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Helicid snails appear to regulate Pb more closely than other toxic metals, though it is reported as the least toxic. No regulatory mechanism has been described in animals, and the possible role of Mg in limiting Pb assimilation is examined here for the first time. Three populations of Cantareus aspersus were fed Pb and Ca with three levels of Mg for up to 64 days. Metal assimilation and production efficiency was calculated for each of 108 snails. Populations differed in their pattern of uptake but soft tissue Pb was unaffected by dietary Mg. The proportion of Pb assimilated did not change as soft tissue concentrations increased, indicating no specific regulatory mechanism. The daily addition of Pb to the soft tissues increases with growth rate suggesting uptake is instead some function of growth or cell turnover. Bioconcentration factors varied with time and are unreliable indicators of an evolved regulatory mechanism for Pb.

An optimal division of a key resource between growth and reproduction is expected to produce consistent life history schedules in habitats where its supply is highly predictable. However, differential growth rates are found between populations and within broods of Cantareus aspersus, a simultaneous hermaphrodite for which the reproductive benefits of a large body size may favour rapid growth. Although energy is usually assumed to be the limiting resource in allocation theory, calcium limits the distribution, growth and reproduction of snails. This is a very consistent resource and populations may have allocation strategies which reflect availability in their habitats. Three experiments compared Ca allocation in the progeny of six populations from Ca-rich and Ca-poor habitats. In the first, 100 d-old juveniles were compared between populations for their shell/soft-tissue dry weight ratio, their allocation of Ca to each compartment, and the variability within broods. The second measured growth, food consumption and shell ratios in growth trials of three populations on low Ca. Thirdly, five populations were compared on abundant or excess Ca. The relationship of shell Ca with soft-tissue levels differs between populations, but shell ratios changed with Ca availability in all populations. Most favoured soft-tissue growth when dietary Ca is low, but one population (LE) always had the highest shell ratios in these trials. Ca in the parental habitat was not a good predictor of juvenile-allocation strategies, but the consistency of LE shell ratios across several broods suggests theirs may be an inherited trait. LE has faster growth rates and a preference for shell building, which probably represents a strategy for early reproduction. The robustness of a snail’s shell may thus be more indicative of its reproductive strategy rather than Ca availability in its habitat.

Previous research suggests that free metal concentrations in soils are the best correlate of the proportion available for uptake, especially by plants. The same may be true for animals which ingest soil, such as earthworms and gastropod molluscs. Using predictions from a simple model proposed by Sauve et al. [Environ. Pollut. (1997) 149] and based on soil pH and total soil concentrations, the relationship between free soil Pb and soft tissue Pb concentrations is evaluated in the garden snail Helix aspersa. Five replicate adults collected from 23 sites in England and Wales were partitioned into two tissue fractions and the relationship between their Pb concentrations and those of their native soil was examined using simple regression analysis. Levels in both fractions rose with free Pb concentrations, but the terms of each relationship and its level of significance were greatly influenced by the most contaminated soils. Neither tissue fraction served as an effective sentinel of free soil Pb and the regression coefficient was a poor summary statistic across sites with a large range of contamination. The same was true of Pb concentrations in the whole soft tissues. It may be that free ion concentrations are not indicative of metal availability to animals with a range of mechanisms to assimilate nutrients, perhaps because of undescribed environmental and physiological factors governing assimilation or limiting soft tissue concentrations.

In an earlier paper examining inherited tolerance to Pb, the shell growth of laboratory-bred offspring of Helix aspersa from contaminated sites was compared with that ofjuveniles from naive populations on dosed and undosed diets. Eight-week-old snails were fed either 500 microg g(-1) Pb or a control food in competitive trials between two populations. In the first series of trials, a parental history of exposure to Pb did not confer any advantage to either of two populations (BI and MI) competing with a naïve population (LE). whether Pb was present in the diet or not. However, in the analysis of their metal concentrations reported here, LE are found to retain higher levels of Pb in the soft tissues than either BI or MI. Compared to their siblings on the unleaded diet, dosed LE and BI juveniles had lower soft tissue concentrations of Ca and Mg. Although the growth in shell height is unaffected by diet, LE and BI juveniles build lighter shells on the Pb-dosed diet, achieving around 75% of the shell mass of their controls. In contrast, the shell weights of dosed MI juveniles are depressed by only 15% and show no change in the essential metal concentrations of their soft tissues. A second experiment using five populations fed only the dosed food show that the shell weight/soft tissue weight ratios are comparable to the dosed snails of the previous experiment. Building a lighter shell thus appears to be the common response of all Helix populations to a high Pb diet, at least amongst juveniles. The reduction in its mass means that less Ca and Mg is added to the shell and, along with the lowered soft tissue concentrations observed in some populations. may be a consequence of an increased effort to excrete Pb. The possibility that the MI population shows a genotypic adaptation. perhaps as some form of modification of its Ca metabolism, is briefly discussed.

The common garden snail has been proposed as a sentinel species for mapping environmental concentrations of toxic metals, since it accumulates high levels in its soft tissues. The capacity of Helix aspersa to quantify site differences is assessed here from a survey of 23 populations around UK and Wales with a wide range of contamination. Five adult snails were collected from each site during 1 week in 1990. Each replicate snail was partitioned into two soft tissue components (the hepatopancreas and the ‘rest’) and their Pb, Cd, Cu and Zn levels compared with those in the soil (as an integrative long-term measure of exposure) and the leaves of one food plant, Taraxacum (as a measure of recent dietary exposure). Median concentrations in one or both tissue fractions correlated with both soil and vegetation levels for each metal except Cu. Whole soft tissue Zn, Pb and Cd concentrations were also correlated with either soil or Taraxacum or both. However, Cd and Zn levels were not independent of tissue dry weight and there were also interference effects between Pb, Cd and Zn in the ‘rest’ and between Cd and Zn in all fractions. Lead in the hepatopancreas correlated with both soil and dietary sources, and was independent of dry weight. There was some correlation of Cd with Pb in this tissue. Outliers largely determined the relationship between hepatopancreatic Pb and both soil Pb and vegetation Pb. If sites with very high levels in each source were excluded from the analysis, the significant correlation of hepatopancreatic Pb with vegetation and soil concentrations disappear. This study emphasizes that sentinels have to do more than simply rank sites by their tissue concentrations: a prime criterion in their selection should be their capacity to quantify the magnitude of site differences.

A range of invertebrates have become adapted to certain toxic metals, such that, in the presence of the contaminant, some measure of their performance, typically growth or reproduction, is superior to that of an unexposed population. Under such a toxic stress, a population with a history of exposure might outperform a naive population in competition for limited resources. This study compared the shell growth of laboratory-bred juveniles from six populations of Helix aspersa with different histories of exposure to Pb. In 10 trials using various combinations of two populations, the snails competed for a limited supply of food that contained either no Pb or 500 μg/g −1 Pb, for 98 days. Each trial consisted of 10 juveniles, five from each population and was replicated four times. Nearly all of the food provided was consumed quickly after presentation. The total amount of shell growth within each replicate (the sum of the mean growth of the two populations) was highly consistent between trials so that the total amount of shell built was limited by food availability. The presence of Pb in the diet caused no measurable depression of shell growth and exposure history did not appear to confer any competitive advantage or disadvantage in any of the trials. One population consistently grew faster than its competitors in every trial of which it was a part. Shell growth tended to be greater in smaller juveniles. Snail activity is known to be inhibited at high densities and this may have contributed to the lower incremental growth in individuals kept at the higher densities. The competitive advantage enjoyed by one population may be primarily determined by their activity or perhaps their Ca metabolism.

Sentinel species are biological monitors that accumulate a pollutant in their tissues without significant adverse effects. Primarily used to measure the amount of a pollutant that is biologically available, they may also increase the sensitivity of an analytical procedure or summarise a complex pollution signal. This paper examines the validation of sentinels, referring particularly to the use of invertebrates in monitoring toxic metal pollution in terrestrial ecosystems. Few studies measure the capacity of a sentinel to quantify changes in ambient levels. Definitions of 'bioavailability' also differ between workers and few note that this may be a highly species-specific measure. Validation requires that the temporal and spatial scales over which a sentinel integrates a pollution signal are known. The sentinel has to be calibrated against source concentrations and this relationship shown to be consistent over the normal range of exposure. This requires some consideration of the environmental and biological determinants of pollutant assimilation. Differences between populations can confound simple comparisons between sites based on native populations. Transplanted individuals, matched for age, sex and physiological state, might be used when the aim is not to assess bioavailability to the resident population. A simple measure of their capacity to detect differences in ambient pollutant levels is proposed to evaluate candidate species, to assess their consistency and capacity to equilibrate with their source. A small survey of earthworms from a well-defined gradient of Pb pollution is used to illustrate problems of interpreting tissue concentrations in sentinels.

The reproductive performance of two populations of Helix aspersa with different histories of exposure to lead was assessed by their egg production, hatching success and the metal content of the eggs. By using laboratory-bred parents raised in a Pb-free environment, the possibility of an inherited response to a high Pb exposure was assessed. Following a period of dormancy, six individuals in each population were paired for copulation and allowed to oviposit. During this time they were fed a diet with 500 pg.g-' Pb, for a total of 14 days. After oviposition, the egg mass from each pairing was weighed, counted and samples analysed. Metal concentrations in four tissue fractions of the parents (shell, hepatopancreas, reproductive system and 'rest') were also analysed. There was no significant transfer of Pb to the eggs in either population. There were differences in their egg masses-the population from an uncontaminated site, Rottingdean (RD), laid, on average, twice the number of eggs of the snails from Minera (MI), a contaminated site in North Wales. However the latter had a higher average percentage emergence (90% compared to 55% for RD). There was also considerable hatching asynchrony in the RD offspring, while hatching within each MI egg mass was completed within two days. This is discussed in relation to the Ca status of the parent and possible differences in reproductive strategy between the two populations.

The eggs of terrestrial gastropods represent a major investment of resources by the parent, not least in the provision of essential metals. In this experiment 11 populations of Helix aspersa were collected from habitats with different histories of lead exposure and allowed to reproduce in the laboratory. After 4 months on a low metal diet, five four-egg samples were taken from a single clutch of each population and analysed for their metal content. Within-clutch and between population variation in the eggs were measured. No Pb or Cd was detected in any of the eggs. Both trace and essential metals show considerable variation between eggs within the same clutch. Cu levels were consistent across populations while egg Zn was closely correlated with soil Zn. Median Ca and Mg levels show little relation to soil concentrations. However, Ca provision declines with soil Pb across the populations. While the metal content of the eggs appears to be closely regulated, an interaction between Pb and Ca in the parental tissues may mediate the supply of Ca to the eggs.

Populations of snails inhabiting areas with different histories of Pb contamination differed in their deposition of Pb in shell relative to soft tissues. Genetic variation, measured using isozymes, was not related to Pb history nor geographic distance between populations. Shell characteristic* were significantly different among sites; shell dry weight was strongly related to soil calcium levels. Shells of snails from areas with long histories of Pb contamination were significantly more robust (greater shell width/shell height ratio) than snails from other locations. H. aspena adaptation to Pb contamination may involve significant changes in shell characteristics but these do not correlate with genetic traits assessed with allozymes

The assimilation efficiency of Pb was measured in the candidate sentinel Cantareus aspersus, to assess its capacity to consistently represent the bioavailability of Pb. Three populations were compared from habitats with different levels of toxic and essential metals. For each, 36 snails were fed a laboratory diet with known concentrations of Pb, Ca and Mg over 64 days. Lead bioavailability was measured as the percentage assimilation by the soft tissues (mass in tissues as a fraction of that consumed), and its variation was assessed between populations, exposure periods, dietary Mg and replicates. The indicated bioavailability was consistent over time and Pb consumed, but differed between populations and diets. Population differences were due, in part, to their different growth rates: soft tissue growth was positively correlated with Pb mass and, in the most rapidly growing populations, this accounted for more than 40% of the variation in soft tissue Pb. Populations did not differ in their consumption of Pb, but the highest Mg diet induced higher food consumption and a reduced assimilation of Pb. Tissue growth determines retention and contributes to the variation in the bioavailability indicated by these populations, as do essential metals in the diet. Such variability has important implications for monitoring programmes using resident populations of sentinels to map the bioavailability of toxic metals or integrate a pollution signal over time.

The loss of Pb from snail soft tissues may depend on the excretion of Ca, and involve the mobilization of shell Ca. Most sub-adults in three populations of Cantareus aspersus (syn. Cornu aspersum Muller) either failed to add, or lost, shell mass on a diet with 250 μg g −1 Pb. Their failure to mineralize shell extensions occurred irrespective of food consumed, time or dietary Mg. Budgets of metals for 36 individuals in each of two populations showed that Ca loss would account for the reduction in shell mass. Lead concentrations were higher in the reduced shells but this may be a consequence of their smaller mass, rather than its cause. In both populations shell reduction correlated with the total mass of Pb assimilated. Any shell growth may have been dependent on the initial Ca reserve in each snail. Differential movement of Mg, Pb and Ca occurred between the shell and soft tissues.

Lead sequestration in shell was examined for English and Welsh populations of the common garden snail (Helix aspersa) with different Pb exposure histories. Isotopic Pb ratios provided signatures for Pb source and a means of implying duration of population exposure from decades to millennia. Total Pb concentrations were used to quantify the intensity of exposure experienced by the populations. Snails from populations with long histories of exposure (millennia) to high Pb levels had proportionately more Pb in their shell than soft tissue compared with snails from other surveyed populations. These observations suggest that Pb sequestration in shell has been enhanced in H. aspersa populations with long and intense exposure to Pb.

Lead and calcium nitrate in a potato base were fed to Porcellio scaber; three levels of each cation were used in a 32 factorial design experiment. The amounts accumulated were determined by atomic absorption spectrophotometric analysis of nitric acid digests of whole animals. Both cations increased markedly in the woodlice with increasing dosage and a clear correlation was found between the rates of uptake of the two.

Snails from a contaminated urban car park were fed an experimental diet for 64 days. One group received a high Pb diet for the duration, and another group were removed from the high Pb diet after two days. Shells were analyzed at various intervals to measure Pb uptake and loss. Regression analysis of twenty-eight shells for each treatment suggests that the weight of Pb in the shell increased linearly with time, but loss from the shell was correlated with soft tissue Pb levels. The shells of fourteen snails from an uncontaminated site accumulated Pb less quickly on the high Pb diet, and had lower Mg and Ca concentrations. All snails lost Mg from the shell following the initial dose, possibly as part of a detoxification mechanism for heavy metals. The shell may lose Pb over a number of days, serving as a short-term sink for Pb which it releases when soft tissue concentrations fall.

Helix aspersa were collected from a car park with a high degree of Pb and Zn pollution over a period of 102 days following the end of aestivation. Analysis of whole soft tissues demonstrates a high variability in both Pb and Ca levels in the short term, possibly associated with shell formation. Pb, Zn and Ca concentrations are also shown to vary with age-class. No simple relationship between total Ca and total heavy metal levels was found, and it is concluded that Ca does not account for the high variability between individuals. The rate of loss of each heavy metal is shown to vary with age-class.

Soft tissue concentrations of lead, calcium, and magnesium were measured in the garden snail,Helix aspersa, over a 64-day dosing regime. Snails were compared from an uncontaminated site and from a grossly polluted car park. In each case, 25 snails were given a diet with 500 μg/g Pb (as PbSO4), and 25 were removed to a Pb-free diet after two days on the Pb dose. The snails from the car park maintained a higher concentration of Ca in the soft tissues, and assimilated Ca faster on a high Pb diet. Concentrations of Pb and Ca can be correlated for the soft tissues of the dosed car park snails. No such relationship was found for the snails from the rural site. The rate of Ca assimilation may determine the excretion rate of intracellular granules, where Pb is bound. Population differences in Pb uptake might result from variations in Ca metabolism, which, in the car park snails, could represent an adaptation to high ambient Pb.

Two populations of the garden snail Helix aspersa, from an urban car park and from a semi-rural site, were fed a diet containing 500 μg g−1 of Pb as PbSO4. After 2 days, half of each population was removed to a Pb-free diet and half continued on the dosed food, both for 64 days. The snails from the contaminated site had a significantly lower uptake of Pb compared with those from the rural, uncontaminated site. The car park snails also lost Pb more rapidly from their tissues. A second experiment evaluated the effect of a previous exposure to a high Pb diet on Pb uptake. The results suggest that the differences between the two populations are not due to a physiological adaptation, but rather that the car park snails represent an ecotype adapted to a high Pb diet.

(1) Helix aspersa Muller was studied as a major herbivore in an ecosystem contaminated with lead. An artificial food was used to test diet selection according to Pb or Ca levels and to construct a budget of Pb uptake and loss, at various doses. (2) Dosed agar gels containing PbSO4 and CaSO4 were presented to juvenile snails. No preference for or aversion to Pb and Ca-rich food is demonstrated. (3) The rates of uptake of Pb by H. aspersa are shown to be comparable to those estimated by extraction with 10% acetic acid. Tissue and faecal concentrations are related to dietary consumption, and evidence suggests that tissue turnover of the metal increases with consumption. (4) The proportion of Pb available to other trophic levels in either the tissues or the faeces rises with the snail's consumption of the metal. Comparisons are made with snails from a contaminated car-park. (5) A range of ecological and physiological factors are known to make the concentrations of Pb in snails time-dependent. H. aspersa may therefore serve to 'buffer' the supply of Pb to other trophic levels.