Dispersion and phagokinesis in the echinoid Evechinus chloroticus (Val.)

Movement in the echinometrid sea urchin Evechinus chloroticus (Val.) was investigated in relation to the availability of a preferred food species, the laminarian alga Ecklonia radiata (C. Ag.). Patterns of microhabitat occupancy, dispersion, and movement were described in two habitats: Coralline Flats, dominated by encrusting coralline algae; and Sediment Flats, occupied by turfing corallines, Ecklonia and the mussel Modiolus areolatus (Gould). In both habitats, sea urchins were positively associated with encrusting coralline algae, despite the fact that encrusting corallines occupied a minor proportion of the substratum in the Sediment Flat habitat. Sea urchins were significantly clumped in both habitat types, and more so in sites within the Sediment Flats habitat. Movement was not directional in either habitat, but the magnitude of movement was almost twice as high in sites from the Coralline Flats habitat, where kelp was absent, as in the Sediment Flats sites, where kelp was present. Behaviour in response to the presence of Ecklonia in both habitats was investigated experimentally. In both habitats, urchins exposed to a “drift” kelp plant at a distance of 1–2 m exhibited net movement approximately twice as great as that shown by control urchins, not exposed to a kelp plant. Despite the relatively small distances the urchins were from the kelp, there was no evidence of directionality in movement. We conclude from the present study that further understanding of the feeding behaviour of urchins requires that a distinction be made between attached and drift algae, and between directionality and magnitude of movement, and that the interaction of these and other factors are best investigated under field conditions.