Abstract :
[en] Pycnogonids, or sea spiders, are ubiquitous arthropods present in most marine
ecosystems. However, because of their usually low population densities, they remain
understudied by zoologists despite presenting numerous unique biological features.
The species Ammothea hilgendorfi (Böhm, 1879), native to the Northern Pacific Ocean,
was introduced in Europe in the late 1970s and its presence has now been reported in
several seas. Recordings of the species in Europe are sporadic and mostly do not
exceed a handful of specimens. Hence, the populations have only been considered
as introduced in the invasion process. The goal of this study is to investigate the
first population of A. hilgendorfi at the Belgian coast (North Sea) during a 24-month
monitoring through different proxies (phenology, biometry, developmental cycle).
For the first time, our results demonstrate that this population of pycnogonids can be
considered as established as it is able to complete its entire life cycle, including all
larval instars, which are ectoparasites of hydroids. Moreover, it has developed in very
high densities of individuals and has outnumbered by far the native sea spiders, which
have almost disappeared, so it can also be considered as invasive sensu stricto. Their
average density increased from the first year to the second, which indicates that the
invasion has likely not reached its capacity yet. Since research on sea spiders has
been very limited, this work emphasizes the importance of studying these animals,
especially when the lack of knowledge undermines the assessment of their impact and
their invasiveness potential.
