Mechanisms for invasion success by Didemnum vexillum (Chordata: Ascidiacea): observations versus manipulations

Abstract

The widespread success of the invasive colonial ascidian Didemnum vexillum has led to descriptions of the species as fast-growing and competitively dominant, with few predators in its introduced range. However, recent experiments assessing the competitive strength and predation resistance of D. vexillum suggest that this ascidian is not a stronger competitor compared to other non-native colonial ascidians (e.g., Botrylloides violaceus, Botryllus schlosseri). To examine this apparent contradiction, juvenile D. vexillum colonies of varying size and age from an invasive population were exposed to competitive interactions with other epifaunal invertebrates and predation by mobile benthic invertebrates. Results indicate that both competition and predation had a significant negative effect on D. vexillum growth as well as on the composition of and percent cover of D. vexillum in 6- to 8-week-old experimental communities. In smaller colonies, predation also had a significant negative effect on D. vexillum survival. The discrepancy between field observations describing D. vexillum as a superlative competitive dominant and recent manipulative experiments indicating the opposite may be a function of the intrinsic life history characteristics of D. vexillum: (a) frequent colony fusion in non-native regions, (b) transport of larvae over relatively long distances via tendril detachment, (c) unique formation of colony mats on pebble substrates, and (d) calcified structures embedded in the tunic. The combined effects of these characteristics could make non-native populations of D. vexillum appear to be competitively dominant species with few predators, even though manipulative experiments on individual colonies suggest otherwise.