This video says about itself:
Nudibranch (Archidoris pseudoargus) laying eggs
20 April 2014
On February 7, 2015 the rare sea slug Archidoris pseudoargus was seen in the northwestern Oosterschelde estuary. In the past 30 years, this slug had been found in the waters of Zeeland province only five times previously. What makes the observation even more special is that for the first time in the Netherlands also Doridicola agilis, a parasitic copepod, was found in the gills of a Archidoris pseudoargus slug.