D.I. Zandee

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.

Effect of cadmium on the reproduction strategy of Daphnia magna

Abstract Survival, number of neonates per female, first day of reproduction and neonate size were investigated in Daphnia magna, exposed to cadmium concentrations ranging from 0 to 50.0 ppb. A 25-day LC50 of about 10.0 ppb was found. The onset of reproduction was delayed at concentrations of cadmium above 5.0 ppb, but was not affected at lower levels. Cadmium concentrations of 0.5, 1.0 and 5.0 ppb were shown to increase the average number of neonates per female significantly. The size of the neonates, however, decreased significantly with increasing cadmium concentrations, thereby probably offsetting their survival probability. The reproduction strategy of D. magna seems to be changed by low concentrations of cadmium. Compared with unstressed animals cadmium-stressed animals produce larger broods with smaller neonates, but at higher cadmium concentrations brood size and body size both decline. Whether the former observation might be considered an adaptation of the stressed animal, or whether ‘hormesis’, i.e., the unspecific effect on the growth control mechanism, might be responsible, is discussed.

Cadmium kinetics in freshwater clams. I. The pattern of cadmium accumulation inAnodonta cygnea

Freshwater clams (Anodonta cygnea) were exposed to cadmium under laboratory conditions. Time courses of Cd accumulation were determined for whole animal and the separate organs. At 5 Μg/L, Cd accumulation proceeded mainly linearly. At 25 Μg/L, the accumulation pattern was biphasic for whole animals and most of the organs: during the first four weeks Cd concentrations increased linearly, remaining at a constant level for the next two weeks. Thereafter, metal concentrations increased strongly until, after 10 weeks, a saturation level was reached. It is argued that the biphasic course of accumulation is not caused by an advancing selection through mortality or by the gradually decreasing mean animal weight, nor by the typical laboratory conditions of absence of food and substratum. The partition of total Cd among the organs gradually changed over the first eight weeks, remaining constant from that time. The ultimate order of Cd concentrations was: gills > labial palps > mantle-edge > mantle, kidney, whole animal > midgut gland > guts/gonads complex > foot. A possible relation between the time course of Cd accumulation and behavioral responses is presented.

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.

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.

A field study on stress indices in the sea mussel, Mytilus edulis: application of the "stress approach" in biomonitoring.

Sea mussels,Mytilus edulis, collected from a relatively unpolluted area of the Eastern Scheldt, were transplanted along contaminated sites of the Western Scheldt for 21/2z and 5 months. Several established stress indices were determined such as accumulation of pollutants, adenylate energy charge (AEC), and condition index. Following field exposure, mussels were also subjected to an additional lethal or sublethal stress. The data show that environmental exposure alters the mussels response to stress,viz., aerial exposure and increased temperature, at the organismal (anoxic survival time), biochemical (AEC), and molecular (heat shock protein synthesis) level. The “stress approach” to assessment of environmental contamination appears to be a promising method to disclose early changes in the organism at a stage when conventional parameters (condition index, AEC) remain still unchanged.

Anoxic survival time and metabolic parameters as stress indices in sea mussels exposed to cadmium or polychlorinated biphenyls

Sea mussels,Mytilus edulis, were exposed to cadmium under laboratory and semi-field conditions and to polychlorinated biphenyls (PCBs) under semi-field conditions. After various exposure times, the animals were subjected to environmental anoxia by aerial exposure. The anoxic survival time was significantly reduced by short-time laboratory or semi-field exposure to cadmium. The effect of PCBs on the anoxic survival time was apparent after six months. In addition, during long-term semi-field exposure to cadmium or PCBs, also other potential stress indices, such as the adenylate energy charge, condition indices, glycogen and metabolite contents, and cadmium-binding proteins were examined.

Bioaccumulation and histochemical localization of cadmium in Dreissena polymorpha exposed to cadmium chloride

Dreissena polymorpha, a freshwater bivalve, was exposed to dissolved CdCl2 (100 μg Cd/l) for periods of 1, 2, 3 or 4 wk. Cadmium accumulation was followed by atomic absorption spectrophotometry and the localization of Cd was investigated with a sulphide-silver technique (SST). During the exposure period the soft body accumulated Cd in a linear way, while the shells reached a saturation level after 3 wk. In exposed mussels treated with the SST, reaction products were present in nearly all tissues. The cellular localization and the amount of reaction products varied according to the type of tissue and the duration of exposure. After one week, reaction products indicative for the presence of free or loosely bound Cd were present in granular structures and in the cytoplasm of all epithelia that had been in direct contact with the Cd-contaminated water. After longer exposure times, the epithelial cells as well as the underlying connective tissue became completely filled with reaction products. In the digestive tract, reaction products were confined mainly to granular structures in the epithelial cells. They became prominent after 3 or 4 wk of exposure. Among the internal organs, the excretory system showed a reaction pattern largely comparable to that of the epithelia which had been in direct contact with the Cd-contaminated water. Apart from the pericardial gland, which represents the site of ultrafiltration, the appearance of reaction products in the cytoplasm of the kidney cells was retarded. In the epithelial lining of the gonads and in neural tissue, diffusely distributed reaction products were observed only after longer exposure times. Gametes and muscular tissue, except for the ventricular muscle, remained free of reaction products. Structural damage related to the Cd-exposure was restricted to the pericardial gland.

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.

Effects of cadmium in freshwater clams. III. Interaction with energy metabolism inAnodonta cygnea

Freshwater clams,Anodonta cygnea, were exposed to cadmium, at 50 ppb (μg/L), to investigate the effects of semi-chronic exposure on energy metabolism. Parameters examined included: adenylate energy charge (AEC), glycogen content, blood glucose and protein concentration, the accumulation of anaerobic metabolic end products,viz. lactate and succinate, and mitochondrial NADH-oxidase activity. In all tissues, AEC was significantly lowered after 12 weeks. Glycogen contents of the separate organs, except for the gills, were diminished to one half those of control animals. Hemolymph glucose increased between 4 and 8 weeks, whereas protein in hemolymph steadily decreased, to about one half the concentration of controls at 12 weeks. Lactate increased in mantle and midgut gland, whereas in gill only an initial accumulation was found after 2 weeks of exposure. Succinate concentrations increased in all organs between 4 and 8 weeks of Cd exposure. Thereafter, no further accumulation occurred. From the onset of exposure to cadmium there was a gradual reduction of NADH-oxidase activity of gill mitochondria, down to 60% of the control value after 12 weeks. The data indicate an impairment of oxidative carbohydrate metabolism that is brought about by a metabolic blockade, rather than by (partial) anaerobiosis as a consequence of shell closure.