Limnoperna fortunei is a freshwater bivalve that invaded South America through Rı´o de la Plata estuary in 1989 and has since become a major macrofouling pest. Along the Parana´-Paraguay waterway, which hosts intense boat traffic, L. fortunei has moved upstream at an average rate of of 250 km per year.

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The globalization of economies and trade have facilitated the spread of exotic species including the five most important freshwater suspension feeding invaders Dreissena polymorpha, D. bugensis, Corbicula fluminea, C. fluminalis, and Limnoperna fortunei. We suggest that the spread of these exotic species has not been a continuous process, but rather punctuated by periods of rapid long distance spread (jump), during which species greatly expanded their geographic ranges. Each jump has been associated with changes in the tempo of some human activity, such as the construction of shipping canals for trade, building of reservoirs for water storage and power production, political boundary changes or changes in political systems, which affected the position or permeability of national borders, human migration, changes in the mode and volume of international trade, or recent industrial practices and environmental laws. We hypothesize that the rate of spread of exotic species depends on the spatial scale of spread and may be accelerated or slowed by various human activities. In general, aquatic exotic species may quickly spread along connected waterways in a new continent they invade and soon reach their maximum range (continental scale). However, it will take much longer to colonize all isolated regions (regional scale) and longer still to spread to all isolated lakes and river systems (local scale). The difference in the rate of colonization across scales may be several orders of magnitude.

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Clearance rates of Limnoperna fortunei (Bivalvia) were investigated in laboratory experiments using monocultures of the alga Chlorella vulgaris. Experimental conditions included two mollusc sizes (15 and 23 mm), and three water temperatures (15, 20 and 25°C) covering the normal seasonal range in the lower Parana´river and Rı´o de la Plata estuary. Filtration rates obtained were, for the larger mussels: 9.9, 13.1 and 17.7 ml mg tissue dry weight À1 h À1 at 15, 20 and 25°C, respectively; and for the smaller ones: 17.7, 20.8 and 29.5 ml mg À1 h À1 . Differences between sizes and between temperatures (except 15 vs. 20°C) were statistically significant. In absolute terms larger animals have higher clearance rates, but as a function of body mass smaller individuals feed more actively. Within the range of experimental values used, filtration rates were positively associated with water temperature. These clearance rates (125-350 ml individual À1 h À1 ) are among the highest reported for suspension feeding bivalves, including the invasive species Dreissena polymorpha, D. bugensis and Corbicula fluminea. High filtration rates, associated with the very high densities of this mollusc in the Parana´watershed (up to over 200,000 ind m À2 ) suggest that its environmental impact may be swiftly changing ecological conditions in the areas colonized.

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Limnoperna fortunei (Dunker, 1857) (Bivalvia) invaded Argentina through the Río de la Plata estuary around 1990 and is presently established throughout five South American countries as a dominant component of the benthic fauna and a major nuisance for industry and power plants. Between 1997 and 2006 we monitored the reproductive activity of L. fortunei through weekly measurements of planktonic larvae in six South American water bodies: Río de la Plata estuary, Paraná and Carapachay rivers, Salto Grande, Itaipú and Embalse de Río Tercero reservoirs. Mean larval densities varied between 4000 and 7000 individuals m -3 ; except in the reservoirs of Itaipú (450 ind. m -3 ) and Salto Grande (869 ind. m -3 ), where the mussel was first recorded shortly before our surveys, and upstream dispersal is limited (Salto Grande). In all cases, reproductive output decreases during the winter. At four of the six sites surveyed larval densities were comparatively high for 8.8-10.2 months per year. A lower food supply is possibly responsible for the shorter reproductive period of 5.9 months at Embalse. At Salto Grande, there is a well-defined mid-summer drop in larval numbers, coinciding with blooms of cyanobacteria. We propose that, in addition to temperature, two major factors may regulate the reproductive activity of L. fortunei: (i) the availability of food; and (ii) blooms of toxic cyanobacteria, significantly shortening the otherwise very long reproductive period. This information is important for the design of antifouling programmes involving the use of molluscicides, and has potential for reduced biocide use. These results provide supporting evidence for some fundamental ecological theories of invasions discussed here.