THE OCCURRENCE AND PHYSIOLOGICAL FUNCTIONS OF MELATONIN IN THE MOST PRIMITIVE EUMETAZOANS, THE PLANARIANS

Asexual planarians of the species Dugesia dorotocephala exhibit a dist inct circadian rhythm of fissioning (asexual reproduction) under the i nfluence of normal photoperiod; fissioning occurs only at night. This rhythm is broken down by continuous illumination, continuous darkness or by decapitation. The fissioning rate increases when planarians are exposed to light for less than 1 hour/day or when they are decapitated . Fissioning of decapitated planarians is suppressed by continuous tre atment with melatonin, whereas fissioning resumes when these are retur ned to normal culture water. Interestingly, fissioning occurs at night when decapitates are treated with melatonin in the daytime, while it is observed in the daytime with night-time melatonin treatment. Endoge nous melatonin was detected by HPLC and RIA. The endogenous melatonin level is always higher in those heads collected in the scotophase than in those collected in the photophase. A type of neurosecretory cell, which may synthesize melatonin, is found in the assembly of photorecep tor cells.