Toxicity testing with aquatic organisms

Toxicity tests on aquatic organisms generally avoid the problems of administering a known dose, or determining how much toxin an animal receives, by measuring the concentration of toxin dissolved in the water into which the organism is placed. In these circumstances, an organism is not given a single dose of toxin but is exposed to it continuously.

With aquatic organisms, direct uptake from water is a route of major importance (eg uptake across the gills of a fish or an invertebrate). Uptake can also occur from food during its passage through the alimentary system and bottom-dwelling organisms are exposed to residues in the sediment. The relative importance of these routes of uptake differs between organisms and between chemicals and depends on environmental conditions. In some cases, all of these routes may operate in one organism at one time.

In conducting toxicity tests it is customary to expose a sample of test organisms to a particular concentration of toxin and measure how long it takes them to die. It may be difficult to detect with precision the moment of death commonly some arbitrary criterion of lethal damage, such as immobilisation or loss of a defensive reflex, is used. Whatever criterion is used, because of variation in sensitivity to the toxin, the test organisms in the sample do not all die at the same time. Instead mortality shows a sigmoid relationship to the period of exposure (Fig ??) It is impractical and normally unrealistic to prolong an experiment until all the test organisms are dead, and the statistic used is the time for  death of 50% of the test organisms, or the median lethal time, which is written as LT₅₀ or LT_m.

The lethal time depends on the concentration of toxin to which an organism is exposed – the higher the concentration the shorter the time – and there may be a lower concentration below which the material is not toxic. The LT50 in itself is therefore not a very useful statistic and it is usual to determine the concentration of toxin at which 50% of the test organisms are killed within a specified time. This time is commonly 48 hrs or 96 hours although for certain organisms such as invertebrate larvae it may be as short as 2 hrs. The toxicity is then recorded as the median lethal concentration:

96 h LC₅₀

or with some other appropriate time indicated.

The median lethal concentration is measured by determining the median lethal time at several different concentrations, the latter usually being set at logarithmic intervals, and then reading the LC50 from a plot of the results. More crudely, the mortality after 96 hours exposure to different concentrations may be recorded; the LC50 then lies between the concentrations that cause more and less than 50% mortality.