Many different polychaete-echinoderm relationships have been described, from tropical to polar environments. Most of these associations have been generally defined as ‘commensal’, with polychaetes guests usually found on the oral surface of their hosts or, in a very few cases, even inside the host's body. Here we present an inquilinistic association involving two Antarctic species, the polychaete Gorekia crassicirris (Willey, 1902) (Polynoidae) and the irregular sea urchin Abatus nimrodi (Koheler, 1911) (Schizasteridae) found in the Ross Sea. This record is only the second worldwide for this kind of association, after that of the polychaete Benthoscolex cubanus which lives in the gut of the spatangoid Archeopneustes hystrix in Caribbean waters. Gorekia crassicirris seems to be a polyxenous species as it was also observed on another schizasterid, Brachysternaster chescheri Larrain, 1985 in the Weddell Sea. Considering that A. nimrodi is absent from that area and that the two sea urchin species have a disjoint distribution, it is possible that a ‘host-switch’ phenomenon occurred at some stage. We review the available literature to compare the Antarctic pairing with the other known examples of similar associations.
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