This paper is the conclusion of the “Spaghetti Project” aiming to revise French species of Terebellidae sensu lato (s.l.) belonging to the five families: Polycirridae, Telothelepodidae, Terebellidae sensu stricto (s.s.), Thelepodidae and Trichobranchidae. During this project, 41 species were observed, 31 of them new for science: eight species of Polycirridae, eleven species of Terebellidae s.s., three species of Thelepodidae and nine species of Trichobranchidae. We provide a comprehensive key for all European species of terebellids with a focus on the important diagnostic characters for each family. Finally, we discuss issues on taxonomy, biodiversity and cryptic and pseudo-cryptic species of polychaetes in European waters, based on results obtained during this project.

The slipper limpetCrepidula fornicataranks among the main invasive species on French coasts and is known to cause ecological impacts on food web structure and nutrient cycling. This study investigated the effect ofC. fornicataon different components of benthic megafauna diversity (species composition, α-, β- and γ-diversity). In Arcachon Bay (France),C. fornicatawas present in 58% of the 221 sampling stations.Crepidula fornicatawas particularly abundant in four of the main benthic megafauna assemblages, suggesting a non-random distribution of its population. The mean number of megafauna species per station (α-diversity) was 11 withC. fornicata vssix without. Conversely, community similarity among stations (β-diversity) was higher whenC. fornicatabiomass increased. Opposing α- and β-diversity trends in the presence ofC. fornicataexplained the similarity of the global number of species (γ-diversity) between both situations (with and withoutC. fornicata). These results highlighted how this exotic engineer species acted on different types of diversity: the presence ofC. fornicataincreased the number of species per sample (α-diversity) but homogenized the benthic community (decreasing β-diversity) in comparison with stations whereC. fornicatawas absent.Crepidula fornicatastock in Arcachon Bay was also monitored and compared with 1999. The spread ofC. fornicatabetween 1999 and 2011 has been limited with a 318 t stock which was not statistically different than the previous estimate performed in 1999. However, the mean length ofC. fornicatacollected in 2011 was significantly smaller, mainly due to a higher proportion of small individuals.

Trematode parasites are distributed worldwide and can severely impact host populations. However, their influence on ecosystem functioning through the alteration of host engineering behaviours remains largely unexplored. This study focuses on a common host parasite system in marine coastal environments, i.e., the trematode Himasthla elongata, infecting the edible cockle Cerastoderma edule as second intermediate host. A laboratory experiment was conducted to investigate the indirect effects of metacercarial infection on sediment bioturbation and biogeochemical fluxes at the sediment water interface. Our results revealed that, despite high parasite intensity, the sediment reworking and bioirrigation rates, as well as nutrient fluxes, were not impacted. This finding was unexpected since previous studies showed that metacercarial infection impairs the physiological condition of cockles and induces a mechanical obstruction of their feet, thus altering their burrowing capacity. There are several explanations for such contrasting results. Firstly, the alteration of cockle behavior could arise over a longer time period following parasite infection. Secondly, the modulation of cockle bioturbation by parasites could be more pronounced in older specimens burying deeper. Thirdly, the intensity of the deleterious impacts of metacercariae could strongly vary across parasite species. Lastly, metacercarial infection alters cockle fitness through an interaction with other biotic and abiotic environmental stressors.