Functional significance of external trap morphology in aquatic Utricularia

Abstract

Utricularia is a genus of carnivorous plants with mechanically activated suction traps. Species are largely generalist, opportunistic predators with very plastic vegetative growth. They variously occupy terrestrial, aquatic and epiphytic habitats and can respond to changes in their environment by altering their investment in carnivory. Their traps are adorned with external appendages, the morphology of which varies greatly among species, carrying both phylogenetic and growth-habit specific signals. While this morphological variation is well documented, little is known about its functional significance. One hypothesis with limited support is that the appendage morphology of aquatic species is under selection for prey attraction. Previous work has shown that appendages of one aquatic clade, antennae and bristles, enhance the capture of one microcrustacean species. There has also been very little work done to quantify the plasticity of aquatic appendage expression, either among conspecifics or in response to environmental variation. Additionally, while studies have examined the effects of biotic and abiotic environmental variation on the growth and investment in carnivory of aquatic Utricularia, the effect of prey-derived mineral nutrition on plant growth has remained confounded with that of ambient nutrition. In this thesis I revisit the prey-capture enhancement hypothesis and look for plasticity in the appendage expression of aquatic Utricularia. Firstly, I conduct appendage ablation experiments on two aquatic Utricularia species with different growth habits, U. australis and U. gibba, to test the aquatic-appendage prey-capture hypothesis with a range of ubiquitous prey animals that exhibit differing feeding and locomotory behaviours. Aquatic appendages only enhance the trapping of prey taxa with specific feeding behaviour. Secondly, I conduct a growth experiment which produces the first experimental evidence of appendage expression changing in response to environmental variation, and demonstrate persistent differences in appendage expression between clones of the same species. Finally, with a second growth experiment, I examine the relative contributions of ambient and prey-derived nutrition to growth and investment in carnivory of U. australis. Prey capture plays a larger role in enhancing plant growth than ambient nutrition. I found little support for the aquatic prey-capture hypothesis. The capture rates of three ubiquitous prey taxa are unaffected by the presence of appendages. The degree of persistent appendage variation in between tested individuals is slight and therefore may not be functionally significant. Antennae and bristle expression is affected by environment but responses are not consistent with being an investment in carnivory.