Based on a review and our own data, we present an overview of the ecological impacts on the trophic web of Mediterranean wetlands by an introduced Decapod Crustacean, the red swamp crayfish (Procambarus clarkii). P. clarkii lacks efficient dispersal mechanisms but is very well adapted to the ecological conditions of Mediterranean wetlands (fluctuating hydroperiods with regular intervals of drought). As an opportunistic, omnivorous species, which adapts its ecology and life history characteristics, such as timing and size at reproduction to changing environmental conditions, it became readily established in most of the Mediterranean wetland environments. High reproductive output, short development time and a flexible feeding strategy are responsible for its success as an invader. Like most crayfish, it occupies a keystone position in the trophic web of the invaded system and interacts strongly with various trophic levels. It efficiently grazes on macrophytes and is one of the main factors, besides the impact of flamingos, cattle and introduced fish, of the change of many water bodies from a macrophyte dominated, clear water equilibrium to a phytoplankton driven turbid water balance. Juveniles feed on protein rich animal food with the corresponding impact on the macroinvertebrate community in competition with other crayfish or fish species. At the same time, it serves as a prey for mammals, birds and fish. Due to its predatory and grazing activity, it efficiently canalises energy pathways reducing food web complexity and structure. Feeding also on detritus it opens, especially in marshlands, the detritic food chain to higher trophic levels which results in an increase of crayfish predators. As a vector of diseases, it has a severe impact on the preservation and reintroduction of native crayfish. P. clarkii accumulates heavy metals and other pollutants in its organs and body tissues and transmits them to higher trophic levels. Due to the long history of its presence, the complex interactions it established within the invaded ecosystems and the socio-economic benefits it provides to humans, prevention and control seem the most promising management measures to reduce the negative impact of this crayfish species. Biological Invasions (2005) 7: 49-73 Ó Springer 2005 ) High fecundity ++ Short generation and juvenile development times ++ Fast adaptation to environmental stress +++ High tolerance to environmental heterogeneity +++ Desirability to and association with humans (edibility, game species) +++ Additional features Omnivory +++ Brood care +++

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We compared the dietary composition, feeding preferences, and food selection of the introduced crayfish, Procambarus clarkii in a natural and a transformed freshwater marsh (rice field) in the lower Guadalquivir basin (southern Spain). Both habitats differed in hydroperiod and food item diversity.Food item diversity in crayfish stomachs is correlated with prey availability, which is significantly higher in the natural marsh. Herrera's index was low in both areas if compared with the theoretical maximum. P. clarkii behaves like a generalist in this area and individual crayfish have very similar feeding preferences.The most frequent items in the stomach were fresh macrophytes, detritus, and sediment grains. However, especially in the untransformed marsh, insect larvae formed an important part of the crayfish's diet, up to 67% by occurrence, with a marked seasonal pattern: crayfish in this area consumed significantly more animal prey in spring than in winter. P. clarkii positively selects for insect larvae (mayflies, chironomids), snails, and microcrustaceans except ostracodes. Negative selection against oligochaetes is explained by their burrowing behaviour and the difficulty to detect them in the stomachs. Cannibalism is stronger in the rice fields as a consequence of high crayfish densities.Animal food for adult P. clarkii is shown to be more important than previously reported. Overestimation of herbivory and detrivory in this omnivorous species might be due to serious methodological drawbacks associated with stomach content analyses. RESUMENPresentamos una comparación de la composición de la dieta, las preferencias alimenticias y selección de los recursos tróficos utilizados por el cangrejo introducido, Procambarus clarkii, en dos ambientes diferentes de marisma de agua dulce de la cuenca del Bajo Guadalquivir (sur de España), natural y transformado (arrozal). Los dos ecosistemas difieren en su hidroperiodo y en la diversidad de las presas.La diversidad de los recursos tróficos encontrados en los estómagos de las dos áreas, estuvo directamente relacionada con la diversidad de especies presentes en el campo, siendo significativamente 1 ) e-mail: paloma.alcorlo@uam.es © Koninklijke Brill NV, Leiden, 2004 Crustaceana 77 (4): 435-453 Also available online: www.brill.nl 436 P. ALCORLO, W. GEIGER & M. OTERO más alta en la marisma natural. El índice de Herrera fue bajo en ambas zonas cuando se compara con el máximo teórico. P. clarkii se comporta como una especie generalista en esta zona y los cangrejos, individualmente, tienen preferencias alimenticias semejantes.Los elementos más frecuentes encontrados en los estómagos fueron, según su orden de abundancia, macrófitos, detritus y granos de sedimento. Sin embargo, en la marisma natural, las larvas de insectos constituyen una categoría muy importante en la dieta del cangrejo (frecuencia de aparición de más del 67%) que fluctúa con un marcado patrón estacional: consumen más presas animales en primavera que durante el invierno.P. clarkii selecciona positivamente larvas ...

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