Study On The Relationship Between Submerged Plants And Herbivous Snails

It is well documented that invertebrate herbivory can influence relative abundance and diversity of macrophyte species in lake communities. Freshwater plants may have wide relationships with numerous kinds of invertebrate herbivores, including crayfish, crab, snail, aquatic insect, and some larva grazers. Some macrophyte tissues are assumed to be protected against herbivore attacks by repellent substances or a hard texture. However, submersed plants generally contain much lower phenolic concentrations in contrast with terrestrial plants, freshwater emergent plants, floating-leaved plants, and even the submersed oceanic plants; and these submersed plants lacking tougher surface and secondary compounds were not so much grazed. Thus, how do these submersed freshwater plants manage to protect their tissues against grazers remains as an interest today. Many interests have been raised on studying the types of interactions between aquatic plants and herbivores, including interspecific and intraspecific variation in plant palatability, in context of the types and theories derived from terrestrial systems previously. A basic theory about plant palatability is the resource availability theory. Which hypothesized that strategy of plants coping with herbivory is related to availability of abiotic resources needed for plant growth (such as water, nutrients and light). Carbon-nutrient balance (CNB) hypothesis indicates that in terms of carbon availability is high relative to nitrogen supply of a plant, an excess of carbon would be allocated to carbon-based defensive substances. The theories have been tested for terrestrial plants and proved to be appropriate. However, they were less tested in freshwater ecosystems, and reliable data are available only for a few freshwater plant species. Whether the theories are appropriate in evolutionary independent freshwater systems needs further confirmation.In terms of seasonal flooding in Changjiang River Basin, light is often a limiting and variable factor for submerged plants in lakes. Based on this information, we chose light as an experimental factor to evaluate the influences of resource availability on intraspecific palatability of freshwater plants by using pulmonate snails. Radix swinhoei (H. Adams) is a freshwater pulmonate snail, which is ageneralist plant-grazer and detritus-feeder. We want to find out whether snails have a preference for plants grown under specific conditions of light intensity and whether the snail performance on plants grown under different conditions of light intensity is consistent with the snail feeding preferences. Palatability to snail herbivory (/?. swinhoei) and C/N ratios were assessed for Vallisneria natans, in three different experimental light regimes (307 umol m'2 s*1, 16 umol m"2 s1, and a variable intensity between the two above). After transplanting, adaptive growth rate had been induced for V. natans, mostly due to the differences in light availability. Snail growth was slightly enhanced by high light, while juvenile survivorship decreased under higher light condition. Higher light intensity as well as prolonged photoperiods increased palatability, growth and C/N ratio. Increased light availability also resulted in greater leaf consumption in choice tests. Such feeding choices show that rank of palatability is high light plants > moderate-high light plants > low light plants. The rank of palatability is in accordance with the resource availability theory, since it showed that plants with adaptive lower growth rate established lower palatability. The results suggest that the availability of resources (light) affects the intraspecific variations in palatability of submerged macrophytes.Some recent studies assume that submersed plants have cheaper tissues with higher water content than other plants, which should not merit protection by expensive second metabolites such as tannins. Some studies consider chemical defenses other than tannins and have identified a number of feeding deterrents from freshwater plants. However, a few species of submersed plants do show higher tannin content. All members of the Haloragaceae including Myriophyllum spicatum contain approximately one order of magnitude higher concentrations of phenolic compounds than many other submersed plants. Most of these phenolics are found to be hydrolyzable tannins. Tannins have long been recognized as a kind of herbivore deterrents and often predict palatabilities of terrestrial plants. The anti-herbivory mechanisms of these secondary compounds about depressing feed utilization efficiency are well held. Tannins in M. spicatum have been shown to have properties of interfering with the growth and development of insect herbivores. Therefore, the role that tannin plays in these phenolic-rich species of submersed plants needs to bereconsidered.We conducted experiments to study the interspecific plant preferences of snail R. swinhoei on 8 common submersed macrophytes species in Liangzi Lake, China, considering the various tannin content quantities of these plant species. The feeding consumption of a tannin-rich plant species (M spicatum L.) is compared against each of 7 other plant species in choice and non-choice tests. The snail R. swinhoei showed distinct feeding preferences when allowed to feed on 8 common occurring species of submersed freshwater macrophytes. 5 of the 7 comparisons in choice tests, and 2 of the 7 comparisons in non-choice tests show that leaves of M. spicatum are an un-preferred food for pulmonate snails. The other comparisons do not showed significant differences. In addition, comparison of tannin content and intake rate of senescent tissues are performed for M. spicatum and one tannin-poor species, V. natans. The results show senescence does not alter plant species preference. The dry matter content (DMC), carbon, nitrogen and ash content are measured for each plant species, but none of them can explain the observed herbivory patterns. No remarkable relationship was found between the tannin content and amount of all 8 plant species eaten, although the correlation coefficient is negative and snails tend to avoid tissues of M. spicatum. The tannin concentration varied considerably among different tissues within M. spicatum (about ± 25%). The range of the variation often exceeds the interspecific differences of other plant species. All these aspects of tannins in M. spicatum seems to conform to the characteristic of quantitative defenses in terrestrial plants, that only high quantities of tannins can deter generalist herbivores.Previous works have shown high dose of total irradiance can depress shoot elongation and increased phenolic content of aquatic plants, and nutrient also contribute to phenolic content. The leaves of M. spicatum contain approximately one order of magnitude higher concentrations of phenolic compounds, which play important roles in interfering with herbivores and absorbing UV-B radiation, than many other submersed plants. We wonder if M. spicatum establish similar responses mentioned above, and how the water-level related factors, including total irradiance, nutrient available, and UV-exposure in the water column, are going to interact witheach other. Thus we conducted experiments to simultaneously evaluate the influences of the three factors on the both, shoot length and phenolic content of M. spicatum. Responses of shoot length and phenolic content of M. spicatum, to different UV-B exposure (0.3 W/m2 and 0 W/m2 additional radiation), nitrogen (3.3 ppm and 0 ppm addition N in solution) and light (230 umol/m2/sec and 60 umol/m2/sec) environments, were assessed with an indoor manipulation experiment. Higher UV-B and higher light both depressed the shoot elongation, while UV-B did not alter the shoot phenolic concentration as remarkably as the light and nitrogen did. The resource availability (Carbon/Nitrogen) would be more crucial for phenolic production, than the functional responses to UV-B of M. spicatum. The three water-level related factors seem to rarely interact with each other in analyses, except the strong UV-B * nitrogen, which show UV-B can more efficiently affected the shoot length in low-N solution.In closure, major questions engaged in this thesis were as following: 1) Testing the resource availability theory among the relationship between snails and submersed plants. Results showed that the resource availability theory is still adaptive in explaining herbivory patterns and palatability variations in submersed habitats. 2) Evaluating the palatability of M. spicatum, considering its relative higher amount of phenolic content among submersed plants. Results showed that herbivorous snails tend to avoid M. spicatum more frequently than other submersed plants. 3) Clarifying the environmental factors affecting phenolic production of submersed plants in comparison with terrestrial plants. Results showed that responses of phenolic production to environmental factors in submersed plants were similar to those in terrestrial plants.