Browsing by Author "Alvarado, JL"

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    Distribution, population structure, population biomass and morphological characteristics of the tunicate Pyura stolonifera in the Bay of Antofagasta, Chile
    (2000) Castilla, JC; Guiñez, R; Alvarado, JL; Pacheco, C; Varas, M
    The present study analyzes the distribution, population structure, population biomass and morphological characteristics of the intertidal tunicate Pyura stolonifera (Heller, 1878) in the Bay of Antofagasta, Chile, where the species shows a reduced geographical range extending around 60-70 km. Three sites along its distribution were studied. At the central site the tunicate presents the widest intertidal belts, reaching nearly 11 m (the mid-intertidal-Pyura-zone shows the higher percent cover). Population dry biomass here averaged up to 20.45 kg.m(-2), representing one of the highest intertidal specific biomasses reported in the literature. Lower values occur towards the borders of its distribution. Population densities were over 1800 indiv..m(-2) at mid-intertidal-Pyura-belts. Crowding and competition potentially affect morphological characteristics and the energy/tissue allocation along the intertidal and geographical gradient of P. stolonifera.
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    Patch recolonization by the tunicate Pyura praeputialis in the rocky intertidal of the Bay of Antofagasta, Chile
    (2001) Alvarado, JL; Pinto, R; Marquet, P; Pacheco, C; Guiñez, R; Castilla, JC
    Patch dynamics of intertidal Pyura praeputialis (Heller, 1838) were studied in experimentally cleared plots at 3 vertical levels (upper, mid arid lower P, praeputialis levels) and 3 sites within the Bay of Antofagasta, Chile. The sites corresponded approximately to the northern limit (Las Conchillas), the center (EL Way), and the southern limit (El Lenguado) of the species geographic distribution in Chile. Replicated 40 x 60 cm quadrats were photographed periodically to follow patch re-colonization. Patch recolonization (recolonized area) followed a sigmoid curve and could be fitted at most sites and levels by applying maximum likelihood techniques to the asymptotic model: y = a/{1 + exp[-(x - b)/c]} where Parameters a and b are asymptotic percentage cover and time for 50 % recovery respectively, and Parameter c defines the shape of the curve (growing phase). Among sites, the highest asymptotic values and fastest recovery times were observed at El Way, in the center of the species distribution. Among levels, the highest asymptotic values were observed in the mid P. praeputialis level quadrats. The perimeter of the patches increased and subsequently decreased as a function of recolonized area. Recruitment observations showed the process of recolonization to be related to the perimeter of the patch to be colonized rather than to the area available. Further more, the recolonization patterns could be predicted using the patch perimeter scaled by a linear coefficient. We believe that the mechanism of recolonization is most probably related to the settlement of recruits to the border of previously settled individuals, possibly mediated by intraspecific self-facilitation mechanisms.
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    Tridimensional matrices of mussels Perumytilus purpuratus on intertidal platforms with varying wave forces in central Chile
    (1996) Alvarado, JL; Castilla, JC
    Perumytilus purpuratus is the dominant competitor for primary substrate in the mid-intertidal zone of central Chile's rocky shores. This mussel is capable of forming dense matrices (beds) that support rich infaunal and epiphytic communities. Three platforms, exposed, semi-exposed, and sheltered, were sampled to determine aspects of the mussel P. purpuratus size structure and temporal dynamics. Palumbi's dynamometers were used to measure intertidal wave forces. Mussel samples from the 3 platforms were collected approximately every second month between April 1982 and June 1983. For sampling, mussels were removed sequentially in order to maintain the stratified structure (layers) of the mussel matrices. Populations of P. purpuratus show a bimodal size distribution when considering their entire size range. Although this bimodality is consistent through time and space, our results show important differences for the size distributions both spatially and temporally. Spatially, the differences could arise as a result of varying wave exposures. Adults showed a higher mean size with decreasing exposure. Temporally, the differences arise due to changes in the number of recruits entering the population. Within platforms, the observed differences when comparing upper and lower mussel layers could arise as a result of differential survival and growth rates between them. We also suggest that retention of recruits is higher in lower layers due to increased complexity of the matrix and reduced water flow. The sampling method developed to collect separate mussel layers proved to be crucial for the recognition of population dynamics processes occurring within the stratified structure of mussel matrices.