Changes in the diet of the native sea urchin Arbacia dufresnii at different scenarios of the Undaria pinnatifida invasion (Patagonia, Argentina)

Undaria pinnatifida is one of the most successful marine invasive species worldwide. Since its first detection in Nuevo Gulf (Argentina), changes in the species richness and diversity of native algae and macrofauna have been reported. Knowledge of the invasive alga-native grazer interaction is key to better understand ecosystem dynamics and the potential effects of Undaria on local food webs. The aim of this study was to compare the diet of the sea urchin Arbacia dufresnii, among different scenarios of invasion of Undaria in two northern Patagonian gulfs. To this end, we determined the relative contribution of kelp to the diet of the sea urchin by comparing the gut content and stable isotopes in different tissues of A. dufresnii from sites where the invasion of Undaria was advanced, relatively recent, or not recorded. Because A. dufresnii has a plastic feeding habit and Undaria represents a high input of algal biomass, our hypothesis was that A. dufresnii feeds on Undaria and that the contribution of the invasive kelp to the diet of the sea urchin will be greater according to the time elapsed since invasion. Our results confirmed that A. dufresnii fed on Undaria and assimilated in gonads and muscle. The contribution of Undaria to the sea urchin diet was related to the stage of invasion. In the site with an advanced stage of invasion, A. dufresnii consumed the kelp both in spring and summer, when sporophytes were mature and senescent, respectively. However, in the recently invaded site, A. dufresnii consumed Undaria only in summer, when the kelp was senescent. Arbacia dufresnii is an omnivore species with the plasticity to adapt its diet to a variety of resources depending on the availability in the environment, prioritizing faunal items. This work reinforces the importance of complementing gut analysis with stable isotope analysis to determine the contribution of items in the diet. Although the impacts of an invasive alga on food webs and ecosystem structure are difficult to evaluate, performing these analyses in different scenarios may allow better understanding the local food web structure and assessing the possible ecological consequences over the invaded habitat.