Introduction

The red swamp crayfish, Procambarus clarkii (Girard, 1852), is an economically important species in China. In 2018, the output of P. clarkii in China was 1.64 million tonnes, so it ranked first in the output of freshwater crustaceans in China; in addition, the output was nearly double the annual output in 20161,2. In 2018, the total area for P. clarkii cultivation in China was 1.12 million hectares, and, of this, paddy fields accounted for 0.84 million hectares. Paddy fields accounted for 40% of the total area used for rice cultivation and fisheries in 20183. Rice–crayfish co-culture has a lot of economic benefits, with an increase in net income by 6302.7 USD per hectare4. According to our field survey (data not provided), farmers in some places such as Honghu City in Hubei Province and Yancheng City in Jiangsu Province often widen rice ditches and pay more attention to P. clarkii cultivation than rice production, which is against the principles of rice–fish co-culture (Technical specifications for integrated farming of rice and aquaculture animal, SC/T 1135-2017).

Since the twenty-first century, rice–fish co-culture has been widely practiced in paddy fields in Asian countries, especially in China2c).

Figure 2
figure 2

Feeding behaviour of P. clarkii (using L. chinensis as an example). (a) Feeding in the side direction, (b) Feeding in the front direction, (c) no motion.

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Discussion

P. clarkii is one of the most important invasive species found worldwide, and its ecological plasticity allows it to live in different types of environments. The great ecological plasticity of P. clarkii is also expressed in its feeding habits attributable to its polytrophic feeding behaviour18. Previous studies have shown that P. clarkii can establish a food chain suitable for its growth needs on the basis of the food sources in its living environment19. Therefore, is the fact that crayfish can reduce weed biomass in paddy fields related to this ecological characteristic? In this study, P. clarkii showed a strong appetite for some weeds, such as L. chinensis,and the PFI of L. chinensis was more than 2%. The results of the quantitative feeding and behaviour observation experiments were highly consistent. The P. clarkii specimens mostly preferred to eat L. chinensis, but hardly ate E. crusgalli. No significant differences were observed in the feeding amount with respect to L. prostrata and E. prostrata,however, according to the behaviour experiment results, the P. clarkii specimens preferred E. prostrata.

Previous studies on the diversity of weed communities in rice fields have shown that the density of L. prostrata and E. prostrata in the rice–crayfish co-culture system has significantly reduced when compared with rice monoculture, and the biomass of these 2 weeds continues to decrease with the increase in the duration of rice–crayfish co-culture14,15,20. In this study, the results showed that the feeding ability of L. prostrata and E. prostrata by P. clarkii was better and probably achieved by direct ingestion. However, the results obtained for L. chinensis and E. crusgalli were inconsistent with those of previous field studies. The biomass of L. chinensis and E. crusgalli decreased first (< 4 years) and then increased (7–8 years) with an increase in the duration of rice–crayfish co-culture, and both weed densities have been found to be lower in the rice–crayfish co-culture system than in the rice monoculture system15,20. This showed that P. clarkii can also control L. chinensis and E. crusgalli. However, in the present study, almost no P. clarkii specimen fed on E. crusgalli. In our study, increased feeding on L. chinensis was observed, relative to all the other weeds examined. Previous field investigations have reported that the biomass of L. chinensis was still significantly higher than that of L. prostrata and E. prostrata15,20, suggesting decreased consumption of L. chinensis. Therefore, it is unclear how P. clarkii controls weeds, especially L. chinensis and E. crusgalli, as it seems that the weeds were not controlled by direct feeding. P. clarkii can directly feed on agricultural seeds such as rice seeds, which contain high protein and/or energy21,22. Therefore, another possibility is that P. clarkii inhibits weed growth by ingesting weed seeds or suppressing weed seed germination through burrowing.

P. clarkii is an opportunistic, omnivorous feeder, and its diet includes submersed macrophytes, algae, invertebrates and detritus18. Generally, P. clarkii likes to feed on aquatic plants, but there as a few studies on P. clarkii feeding on terrestrial plants. During P. clarkii cultivation, fishermen generally grow aquatic plants such as Hydrilla verticillata and Elodea nuttallii. The large amount of L. chinensis consumed by P. clarkii in the present study shows the unlimited potential of P. clarkii to control weeds in rice fields. Both L. chinensis and E. crusgalli are gramineous plants, but the feeding selectivity of these 2 weeds with respect to P. clarkii was very different. L. chinensis seedlings are tender23, which may make its taste closer to that of aquatic plants. In addition, olfaction plays an important role in the feeding process of P. clarkii24, and it is possible that the odour of L. chinensis attracts P. clarkii to a greater extent than that of E. crusgalli. The results of the quantitative feeding experiment showed no significant differences in the feeding amount of the P. clarkii males and females in the L. prostrata and E. prostrata groups,however, the results of the behaviour observation experiment showed that the feeding frequency of the P. clarkii males was higher than that of the females in the L. prostrata and E. prostrata groups. P. clarkii males are more aggressive than the females25, so it is possible that the differences in their behaviour are because the P. clarkii males frequently move, search for food and eat.

All the weeds used in this study were newly grown seedlings. The appearance of weeds changes greatly in different growth stages. Therefore, the conclusions drawn on the basis of the P. clarkii specimens feeding on the weed seedlings in this study are not necessarily applicable to weeds in other growth stages. Freshwater crayfish have a dietary protein requirement of at least 30–35% for optimal growth26. The percentage of crude protein content in dry L. chinensis and E. crusgalli is 8.44% and 11.84%, respectively27,28. Although moulting of P. clarkii was observed in the weed groups in the present study, the P. clarkii specimens in the weed groups died with the extension of culture time, whereas the P. clarkii specimens in the feed group continued to grow well. Obviously, it is impossible for P. clarkii to feed on only weeds in a rice field, as the nutrition in weeds cannot completely meet the requirements for important life history events of P. clarkii, such as moulting and reproduction. However, the high ecological plasticity of P. clarkii makes its use in controlling weeds in rice fields possible. In a rice field, which has high amounts of plant material and low macroinvertebrate diversity, animal food is less important18. Therefore, the use of P. clarkii in controlling weeds in a rice field needs to be studied further.

Conclusions

This study provides direct evidence that crayfish feed on common weeds in paddy fields, suggesting that crayfish may be used to reduce the weed biomass in paddy fields. P. clarkii preferred to eat L. chinensis and hardly fed on E. crusgalli. The mean percentage of daily feed intake to body weight per P. clarkii for L. chinensis was more than 2%.