For marine invertebrate animals, in particular, examples are given of rhyth
mic patterns of locomotor, reproductive and moulting behaviour which are of
lunar and semilunar periodicities. Some of these 29.5 and 14.8 day rhythms
are shown to persist in constant conditions in the laboratory, indicative
of internal biological clock control induced by intense adaptive selection
pressure. In some cases phasing of the rhythms is directly by moonlight but
, in other cases phasing is indirect, associated with lunar monthly variati
ons in the amplitude of ocean tides, that is the neap/spring cycle.