Population densities of lugworms Arenicola manna on tidal flats are remarkably stable and even. In the Dutch Wadden Sea and on the Swedish west coast, we studied processes which might explain the low variability in tune and space in lugworms. Local enhancement of densities resulted in a return to original (and surrounding) densities within 1 yr. Juveniles settled mainly in areas with low adult densities. This was so both in natural areas and in experimental plots with either enhanced or reduced

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... adult densities. Only some of the natural areas with low adult and high juvenile densities had the special environmental conditions of the so-called Brutwatten or nursery areas. Juveniles were smaller in areas where their densities were high than where their densities were low. Short periods of juvenile migration in winter reduced the locally high densities of small juveniles. Recruitment of juveniles was higher in years with low than with high mean densities of adults. We consider the roles of lugworm bioturbation and food lirmtation in controlling lugworm densities.