Population Dynamics and Habitat Suitability of the Intertidal Sea Anemones Anthopleura elegantissima and A. xanthogrammica

Abstract

Population dynamics of the intertidal sea anemones Anthopleura elegantissima (Brandt) and A. xanthogrammica (Brandt) were investigated by long—term monitoring and experimental manipulation of populations in Washington State. Sizes of individual A. xanthogrammica and A. elegantissima fluctuated seasonally, increasing during the spring and early summer and shrinking or reaching a pleateau the rest of the year. Data from marked individual A. xanthogrammica showed that adults (>6.5 cm) moved very little and may persist for at least several decades. Only 3 of 160 mapped adult anemones disappeared over 2 yr. Juveniles moved more often and thus appeared and disappeared at much higher rates. Growth of A. xanthogrammica in control and experimental removal areas was compared by a new technique. One—year growth increments were used to compare growth in each size class by analysis of variance and were also used to plot long—term growth trajectories and expected reproductive success (habitat suitability, Fretwell 1972). Growth was much more rapid in areas with experimentally reduced density than in control areas. The control area with the lowest density had the greatest individual growth rates. Differences in growth trajectories and expected reproductive success between control areas and the experimental areas indicate that adult density affects growth of most size classes. The mechanism is probably both noninterference competition for prey and direct interference by adult tentacle crowns blocking capture by smaller individuals. Clonal aggregations are formed by longitudinal fission of A. elegantissima. (A. xanthogrammica never reproduce asexually.) Monitored clonal aggregations of A. elegantissima showed that fission rate is greatest during the fall and winter, averaging 0.17 divisions per clonal individual per year in all areas. Only the larger individuals in each clone divided and a single individual never divided more than once in a given year. Rate of fission was independent of mean individual size within clones. Therefore, clones in more suitable habitats produced larger individuals and thus had greater gonad (offspring) production. Clones in the high intertidal were composed of very small individuals, had the lowest rates of fission, and were clearly occupying a marginal habitat.