive gland from May to October but reaching a significant peak in SeptemberOctober, followed by a significant drop in November, The dry weight magnesium concentration showed a gradual increase from May to November, May being significantly lower than October-November. Manganese
The dry weight manganese content is significantly higher in September-October than any other month sampled. This is followed by a significant drop in November. The dry weight manganese concentration showed a gradual but insignificant increase from May to November. Copper
The dry weight copper content of Arion digestive gland is significantly the highest in OctoberNovember. The copper concentration is significantly the highest in May but not significantly different in the other months sampled. Zinc
There is a gradual increase in the dry weight zince content from May to November. The zinc content is significantly higher in October-November than during May-June. The zinc dry weight concentration is significantly higher in November than any other month sampled. DISCUSSION
The increase in weight of the digestive gland of in relation to total body weight is not un-
Arion
expected since in numerous animals the size of internal organs is selected is related to body weight. The rat liver has been reported to show proportional changes in weight directly related to body weight (Yiengst et al., 1959). Except for November, there was a tendency for water content of the digestive gland of A. ater to fall from June to October. According to Pasanen and Koskela (1974b) the water content of Rana temporariu liver was lowest in September-October and reached a maximum in June, showing a seasonal variation. In the rat, the water content of the aorta, muscle and liver showed a marked decrease with age (Mori and Durisseau, 1960). The elevated water content of the digestive gland of mature Arion recorded in early November could be due to the extremely emaciated and swollen state of the slugs during this time of year when they are reaching the end of their life expectancy. The life span of A. ater is 1l-12 months and the fertile eggs hatch in October (Lusis, 1961; Smith, 1966). It is difficult to compare seasonal variations in the metal levels of Arion with published work on the shrew (Hyvarinen, 1972), R. temporaria (Pasanen and Koskela, 1974a) and fish (Hyvarinen and Valtonen, 1973) since breeding cycles and longevity are different but it is worth highlighting some of the more interesting comparisons. In Arion there was a general trend towards an increase from May to October in the digestive gland content of the five metals studied. This can be attributed to an increase in the weight of the digestive gland from May to October. In the frog an increase in the total liver content of copper, zinc, magnesium and calcium occurred beween May and October (Pasanen and Koskela, 1974a). Except for zinc and copper, there was a significant drop in the metal content of the digestive gland of A. ater in November compared with October. This may be supported by a descrease in the dry weight of the digestive gland in November. In the frog liver a similar drop in copper, zinc, magnesium and calcium occurred when Ram were wintering (November-March) compared with when they were entering their winter place
Seasonal
changes
in Zn, Mn, Mg, Cu and Ca in slug digestive
(September-October). During wintering the frogs did not eat and the liver size decreased (Pasanen and Koskela, 1974a). The high concentration of copper in Arion digestive gland during May compared with other months has also been reported for the liver of R. temporariu females (Pasanen and Koskela, 1974a). They associated the copper increase with a decrease in the size of the liver and that during spawning (May) copper accumulated in large quantities in the liver of female frogs. This is clearly not the case in A. ater since there was no drop in digestive gland weight during May and the male and female gametes were just beginning to differentiate (Parivar, 1978). In the soft tissues of the American oyster, Crassostrea virginica, there are elevated concentrations of zinc, copper and manganese in the summer. Zinc and copper reached a peak in early August and manganese reached a peak around the first of October. All three metals decreased to winter levels by November. It was suggested that the rapid decrease in zinc and copper during August-September may be related to gametogenesis. Manganese concentrations, on the other hand, were probably correlated with shell growth between July and November (Frazier, 1975). Although not significantly different using a one-way analysis of variance, there was a drop in the concentrations of copper and manganese during August in Arion. During August-September the oocytes of A. ater attain maximum size (Parivar, 1978). From May there is a rapid increase in the body weight of A. ater (Parivar, 1978). This may have resulted in the general increase in the metal’concentration of the digestive gland throughout the year. Since divalent cations are probably not absorbed across the intestine but across the foot epithelium (Ireland, 1982) the accumulation may be simply related to the time the slugs are in contact with the substratum. The concentration of magnesium and calcium in the digestive gland of Helix pomatiu declined markedly between mid-July and the end of September. It was tentatively suggested that the digestive gland supplied the calcium phosphate utilized by the thick winter epiphragm secreted by the mantle (Burton, 1972). According to Pollard (1973) H. pomatia as old as 10 years are not uncommon in natural populations. The change in the mineral concentrations during the autumnal months may be indicative of the snails going into winter quiescence and not termination of their life span. The high concentration of calcium in A. ater in October may be related to ovulation. Maximum oocyte size is attained in August-September and eggs are usually laid during October (Lusis, 1961; Smith, 1966; Parivar, 1978). The oocytes of A. ater contain high concentrations of calcium (Lusis, 1961; Tompa, 1976). In Helix blood calcium levels are elevated by 70% during egg-laying but the source of the calcium required was not reported (Tompa and Wilbut, 1977). The high concentration of calcium in the digestive gland of Arion in October could be indicative of calcium build-up, this extra calcium being the source of the metal utilized in eggs during calcification. The drop in the calcium content and concentration in the digestive gland in November could simply
gland
857
indicate a depletion after egg-laying, or transport of the calcium to other sites. Barfurth (1883) postulated that the digestive gland calcium in Arion was deposited in the skin during winter. An alternative explanation is that the slugs were reaching the end of their life expectancy and the response was age-related. In the rat no change occurred in the liver concentration of calcium but cardiac and skeletal muscle, together with skin calcium, showed a significant drop in concentration associated with ageing (Mori and Duruisseu, 1960; McBroom and Weiss, 1973).
REFERENCES
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