The impact of an introduced crayfish (Procambarus clarkii) on rice fields.

Since the 1960s, the red swamp crayfish, Procambarus clarkii, native to the north-eastern Mexico and to the south-central U.S.A., has been translocated in North America and introduced into all the other continents (except Australia and Antarctica), where it behaves as an \u201cinvasive\u201d species. It is most commonly found in warm fresh waters, such as slowly-flowing rivers, marshes, reservoirs, irrigation systems, and rice paddies. This species is classified as a pest in most wet seeded rice fields, although in some countries, e.g. in Louisiana (U.S.A.), double cropping of crayfish and rice can be practiced with success. In California (U.S.A.) and in Portugal, the impact of P. clarkii on wet-seeded rice fields (Oryza sativa) is high. The reduction of the rice yield highly depends on the density of crayfish populations, the direct impact on rice seeds and seedlings being the result of uprooting, plant fragmentation, and consumption. Both non-germinated seeds and seedlings are heavily consumed, particularly at the beginning of the growing season. Crayfish may exert other direct or indirect biological effects. They lead to changes in some physico-chemical parameters of the water, such as temperature, pH, oxygen concentration, turbidity, and soil structure. Burrowing decreases the stability of the banks and, along with this species\u2019 intense locomotion, increases water turbidity that in its turn reduces photosynthesis in submerged plants. Changes in water and in the waterlogged soil may affect both growth and survival of rice plants. Research is urgently needed to find safe and effective methods able to contain the spread of invasive populations of this species and to mitigate the damages produced