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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 upand-down shore migration with tides. Freshly collected 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 2 spring-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 days after 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 no moulting 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. The coupllng 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.

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SUMMARY:The sublittoral megabenthic assemblages of a northwestern Mediterranean coarse sandy beach exploited for the bivalve Callista chione were studied. The spatial and bathymetric variability of its distinctive faunal assemblages was characterised by quantitative sampling performed with a clam dredge. The taxa studied were Mollusca Bivalvia and Gastropoda, Crustacea Decapoda, Echinodermata and Pisces, which accounted for over 99% of the total biomass. Three welldifferentiated species assemblages were identified: (1) assemblage MSS (Medium Sand Shallow) in medium sand (D50=0.37 mm) and shallow waters (mean depth =6.5 m), (2) assemblage CSS (Coarse Sand Shallow) in coarse sand (D50=0.62 mm) in shallow waters (mean depth =6.7 m), and (3) assemblage CSD (Coarse Sand Deep) in coarse sand (D50=0.64 mm) in deeper waters (mean depth =16.2 m). Assemblage MSS was characterised by the codominance of the bivalves Mactra stultorum and Acanthocardia tuberculata. C. chione was dominant in both density and biomass in assemblages CSS and CSD. The occurrence of the crab Thia scutellata also characterised assemblage CSS, whereas the occurrence of the sea urchin Echinocardium mediterraneum characterised assemblage CSD. A depth breaking point of around 10 m determined the discontinuity between assemblages CSS and CSD, which was related to the closure depth of the beaches in the study area. Species richness was highest in the coarse sand communities; however, Shannon-Wiener diversity and Pielou equitability indexes were higher in the shallow fine sand community.Keywords: sublittoral, megabenthos, faunal assemblages, sediment characteristics, biodiversity, NW Mediterranean, coarse sand, Bivalvia, Crustacea. RESUMEN: Comunidades faunísticas de un hábitat sublitoral de arena gruesa en el Mediterráneo Noroccidental. -Se han estudiado las comunidades megabentónicas sublitorales de playas de arena gruesa afectadas por la pesquería del bivalvo Callista chione en el Mediterráneo noroccidental (Maresme). Se ha caracterizado la variabilidad espacial y batimétrica de las comunidades faunísticas presentes mediante un muestreo cuantitativo utilizando un rastrillo de bivalvos. Los taxones estudiados fueron Mollusca Bivalvia y Gastropoda, Crustacea Decapoda, Echinodermata y Pisces, los cuales representaron más del 99% de la biomasa total. Se identificaron tres comunidades bien diferenciadas: (1) comunidad MSS en arenas medias (D50=0.37 mm) y poco profundas (profundidad media =6.5 m), (2) comunidad CSS en arena gruesa (D50=0.62 mm) en aguas someras (6.7 m), y (3) comunidad CSD en arenas gruesas (D50=0.64 mm) en aguas más profundas (16.2 m). La comunidad MSS se caracterizó por la codominancia de los bivalvos Mactra stultorum y Acanthocardia tuberculata. C. chione dominó en densidad y biomasa en las comunidades CSS y CSD. La presencia del braquiuro Thia scutellata también caracterizó la comunidad CSS, mientras que la presencia del erizo Echinocardium mediterraneum caracterizó la comunidad CSD. Se detectó una discontinuidad entre las comunidades CSS...

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