P.A. Voogt

Cadmium resistance in Daphnia magna

Abstract The potential of the water-flea Daphnia magna to develop resistance to cadmium was investigated. Three successive generations of daphnids were exposed to sublethal cadmium concentrations, and thereafter the cadmium resistance was assessed in acute LC50-tests. Resistance to cadmium was found to be acquired during a single generation. The acquired resistance was lost within 21 days if neonates of cadmium exposed daphnids were placed in cadmium-free test solution. Cadmium resistance thus proved not to be a genetic phenomenon. During a cadmium challenge absorption of cadmium was more rapid in cadmium pre-exposed animals than in control animals. The extent to which the increased resistance might be attributed to the synthesis of metallothionein-like proteins is discussed.

Effects of cadmium on consumption, assimilation and biochemical parameters of Daphnia magna: possible implications for reproduction.

1. The effects of cadmium on consumption, assimilation rates and biochemical parameters of Daphnia magna were determined. 2. The consumption and assimilation rates of 14 days 1.0 ppb Cd treated animals tended to decrease slightly, the decline of these rates at 5.0 ppb Cd (14 days), however, was highly significant (P less than 0.001). 3. The assimilation efficiencies of daphnids exposed to cadmium did not significantly differ from control. 4. No notable changes in the biochemical composition of daphnids could be noticed after 7, 14 and 21 days of cadmium exposure. 5. It seems as if not one metabolic process in particular was depressed due to cadmium, but metabolic activities seemed to be inhibited on the whole. 6. Results are discussed in relation with data of a previous study on the reproduction of D. magna under cadmium stress.

Ecdysteroids in Daphnia magna: their role in moulting and reproduction and their levels upon exposure to cadmium

Abstract The role of ecdysteroids in moulting and reproduction of Daphnia magna has been investigated. Further we examined whether previously observed moulting and reproduction disturbances in cadmium-exposed animals could be attributed to changes in ecdysteroid titres. Six concentrations of ecdysone and 20-hydroxyecdysone, ranging from 1 × 10 −8 M to 5 × 10 −6 M, were administered to daphnids exogenously. Treatment with high concentrations of both ecdysteroids resulted in unsuccessful exuviations and a decline in the number of progeny per female. The size of the neonates, however, appeared to be unaffected. 20-Hydroxyecdysone was demonstrated to be more effective than ecdysone. The presence of ecdysteroids in whole-body extracts of adult females was demonstrated using an enzyme-immunoassay. Additionally, ecdysteroid concentrations in daphnids were found to be affected by cadmium exposure at concentrations of 5.0, 10.0 and 20.0 μg/l. We concluded tentatively that changed ecdysteroid concentrations may have caused the moulting impairments in cadmium-exposed daphnids. On the other hand, there was no indication that the endocrine response of D. magna to cadmium exposure was also responsible for the cadmium-induced decline in neonate-size.

7 – Lipids: Their Distribution and Metabolism

Publisher Summary This chapter describes the lipid composition and lipid distribution in molluscs. It describes how molluscs achieve and maintain a characteristic composition of lipids, despite the different lipid composition of their diet. Fatty acids might occur in free form, but they are mostly present in esterified form. They are found in sterol esters, acylglycerols, and phospholipids. Most fatty acids possess straight chains, but considerable amounts of branched-chain fatty acids can be present. The monobranched fatty acids comprise the iso- and the anteiso fatty acids. The common multibranched or isoprenoid fatty acids are 4,8,12-trimethyltridecanoic acid, 2,6,10,14-tetramethylpentadecanoic acid, and 3,7,11,15-tetramethylhexadecanoic acid. Generally double bonds in unsaturated fatty acids are methylene interrupted. However, a number of dienoic fatty acids are known in which the double bonds are non-methylene interrupted. The fatty acid composition in organisms is influenced by several external conditions. Among these are temperature and diet. Therefore, fatty acid composition in storage organs varies with the diet and that in the membranes with the temperature.

Effects of cadmium and PCBs on reproduction of the sea star Asterias rubens: aberrations in the early development

In semifield experiments sea stars, Asterias rubens, were exposed to 25 micrograms Cd/liter or fed with mussels containing 0.6 microgram/g wet wt PCBs (Clophen A50). After 5 months of exposure, Cd concentrations in testes and ovaries were respectively 17 and 50 times higher than those in unexposed sea stars. PCB concentrations were respectively 7 and 9 times higher. With spermatozoa obtained from Cd- or PCB-exposed sea stars, normal fertilization could be achieved. However, maturation of oocytes from Cd-exposed animals was delayed and early development of embryos from Cd- or PCB-exposed animals was disturbed. Due to aberrations during the early development only 24 and 30% of the embryos obtained from Cd- or PCB-exposed sea stars, respectively, had developed to normal bipinnaria larvae after 1 week.

Effects of cadmium and polychlorinated biphenyls (Clophen A50) on steroid metabolism and cytochrome P-450 monooxygenase system in the sea star Asterias rubens L.

Abstract A study was made on the effects of long-term cadmium or PCB exposure on the steroid metabolism and the cytochrome P-450 monooxygenase system of the sea star Asterias rubens. Sea stars were exposed to 25 μg/l cadmium or fed with PCB (Clophen A50)-exposed mussels containing 26 μg σ8-PCBs/g lipid, which was six times higher than the PCB levels in control mussels. Twelve weeks of exposure resulted in cadmium and PCB levels in the pyloric caeca which were comparable to those found in sea stars collected from the Western Scheldt (SW Netherlands) or in animals that had been fed with mussels from this heavily polluted estuary. Cadmium and PCB exposure caused significant reductions in the levels of progesterone and testosterone in the pyloric caeca of male and female sea stars. Steroid levels in the gonads had not changed after cadmium exposure, whereas after twelve weeks of PCB exposure elevated levels of testosterone were found in testes and ovaries. Both cadmium and PCBs caused a decrease in the content of cytochrome P-450 and cytochrome b5 in pyloric caeca musomes. Benzo[a]pyrene hydroxylase activity was elevated by PCB exposure but had not changed after exposure to cadmium. Microsomal NADPH-cytochrome c reductase and steroid conversion activities were not significantly influenced by cadmium or PCB exposure, although a slight reduction of steroid metabolism was observed.

Toxicity of heavy metals to early life stages of Daphnia magna

This study was designed to investigate the susceptibility of the parthenogenetic eggs of Daphnia magna to cadmium, zinc, copper, and lead. Early life stages of D. magna proved to be highly tolerant to heavy metal toxicity in comparison with later stages. This relatively high tolerance might be explained by the structural constitution of the eggs.

Cadmium accumulation and metallothionein-like proteins in the sea star Asterias rubens

Sea stars were exposed for four months to cadmium at 50 μg Cd/L. The cadmium content of stomachs, pyloric caeca, gonads and body wall was determined after 2, 4, 6, 8, and 16 weeks of exposure. The highest accumulation rates were found for body wall and pyloric caeca. The lowest accumulation rate was found for the gonads. In all tissues cadmium accumulation showed saturation kinetics. The body wall contained the largest part of the accumulated cadmium.Sephadex G-75 gel filtration experiments demonstrated the presence of metal-binding, metallothionein (MT)-like proteins with a molecular weight of 10.8 kD in cytosolic fractions of pyloric caeca from unexposed sea stars. The MT-like proteins contained mainly zinc and a small amount of copper. During exposure, the MT-like proteins accumulated cadmium at a relatively high rate, but a steady state was reached after 8 weeks, indicating thatin vivo, the Cd-binding capacity of these proteins was limited. When pyloric caeca cytosol from unexposed sea stars was incubated with cadmiumin vitro, the MT-like proteins could bind larger amounts of cadmium thanin vivo during the Cd exposure. After 16 weeks of Cd exposure, thein vitro Cd-binding capacity had hardly increased.Low-molecular-weight metal-binding proteins were absent in cytosolic fractions of the testes, ovaries and oocytes from Cd-exposed sea stars, even after four months of exposure. The cadmium accumulated in the gonads was bound exclusively to high-molecular-weight proteins, which accumulated cadmium at a relatively low rate, compared to the MT-like proteins in the pyloric caeca. The absence of MT-like proteins in the gonads may explain the low cadmium accumulation rates found for these organs, but as a consequence, the accumulated cadmium is not detoxified.

The △5-pathway in steroid metabolism in the sea star Asterias rubens l.

Abstract 1. 1. The △5-pathway in steroid metabolism in Asterias rubens was studied using incubations of pieces of tissue from gonads and pyloric caeca with either cholesterol, pregnenolone, or dehydroepiandrosterone. 2. 2. This pathway, known from vertebrates, also exists in the sea star. 3. 3. Conversion percentages showed large differences among the precursors used. The underlying causes are discussed. 4. 4. Conversion of intermediate △5-steroids into the corresponding △ 4-steroids can take place at (at least) two sites of the △5-pathway. One is at the C21-steroid and one at the C19-steroid level. 5. 5. The biosynthesis of two novel fatty-acyl steroids is reported: viz fatty-acyl pregnenolone and mono-fatty-acyl androstenediol. The physiological function of these esters will be discussed.

Steroid synthesizing capacity of the dorsal body of Helix pomatia L. (Gastropoda)—an in vitro study☆

Abstract 1. 1. In vitro studies demonstrated the presence of the enzyme 3β-hydroxysteroid dehydrogenase (3β-HSD) in the dorsal body complex, ovotestis and buccal ganglia of Helix pomatia. 2. 2. Results of incubations with tritiated pregnenolone and tritiated dehydroepiandrosterone indicate substrate specificity of the 3β-hydroxysteroid dehydrogenase. 3. 3. The activity of the two 3β-hydroxysteroid dehydrogenases vary independently of each other during different physiological states of the animal. 4. 4. Before oviposition the conversion of dehydroepiandrosterone into androstenedione in all tissues investigated exceeds that of pregnenolone into progesterone. On the contrary, after oviposition this is reversed for the ovotestis. 5. 5. The relative importance of the pathways in steroidogenesis followed in the organs studied is discussed.