C. Zeng · E. Naylor · P. Abello Endogenous control of timing of metamorphosis in megalopae of the shore crab Carcinus maenas Received: 21 February 1996 / Accepted: 27 November 1996 Abstract Batches of hundreds of freshly collected meg- alopae of the shore crab Carcinus maenas (L.) showed persistent circatidal rhythms of moulting to the juvenile crab stage when maintained in constant laboratory conditions. Peaks of moulting occurred around expected times of high tides, with few megalopae moulting at other times. In larvae collected offshore, the highest tidally-timed peak of metamorphosis occurred during the second to fifth expected times of high tide, and metamorphosis of 50% of each batch took about 22 h or longer. In contrast, in larvae collected at the water’s edge, 70% metamorphosed during the first expected episode of high tide, within 6 to 8 h after collection. However, although inshore megalopae moulted before offshore ones, the tidal timing of moulting remained unaltered whether megalopae were collected at neaps or springs, from the water’s edge or farther offshore, in the presence or absence of natural substratum, and under various light–dark and salinity regimes. Metamorphosis of C. maenas megalopae around the times of high tides may enhance settlement into the upper intertidal zone. Early juveniles of the crab apparently prefer that zone as they are most abundant there and, unlike adults, do not undertake up-and-down-shore migration with tides. The present finding demonstrates, for the first time, endog- enous physiological timing of circatidal periodicity in the metamorphic moult of crab megalopae, suggesting that endogenous factors, as well as exogenous ones should be taken into account in considering the process of settlement by crab megalopae. Introduction In crustaceans such as crabs, ecdysis, in which the entire cuticle is shed at one time, is a critical event (Conan 1985; Chang 1995), during which the animals become defenceless and incapable of extensive movement. The event is under endocrine regulation (Skinner 1985; Chang 1989, 1993, 1995), and involves many physio- logical and biochemical changes (Chittleborough 1975; Morgan 1978; Tamm and Cobb 1978; Strong and Da- born 1980; Lipcius and Herrnkind 1982; Chang 1995). There is, therefore, likely to be pronounced endogenous physiological programming of the approximate time of moulting, an event which is no doubt under intense se- lection pressure, particularly at the stage in a crustacean life cycle when pelagic postlarvae metamorphose to the benthic juvenile stage (Tamm and Cobb 1976; Fern- andez et al. 1994). Hitherto, studies on the factors controlling the timing of crab megalopa metamorphosis have emphasized the responsiveness of such larvae to external stimuli and their capability to shorten or lengthen their time to metamorphosis in response to environmental cues (Sulkin and Van Heukelem 1986; Christy 1989; Lipcius et al. 1990; O’Connor 1991; Forward et al. 1994; Wol- cott and De Vries 1994; Brumbaugh and McConaugha 1995). For example, megalopae of the fiddler crab Uca pugilator were shown to moult earlier over sediment than in clean containers (Christy 1989; O’Connor 1991), and those of the blue crab Callinectes sapidus to moult more rapidly in estuarine water and in the presence of seagrass than in offshore water (Forward et al. 1994; Wolcott and De Vries 1994). Such responses are clearly adaptive, with megalopae evidently exercising some degree of habitat selection at the time of settlement in response to exogenous cues (Lipcius et al. 1990). Marine Biology (1997) 128: 299–305  Springer-Verlag 1997 Communicated by J.P. Thorpe, Port Erin C. Zeng 1 · E. Naylor (&) · P. Abello 2 School of Ocean Sciences, University of Wales, Bangor, Menai Bridge, Gwynedd LL59 5EY, North Wales, Great Britain Present addresses: 1 Department of Oceanography, Xiamen University, Xiamen 361005, Fujian, People’s Republic of China 2 Institut de Cie `ncies del Mar (CSIC), Passeig Joan de Borbo ´ s/n, E-08039 Barcelona, Spain