Differences in the gonadal cycle between two ascidians species, Ascidiella aspersa and Ciona robusta, help to explain their invasion success in a cold temperate port

Abstract

Invasive ascidians are one of the most abundant components of the fouling communities of artificial structures. Ascidians’ tolerance to pollution, salinity, and temperature differences, alongside high reproductive and growth rates, facilitate their global spread and establishment. However, the influence of ascidians’ reproductive ecology on their abundance in cold temperate areas is poorly studied. In cold regions, ascidians’ reproductive ecology fluctuates markedly, depending on environmental factors. This might lead to differences in the gonadal cycle of invasive ascidians between the native and invaded area. Our study investigates the gonadal cycle of two invasive ascidian species, Ascidiella aspersa and Ciona robusta, and discusses their potential role in the invasion success in a cold temperate port. Using histological analysis, the gonadal cycle of each species was compared throughout the year and among seasons. Only A. aspersa showed a reproductive cycle different from their native range, with mature gonads during spring and summer. Ciona robusta showed mature gonads throughout the year, similar to its native range. These results were unexpected, as the larger abundance of A. aspersa is constant throughout the year, whereas the less abundant C. robusta shows a peak during autumn. Future research should explore how interspecific competition and predation interact with reproductive ecology, shaping the abundance of invasive ascidians. Considering the high connectivity of southern South America with Antarctica and the future scenarios of global change, knowing how the biology of invasive species change in their invasive range is key to advance and implement management strategies.