Negative impacts of invasive species are widely accepted, but there is increasing evidence that neutral or positive effects are more prevalent than initially recognized, particularly for species which are of different functional / trophic group than the invader. We used a BACI design to examine how fish communities responded to the invasion of Halimeda incrassata, an ecosystem engineer which colonizes sandy habitats in the Western Mediterranean. While invasion did not alter overall species richness or diversity, we detected positive, negative and neutral responses by different fish species, which has important ecological and socio-economic implications. Contrasting responses likely result from different alterations that this alga conferred, putatively increasing prey availability via habitat creation, or limiting burial and camouflage abilities of fish. Our results highlight that effects of ecosystem engineers can be multiple and complex, and that predictions of invasive species are not straight forward.

Notwithstanding impacts of marine debris on fauna by ingestion and suffocation, little is known about debris-related behavior. Lytechinus variegatus is a common sea urchin known for its covering behavior. We hypothesized that L. variegatus would select more marine debris (i.e. litter) than natural material as cover and we also expected that the selected natural and artificial material would be different in weight, sizes and transparency. We haphazardly collected marine debris and natural material on 20 individuals of L. variegatus and on the bottom, around each individual. All sampled material was weighed, measured and classified regarding opacity, nature (natural or artificial). Our results showed that i) sea urchins picked more litter than natural objects, ii) proportional weight of litter carried by urchins was significantly larger than expected by chance, iii) when considering all objects (on urchins and on the bottom) litter was heavier, wider and less opaque than natural material and iv) litter carried by the urchins were wider and less opaque than natural material. We suggest that litter can influence urchin's protection against sunlight, camouflage and ballast and that sea urchins with covering behavior might be used as indicators of marine debris in coastal and deep waters.