Free-living clusters of barnacles are described as nucleations around shell fragments of Ensis directus. The bivalve E. directus is an invader in European seas that exhibits mass mortalities, during which the animals creep partly out of the sediment. Barnacles can then settle on the inside and the outside of the posterior part of these empty, articulated, semi–in situ shells. This posterior, encrusted portion apparently became separated from the remainder of the valve, enabling the barnacles to continue life as a free-living balanulith.

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