From Unknown to Invasive in a Few Years – The Story of the Discovery of the Sea Spider Ammothea hilgendorfi in Belgium

Pycnogonids, or sea spiders, are unique arthropods which, despite their low abundance, are present in most marine ecosystems. Belgium made no exception as three European species have historically been reported in Knokke, a biodiversity hotspot at the Belgian coast. In 2022, however, was discovered a new species, named Ammothea hilgendorfi (Böhm, 1879), in densities too high to be considered as normal in that group. It is in fact native to the North Pacific Ocean (i.e., Japan and the USA) and was introduced in Europe in the late 1970s, including Italy, the Netherlands, France, and now Belgium. Therefore, the goal of this study was to investigate this population of A. hilgendorfi during a 24-month monitoring using different proxies (phenology, developmental cycle…). For the first time in Europe, results demonstrate that this pycnogonid can be considered as invasive as it can complete its entire life cycle, including all larval instars, and has developed very high densities outnumbering by far the native sea spiders, which have almost disappeared. Our results also suggest that the invasion has likely not reached its capacity yet, emphasizing on the importance to study this peculiar clade.