Acute combined exposure to heavy metals (Zn, Cd) blocks memory formation in a freshwater snail.

The effect of heavy metals on species survival is well documented; however, sublethal effects on behaviour and physiology are receiving growing attention. Measurements of changes in activity and respiration are more sensitive to pollutants, and therefore a better early indicator of potentially harmful ecological impacts. We assessed the effect of acute exposure (48 h) to two heavy metals at concentrations below those allowable in municipal drinking water (Zn: 1,100 µg/l; Cd: 3 µg/l) on locomotion and respiration using the freshwater snail, Lymnaea stagnalis. In addition we used a novel assessment method, testing the ability of the snail to form memory in the presence of heavy metals in both intact snails, and also snails that had the osphradial nerve severed which connects a chemosensory organ, the osphradium, to the central nervous system. Aerial respiration and locomotion remained unchanged by acute exposure to heavy metals. There was also no effect on memory formation of these metals when administered alone. However, when snails were exposed to these metals in combination memory formation was blocked. Severing the osphradial nerve prevented the memory blocking effect of Zn and Cd, indicating that the snails are sensing these metals in their environment via the osphradium and responding to them as a stressor. Therefore, assessing the ability of this species to form memory is a more sensitive measure of heavy metal pollution than measures of activity, and indicates that the snails' ability to demonstrate behavioural plasticity may be compromised by the presence of these pollutants.

Low external environmental calcium levels prevent forgetting in Lymnaea.

Forgetting may allow an animal to react more appropriately to current conditions, rather than continuing to exhibit a previously learned, possibly maladaptive behaviour based on previous experience. One theory is that forgetting is an active process, whereby the previously learnt response is replaced by new learning that interferes with the older memory. Hence, we hypothesized that an appropriately timed environmental stressor that blocks long-term memory (LTM) formation would also block forgetting. Lymnaea stagnalis (L.) is a freshwater snail, which requires environmental calcium of at least 20 mg l(-1) to meet its requirements. Low environmental Ca(2+) (i.e. 20 mg l(-1)) in their environment acts as a stressor, and prevents LTM formation. Here, we asked whether a low Ca(2+) environment would also prevent forgetting, concordant with the retrograde interference model of Jenkins and Dallenbach. Snails were operantly conditioned to reduce aerial respiration in hypoxia. When maintained in standard conditions (80 mg l(-1) Ca(2+)), snails demonstrated LTM following training lasting 24 h, but not 72 h; however, when trained in standard conditions then exposed to a low Ca(2+) environment (20 mg l(-1)) immediately following training, they retained memory for at least 96 h, indicating that forgetting had been blocked. Thus, when exposed to low environmental Ca(2+), Lymnaea will fail to form new memories, but will also continue to retain information previously learned and remembered as the low calcium blocks forgetting.