Field experiments on the settlement, orientation and habitat choice ofChthamalus fragilis(Darwin)

Abstract

Field experiments using stems of Spartina allernifolia as substrata indicate that Chthamalus fragilis, one of the few chthamalids to be so tested, shares with all the balanids so far examined, the advantage of settling more readily among conspecifics than on bare substrata. Despite a general preference for concavities, C. fragilis cyprids settle on the convex stems of Spartina, particularly if they already bear settled individuals. Initial settlement usually occurs in the axillary region, the prime location, which is shaded by the blade. Later colonisers tend to settle further up the stem as space in the axillary area becomes filled up. As in other barnacles, all individuals appear to settle so that the carina points upwards and the adult eyes point to the shaded side.