Editors' ChoiceCircadian Rhythms

Swim Away from the Light

Sci. Signal.  07 Oct 2014:
Vol. 7, Issue 346, pp. ec278
DOI: 10.1126/scisignal.2005997

The secreted hormone melatonin is found in almost all prokaryotic and eukaryotic species. Melatonin is produced during dark periods and influences circadian physiology and behavior by reacting directly to light and signaling through G protein–coupled receptors. Tosches et al. found that melatonin controls the daily rhythmic behavior of zooplankton, which swim upward at dusk and downward throughout the night and into dawn to avoid damaging exposure to ultraviolet light. In Platynereis hiomt, genes encoding melatonin-synthesis enzymes, phototransduction proteins, and components of the circadian clock were expressed in ciliary photoreceptors in the dorsal brain. The expression of melatonin-synthesis genes increased during the night. P. hiomt swim using cilia, and ciliary motions associated with swimming were increased during the night and were inhibited by a melatonin receptor antagonist. During the day, the absence of light alone was not sufficient to mimic nighttime ciliary behaviors, but adding exogenous melatonin increased ciliary motions. The band of cells containing the motile cilia was directly innervated by cholingeric neurons and exposing larvae to acetylcholine enhanced ciliary motions during the day. Analysis with a fluorescent calcium indicator revealed that these neurons showed rhythmic activity at night or in response to exogenous melatonin during the day. Application of cholinergic or melatonin receptor antagonists abrogated rhythmic excitatory postsynaptic potentials in the ciliary band cells induced by melatonin. Thus, the conservation of the control of circadian behavior by melatonin signaling suggests that vertical migration in the oceans may be the evolutionary origin of animal circadian rhythms.

M. A. Tosches, D. Bucher, P. Vopalensky, D. Arendt, Melatonin signaling controls circadian swimming behavior in marine zooplankton. Cell 159, 46–57 (2014). [PubMed]