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TOXICITY TEST USING THE GASTROPOD PHYSA ACUTA

Initial tests by Aquatonics Ltd using a variety of different freshwater taxonomic groups (caddisfly & mayfly nymphs, and an amphipod and isopod crustacean) showed that they were relatively insensitive to a minewater collected from a tributary of the River Lynher, Cornwall. A gastropod snail (Physa acuta) was sensitive to the minewater, and a toxicity testing protocol was devised using this species.

Physa acuta Draparnaud 1805 is a small gastropod snail measuring 10-17 mm when adult. In the UK it is an introduced species, thought to orginate from the Mediterranean (Macan, 1977).

The test is very simple and cheap, and produces results within about 20 minutes. Although our tests so far have been carried out in the laboratory the test could easily be used in the field for rapid testing of minewaters and streams affected by metallic discharges.

The testing protocol was as follows:

A plastic petri dish (46 mm diameter) is filled to half its depth (approximately 9 mm) with the water to be tested.

1. Five Physa acuta are introduced to the petri dish and the stopwatch started.

2. At 1 minute intervals the number of Physa acuta in the following categories are recorded:

(a) actively moving, gliding over the surface of the dish

(b) showing contorted movements, with flexing of the foot. These flexions are intense at first and diminish as the specimen succumbs to the toxicant

(c) showing no movement

3. The time taken for >50% (3 out of 5) and 100% of the Physa acuta to reach categories (b) or (c) is noted and reported as the Effective Time (ET) for 50% of the specimens to be flexing (ET50 flex) or show no movement (ET50 movement).

The experiment is normally terminated at 20 minutes, but could be extended to an hour if some specimens are showing signs of distress (flexing) and the EC50 for no movement is required.

Figure 1 shows the results for one of the minewaters tested.

REFERENCE

Macan, T T (1977). A Key to the British Fresh- and Brackish-water Gastropods with Notes on their Ecology. Freshwater Biological Association Scientific Publication No 13. Fourth Edition.

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