Selecting Organisms

The goal of toxicity testing may be generalized into two categories according to the target of interest: humans or all other living things. Humans themselves may be used as subjects in toxicological studies under carefully proscribed conditions. For example, in human clinical trials, subjects may be administered dosages below the expected NOAEL and then have their blood and urine sampled periodically to determine rates and mechanisms of biotransformation and elimination.

However, for the most part, other mammals are used as surrogates for humans in the determination of adverse effects. The mouse or rat is generally used for determination of the LD50 because they are fairly economical and easy to handle. They are also preferred because a large number of toxicological data have been accumulated using them, which makes it possible to compare results between different toxic substances. The pig is better for purposes of extrapolation to humans because it is phylogenetically closer and more similar to humans in diet as well. Of course, nonhuman primates such as monkeys are closest of all. However, expense often precludes their use.

For environmental toxicity testing, a wider variety of organisms may be used. It may be necessary to consider toxic effects on vertebrates other than mammals (fish, birds, amphibians), on invertebrates, and on plants. Test organisms may be collected from the wild. Whenever possible, organisms should be selected for environmental toxicity testing that:

• Represent a broad range of sensitivities

• Are abundant and easily available species

• Are indigenous to any area that will be affected

• Include species that are important recreationally, commercially, or ecologically

• Are easy to culture and maintain in the laboratory

• Have adequate background information available, such as physiology, genetics, and behavior

Table 20.1 lists some organisms commonly used for acute aquatic toxicity tests. Of these, the most commonly used are the water flea (Daphnia sp.), fathead minnow, bluegill, rainbow trout, mysid shrimp, and sheepshead minnow. Daphnia magna and mysid shrimp have been found useful for predicting the MATC for fish.

Among terrestrial animals, the mammals used include several species of rats and mice, as well as hamsters, guinea pigs, rabbits, and dogs. Common avian species are Northern bobwhite (Colinus virginianus), Japanese quail (Coturnix japonica), mallard (Anas platyr-hynchos), ring-necked pheasant (Phasianus colchicus), American kestrel, and screech owl (Otus asio). A teratogenicity test has been developed using the South African clawed frog, Xenopus laevis. Earthworms have been used to assess the toxicity of soil samples contaminated with hazardous wastes.

Finally, some toxicity tests use a combination of species in a single test. A small-scale ecosystem with at least two interacting organisms is called a microcosm. At a medium scale, they are called mesocosms. These are useful for evaluating ecosystem effects such as biomagnification or predation. They may range from two organisms cultured together in a test-tube microcosm, to a farm pond mesocosm used to evaluate agricultural pesticide effects. Terrestrial microcosms may consist of soil cores with associated microorganisms and invertebrates.

TABLE 20.1 Common Organisms Used in Aquatic Acute Toxicity Tests

Freshwater Vertebrates

Rainbow trout, Salmo gairdneri Brook trout, Salvelinus fontinalis Fathead minnow, Pimephales promelas Channel catfish, Ictalurus punctatus Bluegill, Lepomis macrochirus Frog, Rana sp.; toad, Bufo sp. Invertebrates

Daphnids: D. magna, D. pulex, D. pulicaria, Ceriodaphnia dubia Amphipods: Gammarus lacustris, G. fasciatus, G. pseudolimnaeus Crayfish: Orconectes sp., Cambarus sp., Procambarus sp., or Pacifastacus leniusculus Midges: Chironomus sp., Stoneflies, Pteronarcys sp. Mayflies: Baetis sp., Ephemerella sp., Hexagenia limbata, H. bilineata Snails: Physa integra; planaria, Dugesia tigrina Algae

Chlorphyta (green algae), Selenastrum capricornutum

Cyanophyta (blue-green bacteria), Anabaena flos-aquae, Microcystis aeruginosa Chrysophyta (brown algae and diatoms), Navicula pelliculosa, Cyclotella sp., Synura petersenii Saltwater Vertebrates

Sheepshead minnow, Cyprinodon variegatus Mummichog, Fundulus heteroclitus Longnose killifish, Fundulus similis Silverside, Menidia sp.

Threespine stickleback, Gasterosteus aculeatus Pinfish, Lagodon rhomboides Spot, Leiostomus xanthurus Sand dab, Citharichthys stigmaeus Invertebrates

Copepods: Acartia tonsa, A. clausi Shrimp: Penaeus setiferus, P. duorarum, P. aztecus Grass shrimp: Palaemonetes pugio, P. vulgaris Sand shrimp: Crangon septemspinosa Mysid shrimp: Mysidopsis bahia Blue crab: Callinextes sapidus Fiddler crab: Uca sp. Oyster: Crassostrea virginica, C. gigas Polychaetes: Capitella capitata, Neanthes sp. Algae

Chlorophyta (green algae): Chlorella sp., Chlorococcum sp., Dunaliella tertiolecta Chrysophyta (brown algae and diatoms): Isochrysis galbana, Nitzschia closterium, Pyrmnesium parvum, Skeletonema costatum, Thalassiosira pseudonana Rhodophyta (red algae): Pophyridium cruentum

Source: Based on Rand and Petrucelli (1985); Landis and Yu (1995).

Several standard microcosms have been developed. For example, the standardized aquatic microcosm (SAM) is conducted in 4-L jars and provides a highly controlled environment. Four species of invertebrates, 10 of algae, and one of bacteria are added to a sterilized aquatic medium. Temperature, illumination, and chemical and biological sampling schedule are specified over the 63-day test duration. Another example is the soil core microcosm (SCM). It consists of 60-cm-long by 17-cm-diameter soil columns obtained from the field, containing naturally occurring organisms, and is tested over a period of at least 12 weeks.

Testing should preferably be done with animals of both sexes, and both adults and juveniles. Sometimes tests are done with equal numbers of both sexes, but they are lumped together for measurement purposes.

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