Galleries of the crabs Acanthocyclus under intertidal mussel beds: their effects on the use of primary substratum Galerias de jaibas Acanthocyclus bajo mantos de mitilidos intermareales: sus efectos sobre el uso de sustrato primario

The mid rocky intertidal of Cachagua, like many other rocky sites in central Chile, is dominated by beds of the mussel Perumytilus purpuratus. Under these mussel beds the predatory crabsAcanthocyclus gayi and A. hass/eri build separated blind-ending galleries whose floor is the rock and the roof is the mussel bed. Samples of 1 m 2 of Perumytilus beds (100% cover) showed that the surface area of crab galleries ranged between 17.4% and 34.7% of the total area, and that 80% of the crabs were A. hassleri. In 130 of the 157 galleries analyzed, one or two conspecific crab11 were found. In the remainder, up to six crabs were found together. Comparisons between the floor of crab galleries and similar rocky areas under the mussel bed, but without galleries (controls), showed that both density and cover of Balanus jlosculus, Phymactis clematis and bryozoans were significantly higher in galleries. Similarly, the total number of species was greater on the floor of the galleries than in the controls. Although from outward appearances the cover of P. purpuratus remained at 100%, crab galleries allowed more sessile species to occupy the mid intertidal fringe. This occurs because of the introduction of species typical of the lower intertidal, which would otherwise be excluded by mussels.