As with other simultaneous hermaphrodite nudibranchs, mating is usually performed with mutual penetration by a couple meeting head to head with their genital apertures behind their right cephalic tentacles adjacent to each other. Sometimes, as in this image, they mate both facing the same direction with one lying prone and the other supine so that their genital apertures are proximate to each other. 1: yellow penis.
ELWS, Menai Strait, Wales. February 2013.