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Abstract The colonial ascidian Didemnum vexillum is a high-profile marine invader, with a geographically widespread distribution after introductions to several temperate regions. D. vexillum has been the focus of several eradication and control programmes globally and the need for specific biological knowledge that relates to establishment processes, persistence, impacts and potential for spread remains. The present study describes temporal patterns of D. vexillum percent cover on experimental substrates over 1.5-years in relation to seasonality of substratum availability, in conjunction with key physical (i.e. temperature and sedimentation) and biological factors (i.e. interspecific competition) at two sites in New Zealand. Colonies showed large fluctuations in percent cover between the two study sites and with reference to timing of bare substratum availability. Colonies generally exhibited an initial lag phase, with peak levels of D. vexillum cover reached during the second summer or autumn post-deployment. The long-term competitive dominance of colonies founded from the reattachment of fragments, as opposed to ambient larval recruitment alone, was also investigated. Increases in colony size as a result of ambient recruitment alone were initially much slower. However, after 12-months colony cover exceeded that of plates inoculated with fragments, suggesting the benefit of the apparent competitive dominance conferred by fragment inoculation is restricted to the early establishment phase. This information will provide increased understanding of the population dynamics of this species, as well as assist in the implementation of effective management strategies through knowledge of environmental drivers of prolific infestations. Highlights • Higher levels of colony cover after substrate deployment in summer or autumn. • Initial lag in colony growth with peak cover reached during the following summer. • Increases in colony size from ambient recruitment alone initially much slower. • Competitive dominance of fragment inoculation restricted to establishment phase. [ABSTRACT FROM AUTHOR]

The colonial ascidian Didemnum vexillum has become relatively widespread in New Zealand, since its initial discovery in 2001. Despite the potential economic and ecological impacts of D. vexillum, there are still considerable knowledge gaps surrounding its key biological attributes. The ability to obtain larvae and culture colonies in the laboratory is crucial to research into larval longevity and dispersal potential, and the factors affecting colony survivorship and growth. Here we present methods for spawning and culture of D. vexillum under laboratory conditions. A 'light shocking without cycles' technique was used to stimulate larval release in adult colonies, with > 500 larvae being produced from ∼100 g of tissue at the peak of the reproductive season. Following release, the larvae were allowed to metamorphose and the juveniles were cultured under controlled conditions for four weeks. Recruit survival during the four weeks of culture was > 85 % with the majority having formed small colonies of 4 to 6 zooids with a dense cover of white spicules throughout the tunic. The most effective laboratory spawning conditions are described with respect to light and temperature. The ability to obtain D. vexillum larvae on demand will enable increased research into several aspects of this species' reproductive biology and ecology. [ABSTRACT FROM AUTHOR]

The number and distribution of non-indigenous species in coastal habitats is increasing. Our ability to prioritise the management of this threat is limited by our understanding of their impacts. We investigated the density dependent effects of the non-indigenous solitary ascidian Pyura doppelgangera on native mussels and rocky shore communities in northern New Zealand. Minimal recruitment of P. doppelgangera was recorded during a 1.5-year experiment. Mussels showed no sign of overgrowth or spatial competition with P. doppelgangera , and their physiological condition was not impacted. We found marginal effects of the ascidian on community development, associated with small increases in diversity. We concluded that P. doppelgangera is not an aggressive competitor nor a threat to native communities, as previously thought, and that it has a very limited natural recruitment and spread potential. Reports from local Māori and a literature review suggest that P. doppelgangera has been present in the area for longer than previously thought, raising questions about its 'introduction' status and its current designation as a pest. • Minimal recruitment of Pyura doppelgangera was recorded over 1.5 years. • Mussels showed no sign of overgrowth or competition, and their condition was not impacted. • We found marginal effects of the ascidian on community development. • We concluded that the ascidian is not a threat to native communities. • We raised questions about its introduction status and its current designation as a pest. [ABSTRACT FROM AUTHOR]