Investigating Circadian and Circalunar Systems in Cnidarians

578764-coral-spawning-at-ningaloo-reefOur lab is particularly interested in how broadcast-spawning in corals is temporally regulated, and how solar and lunar illumination control the time at which spawning occurs.  Perhaps the most well known synchronized spawning event is the mass spawning behavior of corals on the Great Barrier Reef where over a hundred species spawn within a few hours after sunset on the nights surrounding the full moon once a year.  The time (hour and minutes) of gamete release is directly controlled by sunset time and not an entrained clock (Brady et al., 2009; Levitan et al., 2011).  However, it has been suggested that the level of lunar irradiance could provide a cue to synchronize the night of spawning in cnidarians (Vize et al., 2012).

Circadian clocks allow organisms to anticipate environmental changes with seasonal cycles of day and night.  There are many similarities between the bilaterian and the cnidarian clocks suggesting that molecular components of the endogenous clock are highly conserved (Reitzel et al., 2013).  The potential that lunar illumination may interact with the circadian clock to set the date of spawning is still open to question. To address this issue we will exploit environmental, pharmacological and genetic means to test the interdependence of circadian and circalunar timekeeping.

Corals detect light via photoreceptors that alter signal transduction pathways, controlling the levels of second messenger calcium, which is responsible for entraining light-driven circadian rhythms. When Hilton et al. (2012) treated coral larvae kept in the dark with thapsigargin, a compound that raise the level of cytoplasmic calcium, patterns of proteins shifted to a day pattern. Melatonin has been found in all multicellular animals, including the cnidarians (Anctil et al., 1991),where a circadian pattern of production was observed (Peres et al. SICB 2012). My working hypothesis is that light induced calcium inhibits melatonin production, and that low levels of melatonin production in response to the full moon set the spawning date, which occurs as night time illumination tapers off. Pharmacologically altering night time melatonin levels in coral and anemones should generate the same responses as the phase of the moon.

The field of cnidarian circadian biology provides an opportunity to test the hypothesis on roles of circadian clocks regulating physiology as well as the evolutionary conservation of circadian regulatory pathways. Spawning synchrony is a complex phenomenon dependent upon the physiological response to several environmental cues which vary both spatially and temporally. By experimentally manipulating cycles of lunar illumination and molecular pathways (using thapsigargin and melatonin) will allow us to determine whether corals display circalunar transcription cycles or if circadian cycles intersect with lunar illumination in some other fashion.  If our results demonstrate circadian and circalunar rhythms are responsible for setting the spawn date, management during lunar cycles prior to spawning will be essential. Findings could potentially encourage researchers to focus on the development of mitigation measures in areas of high risk. This may include monitoring and limiting run-off and human activities such as artificial illumination during period prior to spawning.

References:

Anctil, M., Pani, A.K., Ali, M.A. (1991) Modulation of rhythmic contractions by melatonin via cyclic GMP in the coelenterate Renilla keollikeri. J. Comp. Physiol. B 161:569-575.

Brady, A.K., Hilton, J.D., Vize, P.D. (2009) Coral spawn timing is a direct response to solar light cycles and is not an entrained circadian response. Coral Reefs 28:677-680.

Hilton, J.D., Brady, A.K., Spaho, S.A., Vize, P.D. (2012) Photoreception and signal transduction in corals: proteomic and behavioral evidence for cytoplasmic calcium as a mediator of light responsivity. Biol. Bull. 223: 291-299

Levitan, D.R., Fogarty, N.D., Jara, J., Lotterhos, K.E., Knowlton, N. (2011) Genetic, spatial, and temporal components of precise spawning synchrony in reef building corals of the Montastraea annularis species complex. Evolution 65(5):1254-1270.

Vize, P.D. (2009). Transcriptome analysis of the circadian regulatory network in the coral Acropora millepora. Biol. Bull. 216: 131-137.

Vize, P.D., Hilton, J.D., Brady, A.K. (2012) Biological clock driven circadian transcription cycles in Acropora millepora. Proceedings of the 12th International Coral Reef Symposium.

Xu, Y., Padiath, Q.S. Shapiro, R.E., Jones, C.R., Wu, S.C., Saigoh, N.,Saigoh, K., Ptacek, L.J., Fu, Y.H. (2005) Functional consequences of a CK1 mutation causing familial advanced sleep phase syndrome. Nature 434: 640-644.

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