Crayfish plague affects juvenile survival and adult behaviour of invasive signal crayfish.

The spread of invasive, non-native species is a key threat to biodiversity. Parasites can play a significant role by influencing their invasive host's survival or behaviour, which can subsequently alter invasion dynamics. The North American signal crayfish (Pacifastacus leniusculus) is a known carrier of Aphanomyces astaci, an oomycete pathogen that is the causative agent of crayfish plague and fatal to European crayfish species, whereas North American species are considered to be largely resistant. There is some evidence, however, that North American species, can also succumb to crayfish plague, though how A. astaci affects such 'reservoir hosts' is rarely considered. Here, we tested the impact of A. astaci infection on signal crayfish, by assessing juvenile survival and adult behaviour following exposure to A. astaci zoospores. Juvenile signal crayfish suffered high mortality 4-weeks post-hatching, but not as older juveniles. Furthermore, adult signal crayfish with high-infection levels displayed altered behaviours, being less likely to leave the water, explore terrestrial areas and exhibit escape responses. Overall, we reveal that A. astaci infection affects signal crayfish to a much greater extent than previously considered, which may not only have direct consequences for invasions, but could substantially affect commercially harvested signal crayfish stocks worldwide.

Terrestrial emigration behaviour of two invasive crayfish species.

To disperse between isolated waterbodies, freshwater organisms must often cross terrestrial barriers and many freshwater animals that are incapable of flight must rely on transport via flooding events, other animals or anthropogenic activity. Decapods such as crayfish, on the other hand, can disperse to nearby waterbodies by walking on land, a behaviour that has facilitated the spread of invasive species. Overland movement could play a key role in the management of non-native crayfish, though to what extent terrestrial emigration occurs in different species is poorly understood. Here, we directly compared the terrestrial emigration tendency of two non-native crayfish species in Great Britain; red swamp (Procambarus clarkii) and signal (Pacifastacus leniusculus) crayfish. We found that both species emigrated from the water and that there was no significant difference in terms of their terrestrial emigration tendency, suggesting that there is a risk both of these species will migrate overland and disperse to new habitats. This study shows that terrestrial emigration is an important behavioural trait to consider when preventing the escape of crayfish from aquaculture and further spread of invasive species.

Terrestrial dispersal of invasive signal crayfish during vulnerable life stages.

1. Aquatic invertebrates commonly disperse between waterbodies by flight, though some decapods can emigrate from the water and walk overland. 2. The signal crayfish (Pacifastacus leniusculus, Dana) is a highly invasive species that can survive for days to weeks out of water, though it is unclear to what extent vulnerable life stages, such as ovigerous females or juvenile crayfish, terrestrially emigrate. Understanding this behaviour is important to consider during the management of crayfish stocks, since a single ovigerous female could potentially introduce hundreds of hatchlings to a new waterbody, which could themselves disperse overland. 3. Here, in a laboratory study, we examined the terrestrial emigration tendency of juvenile crayfish and compared the terrestrial emigration behaviour and overland walking speed of ovigerous and non-ovigerous females. 4. Size had a significant influence on juvenile crayfish terrestrial emigration tendency, where only those larger than 16.6 mm (carapace length) left the water. Ovigerous and non-ovigerous female signal crayfish showed no significant difference in terrestrial emigration tendency nor overland walking speed. 5. This is the first study to directly examine the terrestrial emigration behaviour of crayfish during these different life stages. These findings highlight the importance of considering animal behaviour during management strategies.