Limnoperna Fortunei Colonies: Structure, Distribution and Dynamics

Abstract

Settlement of new recruits of Limnoperna fortunei occurs preferentially on areas already colonized by conspecifics, and on surfaces with well-developed periphytic biofilms. Hard substrata (immobile rocks, wood) are preferred by the mussel, but colonization can also take place on muddy areas stabilized by roots or fibrous debris, on floating and submerged plants, and on mussel shells, crustaceans, etc. Colonization starts in crevices, angles and other sites inaccessible to large predators, but it often extends over open areas as well. Mussel beds rarely exceed 7–10 cm in thickness, with most adults being at least partially attached to the substrate. Juveniles often settle on larger shells. Densities of over 200,000 ind./m2 have been reported occasionally, but such high numbers are invariably dominated by specimens < 2 mm in length. Densities of adult mussels (>5-7 mm) are usually below 10,000 ind./m2. The only site where densities were estimated over an entire water body, the reservoir Embalse de Río Tercero, yielded an average of 959 ind./m2. Mussel colonies are usually most abundant and dense along the coastal fringe, where rock outcrops are common. Deeper areas are covered with clay and silt, and are therefore unfit for mussel colonization. Data at hand are still insufficient for describing multiannual trends in mussel abundance in South America; however, ancillary evidence suggests that, after having peaked 7–10 years after introduction, densities have been waning. Size structure of individuals in mussel colonies depends strongly on the time of the year. During periods of peak recruitment (spring to late summer) juveniles < Size structure of individual > 2 mm in length can represent >90% of the population, whereas during the winter they normally account for 10-15%.