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Over the last three decades, the Mediterranean mussel Mytilus galloprovincialis has invaded rocky shores on the west coast of southern Africa, and now makes up 74 % of intertidal mussel biomass. It competes directly with an abundant indigenous limpet, Scutellastra argenvillei, which attains high densities and biomass due to collective feeding on kelp. This competition has reduced the limpet's access to kelp, and dense aggregations of the limpets are now found only in small patches (termed 'established patches') that are remnants of the previous limpet belt. Lower densities of the limpets occur on mussel beds or in patches cleared of mussels by wave action ('cleared patches'). We investigated the effects of Mytilus encroachment on the movement patterns of S. argenvillei by comparing its homing and movements in these 3 situations. Short-term (12 h) homing frequency was similar on established patches (86.5 %) and newly created cleared patches (81.3 %) but significantly less on the mussel bed (60.4 %). On a monthly basis, homing frequency was lower, but significantly different in all 3 habitats (51.0, 29.2 and 10.9 %, respectively). Distances moved did not differ between habitats during the 12 h observations, but on a monthly basis were significantly greater on the mussel bed than in the other 2 habitats. There were no significant differences between the distances moved by juveniles versus adults for either 12 h or 1 mo periods. Small limpets on mussel beds frequently moved into cleared patches (29.8 %) while other limpets virtually never changed habitats. Mytilus encroachment has thus increased the mobility and reduced the homing frequency of S. argenvillei, probably because it limits access to kelp and increases dependency on alternative food sources. These changes in behaviour are associated with radical reductions of the density and mean size of S. argenvillei in areas dominated by M. galloprovincialis. Individuals that survive on the secondary substratum provided by mussels are all small and sexually immature. M. galloprovincialis has thus had an impact on both the behaviour and population dynamics of S. argenvillei, with considerable implications for the proposed commercial harvesting of S. argenvillei.

Homing and movement patterns of a South African limpet Scutellastra argenvillei in an area invaded by an alien mussel Mytilus galloprovincialis