Landis Hare

Aquatic insects and trace metals: bioavailability, bioaccumulation, and toxicity.

The uptake of metals from food and water sources by insects is thought to be additive. For a given metal, the proportions taken up from water and food will depend both on the bioavailable concentration of the metal associated with each source and the mechanism and rate by which the metal enters the insect. Attempts to correlate insect trace metal concentrations with the trophic level of insects should be made with a knowledge of the feeding relationships of the individual taxa concerned. Pathways for the uptake of essential metals, such as copper and zinc, exist at the cellular level, and other nonessential metals, such as cadmium, also appear to enter via these routes. Within cells, trace metals can be bound to proteins or stored in granules. The internal distribution of metals among body tissues is very heterogeneous, and distribution patterns tend to be both metal and taxon specific. Trace metals associated with insects can be both bound on the surface of their chitinous exoskeleton and incorporated into body tissues. The quantities of trace meals accumulated by an individual reflect the net balance between the rate of metal influx from both dissolved and particulate sources and the rate of metal efflux from the organism. The toxicity of metals has been demonstrated at all levels of biological organization: cell, tissue, individual, population, and community. Much of the literature pertaining to the toxic effects of metals on aquatic insects is based on laboratory observations and, as such, it is difficult to extrapolate the data to insects in nature. The few experimental studies in nature suggest that trace metal contaminants can affect both the distribution and the abundance of aquatic insects. Insects have a largely unexploited potential as biomonitors of metal contamination in nature. A better understanding of the physicochemical and biological mechanisms mediating trace metal bioavailability and exchange will facilitate the development of general predictive models relating trace metal concentrations in insects to those in their environment. Such models will facilitate the use of insects as contaminant biomonitors.

Steady-state distribution of metals among metallothionein and other cytosolic ligands and links to cytotoxicity in bivalves living along a polymetallic gradient

The present study was designed to assess the environmental effects of metals in a field setting. We explored exposure-->bioaccumulation-->effects relationships in freshwater molluscs exposed to metals in their natural habitat. Indigenous floater mussels (Pyganodon grandis) were collected from ten limnologically similar lakes located along a Cd, Cu and Zn gradient. Ambient free-metal ion concentrations were estimated as a measure of metal exposure. Metallothionein (MT) was measured in mussel gills and metal partitioning among the various cytosolic protein pools was determined by size exclusion chromatography. Various biomarkers were also measured, including malondialdehyde (MDA) concentrations in the gills and in the digestive gland, glutathione-peroxidase and glutathione-reductase activities in the digestive gland, and lipid concentrations in the gonad. Cadmium and MT concentrations in the gill cytosol increased along the contamination gradient, but Cu and Zn levels were independent of the ambient free-metal ion concentrations. The distribution of Cd among the various cytosolic complexes remained quite constant: 80% in the MT-like pool, 7% in the low molecular weight pool (LMW<1.8 kDa) and 13% in the high molecular weight pool (HMW>18 kDa). For these chronically exposed molluscs there was thus no threshold exposure concentration above which spillover of Cd occurred from the MT pool to other cytosolic ligands. However, the presence of Cd in the LMW and HMW fractions suggests that metal detoxification was imperfect, i.e. that P. grandis was subject to some Cd-related stress at low chronic exposure concentrations. Consistent with this suggestion, MDA concentrations, an indicator of oxidative stress, increased with gill cytosolic Cd. In the digestive gland, MDA concentrations were unrelated to any of the measured metals, but glutathione-peroxidase and glutathione-reductase activities increased with gill cytosolic copper. We speculate that cytosolic Cu catalyses the production of reactive oxygen species, to which the organism reacts by increasing activities of the two enzymes, thus preventing the accumulation of reactive oxygen species. Lipid concentrations in the gonad did not decrease with any of the measured toxicological parameters, suggesting that energy reserves for reproduction were not compromised in the metal-contaminated mussels. The results of the present study, where chronically exposed bivalves were collected from their natural habitat along a metal contamination gradient, contrast markedly with what would have been predicted on the basis of experimental metal exposures, and clearly demonstrate the need to study metal exposure-->bioaccumulation-->effects relationships in natural populations.

Burrowing Behavior and Biogenic Structures of Mud-Dwelling Insects

Little is known about the burrows of sediment-dwelling insects because these structures are concealed by the sediment in which the animals live. We used X-ray images to reveal and compare the burrowing behavior in the laboratory of aquatic insects from several orders (Diptera, Ephemeroptera, Megaloptera, Trichoptera). Most of the taxa studied constructed U-shaped burrows. Exceptionally, individual caddisfly larvae (Polycentropus sp.) varied greatly in the type of burrow they constructed, i.e., I-, J- and U-shaped burrows were observed. Most taxa begin new burrows from preexisting ones under the sediment surface, obviating the need for the animals to leave the sediment and thereby minimizing their exposure to potential predators. Nymphs of the mayfly Hexagenia limbata exceeded all of the other taxa studied in terms of the depth and length of their burrows and their burrowing rate. Our data suggest that nymphs of H. limbata could be responsible for 98% of the volume of sediment disturbed by littoral insects in our study lake. Observations on 3 species of Chironomus suggest that the rate of burrow construction can differ among congeners. Individuals of H. limbata and Sialis velata burrowed to greater depth in the autumn than in the spring. Our study illustrates the potential of radiographs to study the behavior of burrowing animals, and the range of variability in burrowing behaviors among genera and species of insects.

Experimental evidence for cadmium uptake via calcium channels in the aquatic insect Chironomus staegeri

Abstract We used the chironomid Chironomus staegeri to investigate the mechanism of cadmium (Cd) uptake in aquatic insects. We exposed C. staegeri larvae to a low nominal Cd concentration (50 nM) for 3 days and measured the effects of calcium (Ca) concentration (0.1–10 mM Ca) as well as the Ca channel blockers lanthanum and verapamil on Cd accumulation. When Ca2+ concentrations were increased above a control (0.1 mM Ca2+) to 1–10 mM, Cd accumulation by larvae was inhibited by from 46 to 88%, respectively. A simple theoretical model of Cd–Ca competition for uptake sites fitted our observations well. Cadmium accumulation was significantly inhibited in a concentration-dependent manner by both La (73% at 10 μM and 92% at 100 μM) and verapamil (59% at 100 μM and 85% at 300 μM). Our findings represent strong evidence that Cd entry into these insects occurs through Ca channels.

Subcellular Distribution of Cadmium and Nickel in Chronically Exposed Wild Fish: Inferences Regarding Metal Detoxification Strategies and Implications for Setting Water Quality Guidelines for Dissolved Metals

ABSTRACT The objective of this study was to investigate metal detoxification in chronically exposed juvenile yellow perch (YP: Perca flavescens) and to field test the commonly assumed threshold toxicity model. Fish were collected from lakes located along a cadmium (Cd) and nickel (Ni) concentration gradient. Ambient dissolved metal concentrations were measured to evaluate exposure and total hepatic metal concentrations were determined as a measure of metal bioaccumulation. Hepatic metal partitioning among potentially metal-sensitive fractions (heat-denatured proteins, organelles) and detoxified metal fractions (metallothionein) was determined after differential centrifugation of YP liver homogenates. Major proportions of hepatic Cd were found in the heat-stable cytosolic peptides and proteins fraction (HSP; including metallothioneins), whereas Ni was mainly found in the potentially metal-sensitive heat-denaturable proteins fraction (HDP). For these chronically exposed fish there was no threshold exposure concentration below which binding of Cd or Ni to the heat-denaturable protein fraction or the organelle fraction did not occur. Metal detoxification was clearly incomplete and P. flavescens was subject to some metal-related stress, as evidenced notably by endocrine perturbations. Similar subcellular partitioning results were obtained when juvenile yellow perch were transferred from a reference lake to a Cd-contaminated lake and Cd accumulation was followed over time; there was no accumulation threshold below which Cd binding to the putative metal-sensitive fractions (HDP and organelles) did not occur. The presence of Cd and Ni in these fractions, even for low exposure concentrations and low hepatic accumulation, contradicts the threshold toxicity model that underpins metal toxicology theory and that is implicitly used in setting water quality guidelines for metals. Chronically exposed YP appear to have settled for a tradeoff between the cost of turning on their detoxification apparatus at full capacity, to completely suppress metal binding to metal-sensitive sites, and the alternative cost of allowing some binding of inappropriate metals to metal-sensitive sites.

Metal Sources for Freshwater Invertebrates: Pertinence for Risk Assessment

Ecological risk assessments are likely to be more effective if they are built upon knowledge of from where and in what manner animals take up contaminants. We discuss the relative importance of various metal sources for aquatic invertebrates. First, we address the question do sediment-dwelling animals take up their metals from the overlying water compartment or the sediment compartment or both (both compartments include water and particles). We find that the overlying water column is more important as a metal source for insects, whereas the sediment compartment is more important for oligochaete worms. We explain this tendency by the behaviors of the animals involved. Second, we ask the question do animals take up their metals from food or water within a given compartment. Through case studies on three widespread freshwater invertebrates, we conclude that for some predatory insects food is their major source of several metals, whereas for the crustacean Hyalella both food and water appear to be important depending on the metal involved and the experimental protocol used to study the question. We conclude that ignoring food as a metal source could severely underestimate metal exposures for some animals. We suggest that integrating these complexities into laboratory tests and risk assessment protocols will improve their meaningfulness and thus their ability to protect aquatic ecosystems.

Mercury accumulation in the burrowing mayfly hexagenia rigida (ephemeroptera) exposed to CH3 HgCl or HgCl2 in water and sediment

The accumulation of Hg and its distribution within nymphs of the burrowing mayfly Hexagenia rigida (Ephemeroptera) were investigated experimentally by exposure of nymphs to radiolabelled Hg in laboratory microcosms containing water and sediment. Mercury was introduced into the experimental units either in the overlying water (twice-daily additions) or in the sediment (single addition prior to introduction of the nymphs). At the whole animal level, Hg accumulation varied according to the chemical form of the Hg added and the contamination source. When added in an organic form (CH3HgCl), Hg was accumulated to a greater extent than when added in an inorganic form (HgCl2); the ratio of accumulation between the two compounds exceeded 60 (organic/inorganic exposure) when Hg was introduced via the sediment but was only about 2 when Hg was added to the overlying water. The distribution of Hg among the various body parts of the nymphs indicated that the Hg burdens in two major target organs—the gills and the gut—depended strongly on the initial contamination source. When Hg was added via the sediment, the contribution of the gut to the total body burden (43% for inorganic Hg, 18% for methyl-Hg) was much greater than that of the gills (3 and 5%, respectively). In contrast, when Hg was added to the overlying water, the contribution of the gills to the total body burden (49% for inorganic Hg, 20% for methyl-Hg) exceeded that of the gut (8 and 17%, for the two forms, respectively).

Burrowing behavior of Chaoborus flavicans larvae and its ecological significance

The form and extent of the galleries created by burrowing invertebrates influence sediment structure, bioturbation, and the fluxes of nutrients and contaminants between the sediment and the water column. An important burrowing insect in soft sediments is the phantom midge Chaoborus flavicans. Larvae of this dipteran migrate between the water column, where they feed at night on zooplankton, and the sediment, where they find refuge from predatory fish during the day. The extent to which these larvae disturb sediment during their daily migrations and the depth to which they burrow are poorly known in large part because of a lack of adequate techniques for studying animal behavior near and below the sediment–water interface. We used 2 techniques to record larval behavior in the laboratory, that is, infrared videos to determine how larvae burrow into sediment and X-ray images of burrowed larvae to determine whether they burrow into deeper anoxic sediment and whether they maintain a tube connected to the overlying water. Our observations indicate that larval burrowing visibly disturbs surface sediments thereby contributing to bioturbation. Once buried, larvae do not remain in the thin surface oxic layer but burrow into anoxic sediment. In anoxic sediment, larvae assume a vertical, anterior-end up, S-shaped posture and do not create a tube connected to the overlying water through which they could pump oxygenated water. Thus, larvae are likely exposed to toxic hydrogen sulfide and contaminants in interstitial water, to which they are likely tolerant.

Influence of exposure time on the distribution of cadmium within the cladoceran Ceriodaphnia dubia

Abstract The internal distribution of a trace metal can be useful in determining in which body organ toxic effects are most likely to occur and the animal function most likely to be affected. In addition, because a metal’s distribution within an organism is likely to influence the rate and efficiency with which it is transferred to a higher trophic level, laboratory measurements of trophic transfer could be influenced by exposure duration. We compared the internal distribution of cadmium (Cd) in a cladoceran crustacean (Ceriodaphnia dubia) destined as food for a predatory insect (Chaoborus) after either a 1 day or a lifetime of exposure of prey to the trace metal. Prey were exposed to 112+109Cd in both water (10 nM) and their food (algae), as might occur in nature. The internal distribution of 109Cd in the cladoceran was determined by whole-animal autoradiography. Both the amount and the tissue distribution of Cd in prey were the same after the short and the long term exposures, suggesting that metal accumulation parameters measured after short-term metal exposures can be valid for this animal. Cadmium was mainly accumulated in diverticula of the anterior midgut, a region reported to be responsible for nutrient absorption. We hypothesize that Cd is accumulated in the diverticula because of their purported role as sites of calcium uptake.

Surviving in anoxic surroundings: how burrowing aquatic insects create an oxic microhabitat

Abstract We used an infrared video system and O2 microelectrodes to record the behavior of 2 widespread burrowing insects, the predatory alderfly Sialis velata and the sediment-feeding mayfly Hexagenia limbata, to determine how they survive in anoxic sediment. Analysis of video recordings showed that both taxa have several common behaviors, including crawling, pushing sediment, turning around, and brushing their legs over their bodies for the purpose of cleaning. In contrast, these taxa differ in how they draw overlying oxic water into their burrows, i.e., H. limbata beats its abdominal gills, whereas S. velata undulates its abdomen. Feeding frequency also differs between the 2 taxa, i.e., the mayfly feeds frequently on sediment within its burrows, whereas S. velata feeds infrequently. Hexagenia limbata nymphs were rarely inactive (<1% of the time) and irrigated their burrows by gill-beating ¾ of the time, whereas S. velata nymphs were usually inactive (60% of the time). By vigorous irrigation of its burrow, H. limbata maintained its surroundings well oxygenated, whereas O2 concentrations fluctuated more widely in burrows of the less-active S. velata. This behavioral difference between the species is consistent with their reported sensitivities to the low O2 concentrations associated with eutrophication; in such situations, H. limbata is likely to be eliminated, whereas S. velata can persist. We estimate that O2 lost from water passing through an H. limbata burrow is taken up mainly by the insect rather than diffusing into the surrounding sediment.