Transect samples were obtained during the low tides of the summers of 1990 and 1991 at a dissipative sandy beach in south central Chile, to investigate spring-neap tide variability in the down-shore zonation of the cirolanid isopods Excirolana braziliensis Richardson and Excirolana hirsuticauda Menzies. Short periods of sediment accretion alternated with periods of erosion during both summers, primarily at the mid and low intertidal levels. Highest temperatures (25-30-degrees-C) were recorded in the driest sediments (0-10% water content). During both summers, Excirolana braziliensis had its highest abundances (2-11 ind. 0.03 m-2) 136-318 cm below a bench mark in the supralittoral. This species occupied the sediments with the highest temperatures (17-24-degrees-C), the lowest water contents (5-15%), the finest (2.1-2.2 phi) and best sorted particles (0.28-0.29 phi). The highest abundances of Excirolana hirsuticauda (8-25 ind. 0.03 m-2) occurred at lower beach levels (202-356 cm below the bench mark) where the physical characteristics (lower temperatures. higher water contents, coarser particles) differed significantly from those measured at the levels occupied by Excirolana braziliensis during the summer of 1990. During the summer of 1991, no significant differences were found between the physical characteristics of the sediments occupied by either isopod species. During both summer periods the vertical distribution of Excirolana braziliensis did not show temporal variability. The same was observed for Excirolana hirsuticauda during the summer of 1991; however, during the summer of 1990 the zonation of this isopod shifted throughout the spring-neap tidal cycle, particularly during the last two samples of that summer. High tide samples carried out at the sediment water interface during the summer of 1990 revealed the presence of just Excirolana hirsuticauda. The abundances of Excirolana hirsuticauda did not differ significantly between day or night or among tidal levels. Finally, laboratory experiments showed that swimming activity of both isopods lacked tidal rhythmicity.
