Heavy metal induced behaviour modulation in mussels: possible neural correlates

Filtering behaviour of bivalves can be monitored by recording the contraction and relaxation of the adductor muscles, which maintain the filtering activity by closing and opening of the shells having rhythmic and periodic pattern with a time scale of several minutes and hours. Heavy metals (Cu2+, Hg2+, Cd2+, Pb2+, Zn2+) applied into the water for one week cause a clear cut alteration in the adductor's activity characterized in general by changing of the duration of activity and rest. Threshold concentrations affecting the animal's behaviour were 0.005; 0.01 0.05; 0.1 and 10.0 mg/l, respectively. With increasing concentration the duration of activity could be reduced up to 10-20 per cent of the control at application of Cu2+, Hg2+ and Cd2+. Rest periods were also influenced by heavy metals. Wash-out in most cases restored the original activity, sometimes a reverse effect was observed. Since heavy metals, if present in the water, cause reduction of the level of 5HT and DA in the ganglia of mussels and they modulate the chemical sensitivity of molluscan neurons and influence the permeability of various ionic channels, it is suggested that heavy metal ions affect directly both transmitter metabolism and membrane excitability and this way alterations of basic neural mechanisms are responsible in the observed modulation of mussels' behaviour.