Endogenous tidal and semilunar moulting rhythms in early juvenile shore crabs Carcinus maenas: implications for adaptation to a high intertidal habitat

Newly recruited early juveniles of the shore crab Carcinus maenas in North Wales are most abundant on the high intertidal of gravelly shores and, unli ke adults, they do not undertake up-and-down shore migration with tides. Fr eshly collected first and second instar juvenile crabs showed persistent ci rcatidal rhythms of moulting when maintained in constant laboratory conditi ons. Peaks of moulting occurred around expected times of high tides, with f ew crabs moulting at other times. The circatidal moulting patterns were sim ilar in crabs collected at different stages of the neap-spring cycle. Daily monitoring of moulting in the laboratory of 23 batches of early crabs, col lected from the high intertidal at 1 to 3 d intervals over 2 spring-neap cy cles, 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 incr easing moulting rates on the days after collection, whilst a decreasing tre nd of daily moulting rate was found if they were collected on days of decre asing tidal amplitude. For crabs collected at minimum neaps, when water did not reach the high intertidal even at high tides, virtually no moulting to ok 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 w hen tides recede, anticipation of the rising and falling of tides through e ndogenous physiological programming to avoid ecdysis at the time exposed to air has clear adaptive value. The coupling of circatidal and circasemiluna r moulting rhythms, and their endogenous control, reported in the present s tudy appears to be the first demonstration of such a phenomenon in a crusta cean.