Three Faces of the Same Coin: Variability in Native and Invasive Populations of the Sea Spider Ammothea hilgendorfi

Pycnogonids, or sea spiders, are a peculiar group of marine arthropods that have historically been considered negligible in marine ecosystems because of their usually low abundance. However, this perception has been challenged by the recent emergence of Ammothea hilgendorfi in Europe. Native to the North Pacific Ocean, it has settled in many countries since the late 70s, including Belgium, where it was recently classified as invasive. While invasion biology often aims at comparing an alien species with a native counterpart, this comprehensive study takes the problem backwards by comparing the invasive population in Belgium with to two native ones, in the USA and Japan, to help understanding the history of this unprecedented bioinvasion. The same protocol was followed in the three locations: first, a population monitoring to assess their abundance, life cycle, biometry, and preferred environments, followed by different experiments focusing on their behaviour (reaction to stimuli), physiology (stress resistance, respirometry, metabolomics) or population structure (genetics). Results suggest that even if they may still be considered the same species, they differ from each other in many aspects, revealing the great plasticity of A. hilgendorfi, and questioning on their potential for further invasions.