Search

We report new results from both broad-beam, heavy-ion and proton experiments for circuit-level RHBD techniques in SiGe digital logic. Redundant circuit elements within the latches are used to significantly reduce single-event upset rates in shift registers and clock paths, without resorting to TMR techniques.

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.

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.

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.

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.

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.