SUMMARY1. Despite the growing number of non-indigenous crayfish species introduced worldwide, little scientific attention has been given to their effects on the recipient communities. Here, we quantified the impact of adult red swamp crayfish (Procambarus clarkii) on key components of the littoral community of a mesotrophic lake in central Italy. We used two densities of crayfish plus a no-crayfish control, and two complementary methods -in situ enclosures and a laboratory study. 2. Both experiments showed that P. clarkii strongly affects the community even at a low density (4 m )2 ), although a high crayfish density (8 m )2 ) showed a more pronounced impact in a shorter time.3. In enclosures, P. clarkii quickly consumed the pond snail Haitia acuta, notwithstanding its anti-predator behaviour, but not the mosquitofish Gambusia affinis. The biomass of the hydrophytes Nymphoides peltata and Potamogeton spp. was strongly reduced by crayfish grazing, coupled with their non-consumptive plant clipping and uprooting, which were particularly intense in the case of Potamogeton crispus. In contrast, Utricularia australis was avoided by P. clarkii, even when other resources were exhausted. 4. The introduction of P. clarkii could reduce biodiversity and increase biotic homogenisation in a short time.

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A B S T R A C TThe effect of latent inhibition on learning about cues related to increased predation risk was studied in a native crayfish, Orconectes virilis, and in an invasive species, Orconectes rusticus. Individuals of both species either were exposed or not exposed to the conditioned stimulus (potential ''predator'' odor 5 goldfish odor) prior to simultaneous exposure to conditioned (potential predator odor) and unconditioned (crushed conspecific 5 alarm odor) stimuli. Crayfish were then tested one day and one week later for inhibition of feeding responses following introduction of goldfish odor. Both species were clearly inhibited in their formation of an association between the goldfish odor and a predator risk stimulus after a training period during which they were exposed only to the goldfish odor. Thus, the effect of latent inhibition was demonstrated in both species. The implications for the learning of cues associated with predation risk are discussed.

The role of multiple signals in sex recognition in the crayfish Austropotamobius pallipes was investigated by analyzing the main channels (chemical or chemical plus visual) used by males during mate search. The behavioral responses of males were recorded when they were exposed to four different treatments: female odor only, male odor only, female odor plus visual cues, and male odor plus visual cues. Males did not respond to chemical cues alone emitted by a female, but required both chemical and visual stimuli. The high-flow conditions of the aquatic habitat occupied, the restricted mating period, and the low density of the population examined may explain the use of both cues in sex recognition by males.