Effects of apomorphine, a dopamine agonist, on Daphnia magna: Imaging of swimming track density as a novel tool in the assessment of swimming activity. 
Apomorphine (APO) is a non-selective agonist of dopamine receptor activating D2-like receptors.  Although Daphnia has been used in neurotoxicology in toxicity testing, little is known on its behavioural and physiological responses to dopamine receptors ligands.  Therefore, the aim of our study was to determine swimming behaviour (swimming track density, speed, turning activity) and physiological parameters such as heart rate, thoracic limb activity and post-abdominal claw movement frequency in daphnids exposed for 1, 2 and 4h to concentrations of 0.3, 3 and 30mg/L of APO.  The results showed the most significant decrease of behavioural endpoints such as swimming track density, speed and degree of turning angles of daphnids exposed for 4h to the highest concentrations of APO.  The study also showed that a decrease of thoracic limb activity was found after 2 and 4h but only at the highest concentration.  Heart rate was not affected by APO which may be a result of a lack of signalling with dopamine receptors in the heart of Daphnia.  Therefore, activity of this organ seems to be not a valuable physiological biomarker in the assessment of effects induced by dopamine receptor ligands.  The study also showed that our new methodological approach, imaging of swimming track density may be a promising tool for studying the effects of neuroactive substances on locomotor system activity of Daphnia magna.