The puzzling issue of lunar syncronicity (or NOT!)

In 1985 and 1986, I observed striking synchronous liberation of medusae by Millepora platyphylla colonies on Guam.  During both years, these events happened in early April (though I must recheck the dates).  In about 2003 or 4 (will check dates later) I also saw signs of ripeness at about this same time of year, though I don't recall whether we had data about medusae.

Two other groups of researchers working in Taiwan and Reunion have studied timing of liberation of medusae in Millepora spp.   No "obvious lunar cycle" was observed by Soong et al.

Synchronized release of medusae from three species of hydrozoan fire corals

K. Soong1 and L. C. Cho1

(1) 

Institute of Marine Biology, National Sun Yat-sen University, Kaohsiung, 804, Taiwan, ROC, TW

Abstract   The release of medusae from three hydrozoan fire corals, Millepora dichotoma, M. murrayi and M. platyphylla, was investigated at three sites in southern Taiwan from February 1994 to July 1995. All three species were gonochoristic, and developed and released several batches of medusae between April and May. The duration of open ampulla appearing on the surface of coralla was short, about three months, and could be used to infer the reproductive season of the fire corals between April and May. No obvious lunar cycles of medusa release were found in these species (emphasis mine). Medusa release started before dark at approximately 17:00 h and continued for several hours. Males began releasing medusae earlier than females. Synchronization of medusa release between colonies, i.e., the probability of occurring on the same nights, was significantly higher within a species than between different species. Hybridization in nature among the three species is, therefore, unlikely due to segregation in the spawning dates. Moreover, the synchronization within each species was often significantly higher within versus between sites. The free-swimming medusae released gametes within approximately one hour, and the spent medusae lived for a few more hours. Medusae may facilitate fertilization rates as a result of an apparently negatively geotactic swimming response that results in medusa aggregation at the surface. No differences in the sizes of medusae, eggs and sperm were detected among the three species; however, some characteristic differences of medusa nematocysts were found.

This deserves greater attention.  My primary result seemed to be that liberation of medusae was synchronous.   Soong et all pooled the species of Millepora, it appears.  Regarding synchronization between species and within species, M. dichotoma released medusae much later in the summer on Guam in 1996.