A review of allodiversity in Lake Naivasha, Kenya: Developing conservation actions to protect East African lakes from the negative impacts of alien species

1. Prey species should evolve the ability to respond to the threats from a given predator with an 30 intensity that matches the level of risk associated with that predator. In this study, we investigated 31 the responses of the North American invasive crayfish Procambarus clarkii to the odour of three 32 fish species at two sites into which crayfish had been introduced; the Malewa River in the Lake 33 Naivasha basin (LN, Kenya) and Lake Trasimeno (LT, Italy). 34 2. Behavioural experiments were carried out on 15 adult crayfish for each site; treatment included 35 three phases, i.e. (1) a 3-min “water” phase, following the introduction of 10 mL of well water, (2) a 36 3-min “food” phase, following the introduction of 10 mL of food solution, and (3) a 3-min “smell” 37 phase, following the introduction of 10 mL of either well water as a control or a test solution, which 38 added to the food solution previously introduced. Test solutions consisted of well water conditioned 39 with (1) tilapia odour in LN and chub odour in LT, and, for both sites, (2) carp odour, (3) bass 40 odour and (4) conspecific alarm odour. While bass is a historical and high-risk predator for adult 41 crayfish, the other tested fishes represent non-historical low-risk predators. Only crayfish from LT 42 live in sympatry with the fish species used in the experiment. Every 15 s, we recorded the time 43 spent in feeding and walking, and the time spent in one of three postures (raised, intermediate, or 44 lowered). 45 3. We found that (1) in both sites crayfish responded with the most pronounced reduction of feeding 46 rate to the alarm odour than to the fish odours; (2) crayfish from LN reacted with the same intensity 47 to the three fish odours tested, independently of the species used; and (3) the odour of bass induced, 48 on crayfish from LT only, a significantly more intense reduction in the time spent feeding than the 49 odour of the other two fish species. 50 4. Procambarus clarkii seems to perceive a generic fish odour that alerts it, independently of 51 whether the test fish species has coevolved or not with the crayfish or if the fish predator poses a 52 high or a low risk. However, where it lives in sympatry with the test fishes it becomes more 53 selective by responding to the historical and high-risk fish only. Our results underline P. clarkii’s 54 behavioural plasticity and learning capacity and stress the need for further studies to elucidate the 55 mechanisms that underly predator recognition by this and other invasive aquatic species.