The original device and firmware “Blue mussel-7T” was designed for distant continuous recording of mollusk cardiac activity. This device allows estimating the effect of nickel ions
(NiCl2×6H2O in 500 µg/l concentration) on the heart rate in blue mussels Mytilus edulis Linnaeus, 1758. A significant decrease in the heart rate 20 minutes after the addition of the heavy metal was shown. Besides, the cross-group variance of the parameter increased sharply (20–100‑fold) after the same time interval. At the same time, individual variance of the heart rate rose 3–10‑fold as soon as 1 minute after nickel had been added. The increase in heart rate variance probably testifies to a stressful nature of the impact. It is suggested to use three indices in toxicological experiments estimating mollusk cardiac responses to heavy metals: heart rate change, individual and cross-group variances of heart rate indices. The new device and firmware proved to have potential for experimenting with the estimation of mollusk cardiac responses to impacts.