Six species of dinoflagellates were investigated for their responsiveness to the indoleamine melatonin (and its homologue 5-methoxytryptamine), the vertebrate hormone for circadian rhythm regulation. Five species were observed forming pellicle cysts in response to 5-methoxytryptamine at concentrations around 10−6M. Cells of Oxyrrhis marina were observed retracting from the theca, part of the responsive pathway to indoleamine, but the resulting naked cells were unable to form a new cell wall and lysed.