ABSTRACT: Newly recruited early juveniles of the shore crab Carcinus maenas in North Wales are most abundant on the high intertidal of gravelly shores and, unlike adults, they do not undertake up-and-down shore migration with tides. Freshlycollected first and second instar juvenile crabs showed persistent circatidal rhythms of moulting when maintained in constant laboratory conditions. Peaks of moulting occurred around expected times of high tides, with few crabs moulting at other times.The circatidal moulting patterns were similar in crabs collected at different stages of the neap-spring cycle. Daily monitoring of moulting in the laboratory of 23 batches of early crabs, collected from the high intertidal at 1 to 3 d intervals over 2spring-neap cycles, further showed a marked circasemilunar moulting pattern superimposed on the tidal moulting rhythms. Significantly more crabs moulted within 24 h after collection when collected during spring tides than when collected at neaps.Moreover, the daily percentage moulting of the crabs on consecutive days after collection clearly followed the trend of predicted tidal height changes. Crabs collected on days of increasing tidal amplitude showed increasing moulting rates on the daysafter collection, whilst a decreasing trend of daily moulting rate was found if they were collected on days of decreasing tidal amplitude. For crabs collected at minimum neaps, when water did not reach the high intertidal even at high tides, virtually nomoulting took place on the following days. Moulting at high tide, particularly during spring high tides, appears to be an adaptation to a high intertidal habitat which is only inundated at certain times during semilunar and tidal cycles. For C.maenas early juveniles, which remain in the high intertidal even when tides recede, anticipation of the rising and falling of tides through endogenous physiological programming to avoid ecdysis at the time exposed to air has clear adaptive value. Thecoupling of circatidal and circasemilunar moulting rhythms, and their endogenous control, reported in the present study appears to be the first demonstration of such a phenomenon in a crustacean.