Early life-history stages competition of two native solitary ascidians Ciona intestinalis and Ascidiella aspersa

Supervisor: Marc Rius (University of Southampton, UK)
Competitions are important interactions allowing us to understand better biological diversity, and determine distribution and abundance of most marine communities. Competition can either be recognized as monopolisation of resources or by the ability of preventing others to use them. Results to these interaction can lead to dominance, when a range of a specie decrease in presence of another with same requirements, or to coexistence, when animals necessiting similar requirements are found in same proportions without dominative position. In marine sessile organisms, competition for space is closely related to the competition for food. In addition, these organisms have to settle on the most suitable substratum in order to increase their chance of successful reproduction, and to be sheltered enough from predation. Several marine taxa live usually in aggregation, as some ascidians. However it also increases strong intra- and interspecies interaction. Settlement and metamorphosis can be interrupted or affected by larval competition, which have been shown already having the possibility on impacting the juvenile performance. In this study competition of two solitary ascidians, Ascidiella aspersa and Ciona intestinalis were examined across multiple life history stages. In most ascidians, the fertilization is specie-specific, and heterospecific sperm has been shown being cable of inhibit fertilization. A. aspersa sperm didn’t show any negative effect on the C. intestinalis fertilization success, and vise-versa. Settlement rate, metamorphosis rate, survival and growth were checked according to different densities and under intra- and interspecies competition. In average, C. intestinalis performed better settlement, survival and growth than A. aspersa. Its significant settlement success, most especially in high density, could be a real advantage in limited space environment. In addition, its better survival and faster growth, participated to its dominance on A. aspersa. However, previous studies confirmed that performance at one life-history stage can reverse. Furthermore, C. intestinalis has been shown being in adult stage, a weaker competitor. Certain mechanisms appeared to allow A. aspersa to reverse from dominated to dominator organism, or vice-versa for C. intestinalis. Thus, this study confirmed that it is important to consider to whole life-history of an organism to understand its participation in biodiversity, and to understand its evolutionary mechanisms.