Study of the Invasive and Native Populations of the Pycnogonid Ammothea hilgendorfi: Structure, Success, and Invasive Traits

Invasive species live outside their historical range, complete their entire life cycle there, then spread and/or become ecologically dominant. Although they pose a major threat to biodiversity, globalisation has driven their numbers to increase continuously. While some of these species are well known and studied, others remain overlooked by the scientific community. This is notably the case for Ammothea hilgendorfi (Böhm, 1879), a marine arthropod belonging to the class Pycnogonida. These animals are characterised by body reduction, organ extension into the legs, and exclusive paternal care. Despite their uniqueness, pycnogonids are rarely investigated because of their typically low abundance. The discovery of a dense population of A. hilgendorfi on the wave breakers of Knokke, Belgium, has raised double concerns: first, because it is not native to Europe; and second, because its density contrasts with the general rule of that group. As such, the present PhD thesis represents the only ecological study of this species thus far and provides a robust foundation for further research. The first goal was to determine whether A. hilgendorfi could be classified as invasive, and the second was to understand how this species became so successful in Belgium. The scientific strategy revolved around the analysis of invasive traits and comparisons between native and invasive populations. This thesis led to pioneering discoveries, including a remarkable ecological versatility, gregarious tendencies, and a form of dormancy, a first for pycnogonids.