The Thick-horned Nudibranch was previously called the Opalescent Nudibranch, but recent taxonomic studies have set it apart from its southern cousin the Hermissenda opalens.  These animals can be utterly ubiquitous in certain areas and rarely seen in others. The racing stripe that runs down the centre of its mantle immediately differentiates it from other similar species, such as the White and Orange-tipped Aeolid. Sometimes this stripe can light up in an iridescent blue, providing a beautiful contrast to the brilliant orange and white of the more colourful individuals.  For more information (and contribution to citizen science!) see inaturalist.org.