Complex Fish-Snail-Epiphyton Interactions and Their Effects on Submerged Freshwater Macrophytes

Responses of survival, antioxidant system and intestinal microbiota of native snail Bellamya purificata to the invasive snail Pomacea canaliculata

Pomacea canaliculata is one of the most successful invader in worldwide, adversely affecting native ecosystem through direct predation or indirect competition, while the mechanism of indirect effects on native species remain poorly understood. To clarify the effects of P. canaliculata on the native near-niche species, Bellamya purificata, a widespread freshwater gastropod in China, was selected as the research subject. The changes of mortality, histology, antioxidant system as well as the intestinal flora diversity of B. purificata were explored in present study. The results showed that the median lethal dose of P. canaliculata culture solution for B. purificata was 23.76 ind/L and a concentration-dependent damage of both the gonad and hepatopancreas were observed, the gonadal villi were dissolved and the hepatopancreas cells were broken at 20 ind/L. Furthermore, different concentrations of P. canaliculata culture solution leading to the antioxidant damage on the enzyme or non-enzyme systems of B. purificata at various degrees. Additionally, a decrease in the diversity of the intestinal flora was observed, accompanied by an increase in the abundance of pathogenic bacteria such as Pseudomonas and Aeromonas after the exposure of the culture solution of P. canaliculata. Last, after being recovered in freshwater for 24 h, the antioxidant damage of B. purificata and the disturbance of intestinal flora diversity were still not recovered especially in the high concentration group. The indirect competitive mechanism of P. canaliculata culture solution on B. purificata were explored from the aspects of tissue, biochemical level and intestinal flora, which enriched the research of P. canaliculata invasion on native snails in China, and provided new insights for the study of the invasion strategy of P. canaliculata.

Submerged Rootless Macrophytes Sustain a Stable State Against Free-Floating Plants

Both non-rooted submerged vegetation dominated by coontail (Ceratophyllum demersum) and non-rooted floating duckweed vegetation (Lemna gibba) can maintain their stable dominance in small ponds and channels. We examined the competitive interactions between them and how Ceratophyllum can sustain its stable state against floating plants in a range of nutrient concentrations. Coontail and duckweed were co-cultured in static and semi-static microcosm experiments, and their impact on the nutrients (N, P, Fe, Mn) in the water column was analysed. Coontail strongly reduced the growth of duckweed under a low nitrogen level (0.2–2 mg N L−1). This reduction seems to be due to the low availability of nutrients in the water as derived from the lower nutrient concentrations in duckweed tissue or high pH in water. High nitrogen levels in semi-static media (5–10 mg N L−1) resulted in an increasingly higher chance to overgrow C. demersum by L. gibba. Field observations revealed that C. demersum dominated over L. gibba in water bodies with total N below 3 mg L−1, while L. gibba dominance over C. demersum occurred above 5 mg L−1 total N. Ceratophyllum occurrence correlated negatively with total N in the water, while Lemna showed a positive correlation. Furthermore, the occurrence of L. gibba was negatively correlated with the frequency of C. demersum. All findings together support the theory that under a certain nutrient range, rootless submerged macrophytes have a strong potential to inhibit the dominance of floating plants in ponds, ditches and channels, and thus, they stabilize the submerged vegetation state.