Gregariousness during settlement in the barnacle Elminius modestus Darwin

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Abstract
A gregarious tendency during settlement, similar to that already demonstrated in oyster larvae, was suspected in Elminius, because cyprids settled in groups during the initial stages of colonization of surfaces (crowded later arrivals showed a spacing-out tendency) and because settlement on test-plates was peculiarly sparse at stations with a muddy bottom, where barnacles were absent.Settlement was much heavier on areas of smooth glass, which already bore recently settled barnacles, than on similar adjoining areas which were bare.When barnacled microscope slides were stuck to one set of glass plates, bare slides to another and the two sets exposed side by side, settlement was consistently much heavier on the plates which bore the barnacled slides. The mean density of settlement on the bare slides and the surrounding plates was uniformly low, except for a greater density immediately adjoining the slides, probably due to sheltering alongside their edges. On the barnacled slides density of settlement was much higher and on the plates immediately alongside still more so, whilst at increasing distances from the slides it became gradually smaller but was still much heavier than on the plates with the bare slides. This suggests that the sensory basis for gregariousness can act at a distance. It may possibly be olfactory.