Crayfish are excellent bio-indicators in the aquatic environment. However, we know little about the effect of environmental toxicants such as polyromantic hydrocarbons on aquatic invertebrates including crayfish. This study investigated the effects of stress induced by handling and exposure to phenanthrene on the heart of the invasive signal crayfish, Pascifastacus laniusculus. A non-invasive method employing infra-red sensor were used to obtain heart rates from the crayfish while the electrocardiogram (ECG) and monophasic action potential (MAP) were recorded by inserting electrodes into a semi-isolated heart preparation. The ECG from crayfish was characterised and measured upon exposure to phenanthrene. The results showed that upon exposure to physical stress, there was an initial decrease in mean heart rate followed by a gradual rise reaching a peak rate. After about 30 minutes, a gradual reduction was observed which stabilized to basal rates after 45 minutes of recording. Phenanthrene significantly impacted the heart rate by reducing the cardiac response to stress and basal heart rates although the pattern of response was same with the stressed animal. The electrical properties showed phenanthrene exposure impacted the ECG waveform and action potential duration which could provide basis for arrhythmia which could suggest the variable heart rates recorded. This study provides evidence to support the role of cardio inhibitors in the crayfish heart that tend to provide a protective function when stressed. At same time, the study is consistent to findings of vertebrates upon exposure to poly aromatic hydrocarbon. Understanding the cardiac response of crayfish could be vital as a tool in cardio-physiological studies.