Red swamp crayfish Procambarus clarkii is one of the most important invasive species worldwide, the only alien freshwater crayfish introduced in China since 1930s (Chen et al., 2017; Oficialdegui et al., 2020). It is a popular aquatic product in China and is cultured in more than 20 provinces with an area larger than 1.46 million hectares (Yuan et al., 2022). Red swamp crayfish is also widely distributed in lakes, rivers, ditches, sloughs, and elsewhere (Yi et al., 2018; Yue et al., 2021). Crayfish have been found to disrupt the clear water state of lakes through physical disturbances and direct consumption of macrophytes (van der Wal et al., 2013; Dercksen et al., 2025). They also negatively impact native species such as amphibians, gastropods, and insect larvae through predation or competition for food (Souty-Grosset et al., 2016; Huang et al., 2025). However, most research on the impacts of crayfish has focused on macrophytes and benthic animals (Ruokonen et al., 2016), while little attention has been paid to other taxa within the freshwater food web.

Crayfish are generally omnivores and their diet varies according to life stage (Correia, 2003; Geiger et al., 2005). The diet of adult crayfish (>8 cm in total length) consists of macrophytes, insect larvae, gastropods, and detritus (van der Wal et al., 2013; Wu et al., 2022), while little is known about the diet of juveniles and sub-adults. Adult female crayfish lay eggs and incubate them on their pleopods until hatching. By the third instar stage, juveniles become free-living (Hamasaki et al., 2023). Juvenile crayfish (<5 cm) are primarily carnivorous, feeding on zooplankton or small benthic animals (Correia, 2003; Weber and Traunspurger, 2017). They are capable of capturing planktonic organisms, giving them great potential to influence the littoral food web (Alcorlo et al., 2004; Geiger et al., 2005). The total length of sub-adult crayfish is 5~8 cm and they share trophic niches with both juvenile and adult crayfish (Correia and Anastácio, 2008; Veselý et al., 2020). In the field, 30%~80% of individuals are juveniles or sub-adults (Dorn et al., 2005), highlighting the importance of investigating their impacts on the food web to understand their effect on the stability of freshwater ecosystems.

Zooplankton play an important role in freshwater ecosystems and are the main food source for many kinds of fish and macroinvertebrates (Li et al., 2022; Neale and Rudolf, 2025). High densities of white leg shrimps Litopenaeus vannamei significantly reduce zooplankton abundance and diversity (Tran et al., 2023). The presence of juvenile and sub-adult crayfish P. clarkii has been shown to greatly reduce the abundances of the crustaceans Daphnia and Cyclops (Correia and Anastácio, 2008), indicating that crayfish have strong potential to feed on and influence zooplankton community.

Aquatic animals including fish, macroinvertebrates, and zooplankton exhibit diurnal feeding rhythms that vary among species and habitats (Williamson et al., 2011; Krylov et al., 2021; Viccon-Pale, 2022). Light intensity influences the circadian rhythm of crustaceans which is regulated by putative pacemakers such as the brain (López-Becerril et al., 2025). The juvenile shrimp L. vannamei exhibit higher feeding rates in light than in darkness (Sanudin et al., 2014). Crayfish P. clarkii often hide in burrows to avoid predators such as fish or waterbirds, which mainly feed during the day (Haubrock et al., 2019). The feeding activity of crayfish follows rhythmic patterns (Viccon-Pale, 2022), some studies reported peak feeding time in the morning from 8:00 to 10:00 and at night from 19:00 to 22:00 (Xu et al., 2012), while others suggested continuous feeding during the day and at night (Zhou and Zhao, 2007). Most studies on feeding rhythms focus on adult crayfish, while little is known about the behavior of the juveniles.

Invasive crayfish have significant impacts on native crustaceans due to their advantages in growth, reproduction and locomotion (Lodge et al., 2012; Guareschi et al., 2024). Native freshwater shrimps in China are generally omnivorous, feeding on zooplankton, dipteran larvae, periphyton and detritus (Tiffan and Hurst, 2016; Zhu et al., 2022). Presence of shrimp may reduce periphyton biomass and increase macrophyte biomass in freshwater lakes (Ye et al., 2019). Shrimps prefer to feed on cladocerans, leading to changes in zooplankton community structure (Mamani et al., 2019). The shrimp Exopalaemon modestus is one of the most abundant and common native shrimp in lakes in the lower reaches of the Yangtze River (Zhao et al., 2023). E. modestus predate on various zooplankton species (He et al., 2021), with cladocerans and copepods constituting a high percentage in their diets (Zhu et al., 2022). This shrimp is generally active at night, with feeding activity peaked near 22:00 (Wen and Xie, 2013). Co-occurrences of P. clarkii and E. modestus have been reported in natural lakes in China (Wu et al., 2022; Zhao et al., 2023). Although field observations indicate a similar trophic niche between the invasive P. clarkii and the native E. modestus (Wu et al., 2022), their differential predation rhythms on zooplankton remain unclear.

In this study, we hypothesized that there is a circadian rhythm difference between invasive juvenile crayfish and native shrimp in their predation of zooplankton, which might lead to a higher zooplankton feeding intensity in juvenile crayfish than in shrimp. To test this hypothesis, we compared the predation intensity and efficiency of crayfish and shrimp on cladoceran Simocephalus mixtus, one of the most common cladocerans in the littoral regions of freshwater lakes (Huang, 2014), at different prey densities under both daytime and night-time conditions. Our results provide insights into the mechanisms through which invasive species adversely affect native ones and partly help explain the invasion success of crayfish.

Invasive species generally have higher feeding efficiency, faster growth rate and/or higher fecundity than native species (Vila-gispert et al., 2005; Van Kleunen et al., 2010). In our study, juvenile crayfish exhibited higher zooplankton feeding intensity than shrimp. The results confirmed the hypothesis that the invasive crayfish and native shrimp differ in circadian rhythm of predation on cladocerans. Invasive juvenile crayfish fed continuously throughout the day and night, while native shrimp showed low feeding activity during the day.

Predation rates of crayfish increased with increasing zooplankton densities in all experiments. As prey abundance rises, encounter and predation rates normally increase (Turesson and Brönmark, 2007). Functional responses describe the relationship between predation rate and prey density (Faria et al., 2023). Juvenile crayfish shows a Type II functional responses when feeding on another cladoceran Daphnia magana, maintaining high feeding rates at low prey densities (South et al., 2019). Prey density also affects the defense of cladocerans against handling of predators, which tends to be stronger at higher prey densities (Jeschke and Tollrian, 2000). We found that feeding rates of juvenile crayfish on cladocerans increased as prey density increased, however, density gradient of zooplankton was not high enough to fit the Type I, Type II or Type III functional responses. Further studies are needed to explore the functional responses of juvenile crayfish and shrimp to variations in density of S. mixtus and other zooplankton species.

We found that the larger sub-adult crayfish predated less on zooplankton than juvenile crayfish at all zooplankton densities. Furthermore, one individual of sub-adult crayfish also predated less on zooplankton than one juvenile crayfish (1.71 versus 4.88 ind./h under high zooplankton density). This aligns with the fact that juvenile crayfish tend to be carnivores, while sub-adult and adult crayfish shift toward higher degree of herbivory (Correia, 2003). Moreover, juvenile crayfish are capable swimmers (Geiger et al., 2005; Kato et al., 2018), whereas the swimming ability of sub-adult crayfish is limited, resulting in low possibility of catching cladocerans (Barbaresi et al., 2004). Predation of juvenile and sub-adult crayfish on cladocerans will influence zooplankton community structure, with potential cascading effects on phytoplankton abundance and water clarity (Jeppesen et al., 2004). Crayfish influence the zooplankton community not only through predation but also by increasing water turbidity (Correia and Anastácio, 2008), and their influence on zooplankton is more complicated under natural conditions. Differences in eyesight between juvenile and sub-adult crayfish also contribute to variations in their catching ability (Fanjul-Moles and Prieto-Sagredo, 2003). As sensitivity to light varies between juvenile and adult crayfish (Ou and Liang, 2017), further studies are needed to explore how light influences their predation rhythms.

Juvenile crayfish showed similar feeding rate but a longer feeding period on S. mixtus compared to shrimps of similar length, indicating higher feeding capacity of this invasive species. In our study, the feeding rate of juvenile crayfish tended to be slightly lower during the day than at night. Shrimp E. modestus mainly fed at night, which aligns with findings of other studies of diurnal variation in the feeding activity of another shrimp of the same genus (E. carinicauda) (Wang et al., 2023). For shrimp L. vannamei, the circadian rhythm of nocturnal feeding is driven by an endogenous clock and persist under different conditions (Santos et al., 2016). Juvenile crayfish fed continuously during the day and at night, showing no apparent circadian rhythm, which might be linked to the fact that crayfish rely on both eyesight and smell for foraging (Zhou and Zhao, 2007). The night-time feeding rates of juvenile crayfish were close to that of shrimp, indicating that juvenile crayfish has high catching ability on cladocerans (South et al., 2019). Cladocerans can identify predators with different feeding habitats and behaviors (Ekvall et al., 2020; Lee and Hansson, 2024). Juvenile crayfish are ambush predators, while shrimp swim at relatively constant speeds at night (Hu et al., 2001; Renai and Gherardi, 2004), which may also explain the observed differences in their predation rhythms on S. mixtus. Size of the predator influence the enzyme digestive activities, which indirectly influence the predation rates and frequency (Gilannejad et al., 2021). Juvenile shrimp at weight 0.4 g showed relatively lower enzyme digestive activities than shrimp at weight 0.1-0.3 g (Wang et al., 2010). For juvenile crayfish, enzyme digestive activities increase steadily through 42 days of development and remain high at weight 0.6 g (Hammer et al., 2000). Our study on early stages of invasive crayfish and native shrimp supports the inherent superiority hypothesis that invasive species possess advantages over native species in feeding behaviors and capacity (Ju et al., 2013). Presence of crayfish has also been found to cause shift in the trophic niche of shrimp (Baudry et al., 2024). Moreover, adult and sub-adult crayfish have the potential to prey on shrimps (Banha and Anastácio, 2011), adding to their invasion success.

To conclude, juvenile crayfish and shrimp were found to consume cladocerans efficiently. Juvenile crayfish fed more at night than during the day and at a faster rate than sub-adult crayfish. Juvenile crayfish also fed for longer periods than shrimp, showing higher feeding capacity for invasive species than their native counterparts (Faria et al., 2025). Future studies are needed to investigate the differences between invasive crayfish and native shrimps in natural lakes, and to explore the overall impacts of decapods on the zooplankton community and the entire freshwater ecosystem.