The influence of size and density of the blue mussel, Mytilus edulis and the common barnacle, Semibalanus balanoides on the foraging behaviour of the sea urchin Psammechinus miliaris was examined in laboratory studies. Urchins of two size-classes offered six size-categories of barnacles exhibited a constant preference for the smallest category ([les ]7.5 mm plate length). Ingestion rates were independent of the size–frequency distribution of mussels presented to large urchins of 38–46 mm test diameter, which showed a preference for small–medium sizes (>13.4–17.8 mm shell length). Large urchins ingested a maximum of six mussels a day and increased their consumption in a density dependent manner until reaching a plateau once more than ten mussels were offered. In most cases, the rate of successful attacks decreased with mussel size.
Pairwise comparison experiments to determine the ingestion rate of fronds of the algae Laminaria saccharina showed that P. miliaris had no preference for old fronds with or without epifauna, but young fronds of the algae were frequently rejected. These results highlight the omnivorous nature and semi-selective behaviour of this urchin and demonstrate the potential for it to have an important role in structuring algal and epifaunal communities.