Most disciplines of biology have evolved along a path of hypothesis testing, but the field of aquatic toxicology is an exception. Society's demand for information in an atmosphere of increasing litigation initially drove, then hastened, the development of the field of aquatic toxicology. Before 1962, pollution concerns were largely focused on sewage treatment facility operations and eutrophication of lakes, streams, and estuaries. With the publication of Silent Spring by Rachel Carson in 1962, however, the warning alarm was sounded, proclaiming that organic contaminants, particularly pesticides such as DDT, posed a threat to the health of natural resources and humans.

One of the most salient features of the development of environmental toxicology as a scientific discipline has been the expressed need for chemical detection and identification simultaneously with the need to determine the biological effects associated with chemicals. Lags in methodological progress, both chemical and biological, limited observable progress in the early years of environmental toxicology and chemistry. Thin-layer chromatography and gas chromatography were first used to detect and identify pesticides and organic chemicals; atomic adsorption spectrophotometry was used to detect metals and other inorganic materials. At the same time, acute lethality (in which the subject organism dies in 96 hours or less) toxicity tests were initially developed and standardized to bracket extreme biological effects.

In 1977, a group of scientists working in various areas within the field gathered to discuss the ideal attributes of a toxicity test. This group, made up of representatives from government, academia, and industry, determined that the following attributes were the most important in a toxicity test and ranked them in order of their significance:

* produces ecologically significant results;

* generates scientifically and legally defensible data;

* is based on methods that are routinely available for widespread application;

* is predictive;

* methods are widely applicable across a range of chemicals; and

* test is simple and cost-effective.

The group evaluated available aquatic toxicity tests and scored them on the basis of the previously listed criteria. A score of 100% (out of a possible 100%) was awarded to acute lethality toxicity tests because of their ecologically significant results (death ... unarguably important), applicability across chemical classes, simplicity and cost-effectiveness, and their scientifically and legally defensible results.

If evaluated today, acute lethality toxicity tests would still fare well for some of the same reasons but also for some different ones. Acute lethality tests allow for the rapid building of comparative data bases in which species can be compared in terms of their sensitivity to the same chemical or by which chemicals can be compared to one another using the same species. Additionally, water quality can be varied to evaluate potential interactions with toxicity (for example, as pH increases, the toxicity of some metals increases). When dealing with new chemicals, new formulations of existing chemicals, mixtures, changing environmental conditions, and so forth, a rapid screening toxicity test is often invaluable. Consequently, acute lethality tests remain a comparative framework for evaluation.

This approach, however, does not address the important and more likely situation encountered in natural systems, namely, longer-term, sublethal exposure and its ecological consequences. Scientists grew concerned about the inadequacy of so-called "kill 'em and count 'em" tests and developed methods to evaluate changes induced by contaminants that affected reproduction, behavior, physiological processes, biochemical function, and survival of young and other sensitive life stages. Although disruptions in these areas might render an organism "ecologically dead," it may not technically induce direct mortality. For example, courtship behavior may take place normally in the presence of sublethal concentrations of toxicants, but the larvae produced might be malformed or unable to make developmental progress. Because researchers needed chronic, sublethal tests to approximate effects more likely to occur in nature, they developed full life cycle (from birth until the organism reproduces) fish tests. Although the results are ecologically meaningful (such as contaminant effects on the number of eggs produced, percent successful hatch, survival to swim-up, and so forth), such tests are difficult and expensive to run without problems. For example, of the native warmwater, freshwater fishes adaptable to laboratory conditions for full life cycle testing, fathead minnows complete the life cycle in the shortest time; even so, a test on them can last 3 to 6 months depending on broodstock and test conditions. Consequently, to decrease the testing time required, partial life cycle testing began to replace full life cycle tests. Such partial life cycle tests decrease the probability of some test condition complications, can bracket sensitive life stages (such as reproduction), and can increase the potential for testing various species of interest even if their full life cycles could not actually or practically be completed in a reasonable time.

In the late 1970's and early 1980's, invertebrate tests became increasingly important because they took less time than full life cycle testing and because disruptions in food-chain dynamics at lower trophic levels can translate into severe ecological consequences for top predators and species of monetary and ecological concern. In addition, invertebrate testing requires less space and specialized equipment than fish testing.

Behavioral toxicity tests, although ecologically relevant if the endpoints measured are interpretable, have met with limited success because of their intrinsic variability when replicated. The very thing that contributes to their sensitive detection capabilities can backfire if the animals are not acclimated properly or standardized test approaches are not appropriately conducted. The expenditure of time and labor required, however, can be offset by the ecologically interpretable results of such tests.

Biochemical and physiological approaches have become important in aquatic toxicology over the last 5 to 8 years. Not only do such approaches demonstrate the relation between exposure and effects, but they can also sometimes explain the toxic action of the contaminant. As researchers have refined biomarker techniques, such techniques have become more specific and sensitive in detecting contaminants. Baseline information on the normal physiological and biochemical states of aquatic organisms has also grown, making perturbations due to contaminants more discernible.

Currently, scientists are emphasizing portable, field-oriented, sublethal yet acute detectors of dose­response. Microtox (trade name for a bioluminescing bacteria assay) and rotifer tests are receiving much attention. Field approaches, originally survey-oriented and aimed at detecting the presence of a problem, have moved more and more toward hypothesis testing and experimental manipulation. This is a great addition to field data because experimental information can be expanded with more confidence past the geographic perimeters of the field site, and cause­effect relations can be more realistically investigated.

The realization that no single test approach meets all needs or answers all questions has become even more evident over the last decade. The fact is that many "tools" are needed and each should be selected and combined with others in diverse configurations depending on the contaminants of interest and the questions being addressed. Continued effort is required to further develop meaningful, cost-effective, and field-friendly methodologies to detect contaminants and their effects on aquatic biota.