Promoteurs: Monsieur Guillaume Caulier et co-promoteur: Monsieur Igor Eeckhaut

Titre de la dissertation:
Study of the invasive and native populations of the pycnogonid Ammothea hilgendorfi: structure, success and invasive traits

Résumé de la dissertation:
Pycnogonids (i.e. pantopods) are a class of marine arthropods scarcely studied due to low population sizes. Among the 1400 species belonging to this taxon, only one, originating from the North Pacific Ocean, stands out as it recently spread outside its natural distribution: Ammothea hilgendorfi (Böhm, 1879). Indeed, it has been frequently collected in Europe from 1978, in the North Sea from 2010, and in Belgium from 2018. From 2022 to 2024, this species was collected in Knokke (Belgium) up to 600 specimens/m², exceeding by far the average density of pycnogonid populations, and dominating the ecosystem. Therefore, it can be considered as an invasive species. Apart from its distribution, virtually nothing is documented about this species (i.e. its biology remains unknown). The goal of this thesis, divided into four parts, is to characterize the biology of A. hilgendorfi to determine the elements that promoted its invasion in Europe (i.e., invasive traits). To do so, populations of A. hilgendorfi were sampled at several places in the world, in the native and non-native range and compared to each other, as well
as with other pycnogonid species jointly collected. Specimens were counted, measured, and sexed to characterize their population structures, niches and phenologies. Their behaviors and reactions to several stimuli were investigated using innovative device and their gregarious and exploratory tendencies were addressed. Finally, their resistance to various physicochemical stresses was assessed, and an extreme physiological state, dormancy, was explored.