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Abstract :
[en] Sea spiders, or pycnogonids, are marine arthropods with unique biology and morphology. Among the 1,400 extant species, only one has become invasive: Ammothea hilgendorfi (Böhm, 1879). Originally from the North Pacific Ocean, it has been documented throughout Europe since the late 1970s, yet it has remained largely overlooked by ecologists. Recent fieldwork in Belgium has revealed a population of this species reaching unprecedented abundances, displaying what appeared to be gregarious behavior. Given that gregariousness enhances the reproductive success of a species if there are no other limiting factors – as is the case here – it should be regarded as an invasive trait that may explain its success. This integrative study employs a combination of fieldwork and ethological experiments using various approaches to investigate the gregarious behavior of A. hilgendorfi. First, their spatial distribution was analysed in the field and in a controlled environment. Then, to decipher the factors driving their gregariousness, the reaction of specimens to different stimuli (i.e., phototaxis and intraspecific chemotaxis) was monitored. Our findings statistically confirmed the gregariousness of that species, driven both by environmental and intraspecific factors. They shed light on the group dynamics and ecological interactions of this alien pycnogonid, contributing to a broader understanding of the first ever invasive population of this taxon understudied for decades.
